Patent Publication Number: US-9413824-B1

Title: Storage constrained synchronization of content items based on predicted user access to shared content items using retention scoring

Description:
BACKGROUND 
     The described embodiments relate generally to improving the performance of computer systems providing content item synchronization, and particularly to improving the synchronization of content items between a client device and a content management system where storage allocation for synchronized content items is constrained. 
     Content management systems enable users to share content items from one client to another client. The clients are computing devices that provide content items to a content management system for storage and synchronization with other clients. The other clients may be operated by another user or may be devices registered or managed by the same user. A user designates which content items or directories containing content items are available to be shared with other users, and thus synchronized to the client devices of such users. Generally, a content management system synchronizes a given content item with all of the client devices that have been designated to share the content item. As a result, each of these client devices may store a very large amount of shared content items. In some cases, the amount of storage taken up on a client device by the shared content items substantially reduces the amount of storage available on the client device for other items, such as unsynchronized content items and applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are concept diagrams that illustrate one embodiment of constrained synchronization. 
         FIG. 2  illustrates a system environment for a content management system that synchronizes content items between client devices. 
         FIG. 3  illustrates the software architecture of a client device. 
         FIG. 4  is an interaction diagram of constrained synchronization for accessing an existing content item in the local content directory. 
         FIG. 5  is an interaction diagram of constrained synchronization for creating a new content item to be stored in the local content directory. 
         FIG. 6  illustrates a system environment for host based constrained synchronization. 
         FIG. 7  is an interaction diagram of constrained synchronization managed by a host device. 
         FIG. 8  is an illustration of a user interface for a local content directory with icons representing remote and local content items. 
         FIG. 9  is a concept diagram illustrating constrained synchronization using predicted content item importance. 
         FIG. 10  illustrates a system environment for a content management system using predicted content item importance for constrained synchronization. 
         FIG. 11  illustrates the software architecture of a client device using idle state triggered content management. 
         FIG. 12  illustrates a system environment for a content management system using idle state triggered content management. 
         FIG. 13  is a flow diagram illustrating the process used in idle state triggered content management. 
     
    
    
     The figures depict various embodiments for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
     DETAILED DESCRIPTION 
     Functional Overview of Constrained Synchronization 
     A general functional overview of a constrained synchronization system and process is now described. As a preliminary condition, users store content items on client devices, and the content items are synchronized with instances of the content items on other clients and with a host system, typically a content management system. A client device stores the content items in a local content directory. Content items stored in the local content directory are synchronized with a content management system, which maintains copies of the content items and synchronizes the content items with other client devices. Each client device executes a client application, which enables the user to access the content management system. The client application further enables the user to configure a maximum storage allocation or size for the local content directory. 
     In one aspect, the client device is configured to selectively determine which synchronized content items remain locally available on the client device, and which are stored in their entirety only on the content management system. In one embodiment, the client device receives a request to access a content item, for example from an application needing access to the content item. The client device determines whether the requested content item is a shadow item or a content item stored locally on the client device. A shadow item is an item that represents or emulates the content item, but does not contain the application data of the content item. Generally, the shadow item replicates the metadata attributes of the content item, such as the name of the content item, as well as various attributes, such as type, path information, access privileges, modification information, and size of the content item, without storing the actual application content, such as text, image data, video data, audio data, database tables, spreadsheet data, graphic data, source or object code, or other types of content data. Because the shadow items only store metadata for the content item, they require only a small amount of storage, e.g., typically about four kilobytes, as compared to a content item that can be hundreds of megabytes or even several gigabytes in size. Thus, using shadow items to represent content items operates to save considerable storage space, thereby improving the functioning of the client device. 
     Where the client device determines that the requested content item is a shadow item, this indicates that the requested content item content is not at present stored on the client device, but is stored on the content management system. Accordingly, the client device downloads from the content management system the content item that corresponds to the requested shadow item. The client device further determines whether storing the content item in the local content directory would exceed the maximum storage size established for that directory. In that case, the client device determines which content item or items in the local content directory can be removed from the local content directory, and replaced with shadow items that represent the content items. Generally, the client device uses one or more attributes of a shared content item to select content items from the local content directory that have been determined as being unattended by the user of the client device or users with access to the content item via the content management system, including latest access time on the client device (e.g., actions of the user of the client device or applications executing thereon), latest access time on the other client devices with which the content items are shared (e.g., actions of the users of those client devices), content item size, and access frequency. Combinations of these factors may also be used to determine unattended content items. The client device selects a number of content items from the local content directory such that deleting these content items creates a sufficient amount of storage space in the local content directory to allow the downloaded content item to be stored therein without exceeding the maximum storage size. In one embodiment, the client device selects a number of content items so that the total amount of storage used by these content items in the shared content directory at least equals or exceeds the amount of storage required to store the downloaded content item. 
     The client device deletes the selected content items, and for each deleted content item creates a corresponding shadow item. The client device stores the shadow items in the directory locations corresponding to the deleted content items. Storage of the shadow items in the corresponding locations enables subsequent retrieval of the deleted content items in a manner that is transparent to the requesting applications. 
     This embodiment provides a constrained shared storage system wherein each client device can maintain access to all content items shared with a content management system while having more space for other content items and applications, and improves the storage efficiency of each client device as well as content management system as a whole. More particularly, the embodiment enables a client device to effectively operate as if it has a significantly larger storage capacity than it does in fact. For example, a client device with only a 10 GB storage allocation for the local content directory can operate as if it had over 4,000 GB (4 TB) of storage allocation for that directory, representing a 400-fold increase in effective storage. In the past, such a solution to limited local storage capacity was made impossible by network connectivity and bandwidth limitations, thus the problem being solved for arises as a result of the recent developments in Internet infrastructure that allows for pervasive connectivity and fast upload and download speeds. 
     Despite the recent developments in Internet infrastructure, the computational, uploading, and downloading times required for the removal of content items, their replacement with shadow items, and their restoration following a user request may still impact device performance. Therefore, alternative embodiments are also described that reduce impact on device performance as visible to the user while still reducing the storage burden on a client device over traditional shared content synchronization methods. In one embodiment, the computation, uploading, and downloading are completed based on a predicted user access to a shared content item represented as a shadow item. To predict a user access to a content item, the client application or the content management system maintains a retention score for each content item; the retention score is a measure of the predicted importance to the user of each content item. Each client device is configured with a retention score threshold such that any content item with a sufficiently high predicted importance (represented by a retention score that exceeds a retention score threshold) is downloaded to the corresponding client device. The retention score may be calculated based on a variety of attributes including latest access time, location, type, size, access frequency, shared status, number of accounts with access, number of devices with access, or number of devices storing the content item. 
     Alternatively, another embodiment allows the storage space occupied by shared content items on a client device to exceed the storage allocation while the activity of the client device is monitored (either by the content management system or by the client application). When a client device is determined as being idle, the client application removes the content items and replaces them with shadow items, as previously discussed, in order to reduce the effective storage space occupied by the content items stored on the client device. In these embodiments, the storage allocation is not maintained at all times and so occupied storage can be reduced according to other content item attributes. Instead of maintaining a storage allocation, for example, all content items with a latest access date older than a specified amount of time (e.g., two weeks) could be removed and replaced with shadow items whenever the client device is idle. This process does not keep the occupied storage space below a storage allocation but would reduce it in a way that might be preferable to the user since the operations are done while the client device is idle and thus not being actively used by the user, thereby improving a client device configured to use a constrained synchronization system by offering a user experience improvement over the previously described embodiments while providing a similar increase in effective storage capacity. 
       FIGS. 1A and 1B  are concept diagrams that further illustrate embodiments of constrained synchronization.  FIG. 1A  illustrates a process of saving a content item in a storage constrained synchronized folder.  FIG. 1B  illustrates a process of opening a shadow item on a storage constrained client device. 
     In  FIGS. 1A and 1B , client device  100 A is one of a plurality of user controlled devices that can be connected and synchronized with content management system  110 . Content management system  110  is a server instantiated to synchronize content from a plurality of client devices using a network. A shared content storage directory  120  is a directory located on the client device  100  that contains content synchronized with content management system  110 . A storage allocation  130  is a parameter value that specifies an amount of storage space allowed for all content items in the shared content storage directory  120 . The storage allocation  130  can be set by the user of the client device  100 A, the operating system of the client device  100 , a client application of content management system  110 , by a system administrator, or by policies established at content management system  110 . An example value for the storage allocation  130  is 10 GB; this means that the user can store up to 10 GB of content items in their entirety (all content item attributes and data) in the shared content storage directory  120 . Content items  140  are saved within the shared content storage directory  120 ; after synchronization between a client device  100  and content management system  110  a version of each content item  140  in the shared content storage directory  120  is also maintained by content management system  110 . 
     The term “content item”, as used herein indicates any file, group of files, or collection of files. Any content item that consists of only a single file may alternatively be referred to as a file. Additionally, terms such as “file table” may be used to refer to both individual files or content items. 
     In  FIG. 1  the shared content storage directory  120  is graphically depicted as a box that contains the content items  140 . The storage allocation  130  is represented by the particular length of the box representing the content storage directory  120 . 
     The first illustration of the client device  100 A and content management system  110  represents a typical state of the two entities. The client device has content items  140 A,  140 B, and  140 C stored within its shared content storage directory  120  (only a small number of content items  140  are shown for the purpose of explanation, as in practice the number of content items  140  can be in the thousands, tens of thousands, or more). Content management system  110  is represented as being synchronized with client device  100 A and so it maintains an identical version of each of the content items stored on the client device  100 A though it does not have a storage allocation  130 . Additionally, content management system  110  supports another client device  100 B, with which the content item  140 A is shared. The presence of content item  140 D in association with the identification of client device  100 B indicates that client device  100 B is also synchronizing this content item  140 D with content management system  110 . Thus, each client device  100  can synchronize content items  140  with only content management system  110  or with content management system  110  and other client devices  100 . 
     Stage  1 . 1  illustrates the operation of a request from client device  100 A to save content item  140 E to the shared content storage directory  120 . However, as illustrated, the addition of content item  140 E to the shared content storage directory  120  would cause the total storage space occupied by the content items  140  to exceed the storage allocation  130 , since the size of content item  140 E exceeds the remaining available space in the shared content directory  120  as limited by the storage allocation  130 . 
     Stage  1 . 2  illustrates the operation of the selection of an unattended content item  140 C to be removed from the client device  100 , so as to make available sufficient storage in which content item  140 E can be stored. Depending on the embodiment, either the client device  100  or content management system  110  determines which content items  140  to select as being unattended. A variety of methods, discussed below can be used to determine which content items are selected as unattended. While only a single content item  140 C is selected in this example, in practice any number of content items  140  may be selected, depending on the amount of storage capacity that needs to be made available. 
     Stage  1 . 3  illustrates the operation of removing the selected content item  140 C from the client device  100 A. In place of each removed content item, the client device  100 A creates a shadow item  160 C that represents the removed content item  140 C, and stores the shadow item in the same location in the shared content storage directory  120  as the removed content item  140 C. Alternatively, the content management system  110  may create the shadow item  160 C and then download the shadow file  160 C to the content storage directory  120 . The shadow item includes attributes that represent the removed content item  140 C, such as the content name, path information, content attributes, and content size, but without containing the actual data of the content item  140 C. By not including the actual data of their corresponding content items, shadow items require considerably less storage. For example, a shadow item typically requires no more than the smallest file size allocation provided by the operating system, such as 4 KB. This small size is illustrated visually in  FIG. 1  using a vertical line, showing that the size of the shadow item is negligible when compared to the content item  140 C itself. For example, while the removed content item  140 C may be many megabytes or even gigabytes in size (very common for audio or video files), the storage required for a shadow item representing such a content item would still be only 4 KB or so. As a result, the client device  100  is able to reduce the amount of local storage used for shared content items to an amount below the storage allocation  130 , and thereby make available sufficient space to store the newly created (or updated such that the new version of the content item is larger) content item  140 E. Information identifying the selected (and removed) content items is maintained on the client device  100 A, to allow these items to be selectively retrieved at a later time. This information is stored locally in client device  100  in a list  150  of stored content items that are remotely stored in remote content item table  366  (as further described below, not illustrated in  FIG. 1A ) in content management system  110 . 
     Stage  1 . 4  illustrates the operation of saving content item  140 E to the client device  100 A once sufficient space has been made available in the shared content storage directory  120 . Once the client device  100 A successfully saves the content item  140 E to the shared storage directory  120 , synchronization with content management system  110  is initiated and content item  140 E is uploaded to content management system  110 . Content management system  110  still maintains full copies of all content items (including shadow items) on client device  100 A. 
     Referring now to  FIG. 1B , client device  100 A and content management system  110  are shown after content item  140 E has been synchronized between the client device  100 A to the client management system  110 . 
     Stage  1 . 5  illustrates the operation of client device  100 A requesting access to content item  140 C (e.g., open content item  140 C using a word processor, or show the content item in a file browser), wherein client device  100  determines that the requested content item is represented by a shadow item. If the content item is stored locally, it is provided to the requesting application on the client device  100 A. In this case the requested content item has been removed from the client device  100 A and is only stored remotely on content management system  110 , so the client device  100  requests content management system  110  to download the requested content item. If there is sufficient space on the shared content storage directory  120 , content management system  110  downloads the requested content item to the client device  100 A; the client then replaces the shadow item  160 C that represented content item  140 C with content item  140 C itself, which allows any requesting application to access the content item transparently. However, in this case, the addition of content item  140 C to the shared content storage directory  120  would exceed the storage allocation  130 , as depicted by content item  140 C extending outside the boundaries of the shared content storage directory  120 . 
     Stage  1 . 6  illustrates the operation of selecting unattended content item(s) for removal from the client device  100 A. In this case, the unattended content item selected is content item  140 A. 
     Stage  1 . 7  illustrates the operation of removing content item  140 A and replacing it with its shadow item  160 A. This removal creates enough space in shared content storage directory  120  for content item  140 C to be downloaded from content management system  110  and appended to its shadow item representation without exceeding the storage allocation  130 . The removed content item  140 A is included in the list  150  of remotely stored content items, and content item  140 C is removed from this list  150 , since it has been restored to the shared content directory  120 . 
     Stage  1 . 8  illustrates that once content item  140 C is resident on client device  100 A it can be opened by the requesting application. Once the processes illustrated by  FIGS. 1A and 1B  on client device  100 A have been completed, normal synchronization can occur between client device  100 A and content management system  110  such that all changes to content items  140  on client device  100 A are mirrored on content management system  110 . All content items  140  (even if represented by shadow items) are maintained on content management system  110  until they are deleted from the shared content storage directory  120 . 
     Overview of System Architecture 
       FIG. 2  illustrates the system architecture of a constrained synchronization system. Details about each component will be further described in a later section, however some elements are introduced here to provide context for the explanation of constrained synchronization. Further, as is apparent to those of skill in the art, the operations and methods used in constrained synchronization necessarily require a computer, and are not performed in any embodiment by mental steps by a human operator. Further, while the operations may make use of the facilitates of a computer to store and retrieve information, transmit and send information, or process information, those of skill in the art appreciate that such operations are not simply generic computer operations since they are herein performed in specific manners on specifically defined data using the algorithms described herein, and thus require configuration of a computer in a manner different from how such computers are provisioned natively by their operating system and standard applications alone. Additionally, the required configuration enhances the storage capacity of the computer, through the steps detailed below, over generic, general purposes computers configured with conventional operating systems and file management systems. 
     Client devices  100  communicate with content management system  110  through a network, not shown, which can be any suitable communication means providing internetworking between client devices  100  located remotely from content management system  110 ; e.g., a LAN, WAN, or WAN. In general, client device  100 A with a client application  200 A installed provides content items to content management system  110 . The client application  200 A contains the programs and protocols necessary for client device  100 A to perform the functions associated with storage constrained synchronization. Therefore, client device  100 A often performs actions requested by the client application  200 A. However because client device  100 A and client application  200 A act together, for ease of description some of these actions are referred to using “client device  100 A” as the operative element. The user of client device  100 A has designated certain of the content items to be shared with client device  100 B, which for example, can be another computer managed by the same user, or a computer operated by a different user. Content management system  110  notifies client device  100 B and synchronizes the designated content items received from client device  100 A with local content stored at client device  100 B. 
     Content management system  110  associates each content item with a namespace corresponding to a set of content items. A namespace designates a directory (or “folder”) in a directory structure into which the given content items are stored. The association of content items with particular namespaces is stored in a namespace table  222 . Content management system  110  associates each client with the namespaces (and content items therein) to which it has access, along with an identification of the specific rights to access, modify, and delete the content items in each namespace. When clients  100  are synchronized to a namespace, the clients store a local copy of content items associated with the namespace and organize the content items according to content location, if available. A user may be associated with an individual client device  100  or with multiple clients  100 ; for example, a user may have a home computer, a work computer, a portable computer, a smartphone, and tablet computer all synchronized together. To share content items, a user designates a namespace to be shared with other users and/or clients. Content management system  110  then synchronizes the content items in the shared namespace(s) across the clients  100  associated with the shared namespace. The content items stored at content management system  110  can include any type of content item, including documents, data, movies, applications, code, images, music, and so forth. The content item may also be a folder or other mechanism of grouping content items together, such as a collection, playlist, album, file archive, and so forth. 
     Each user is associated with an account on content management system  110  that includes information specifying an amount of storage to be used for storing content items on content management system  110 . A client device also has a designated amount of local storage for storing the synchronized content items, which is the size of the shared content storage directory  120 ; this designated amount is the storage allocation parameter  130  described above. For example a user&#39;s account may specify that the user has 50 GB of storage available on content management system  110 , but has a storage allocation on the client device  100  of only 10 GB. In circumstances such as this, when the user modifies a shared content item that is stored locally, the content item may increase in size, and thereby exceed the storage allocation on the client device  100 . Similarly, the user may exceed the storage allocation on the client device  100  by creating and storing in the shared content directory  120  a new content item to be shared and synchronized with content management system  110 . In these cases, the amount of shared content items exceeds the storage allocation for the client device  100 , in which event the client device  100  is storage constrained and can no longer maintain a local copy of all content items synchronized by content management system  110 . 
     Either the client device  100  or content management system  110  is configured to select one or more content items to remove from the local storage while still maintaining them remotely on content management system  110 , so that they can be subsequently retrieved and restored to the client device  100 . Generally, the content items that are selected are those that are least recently accessed, either on the particular client device  100  on which the request to access the content item is made, or across all client devices  100  on which the content items are shared; other methods of selections are discussed further in a following section. In a client-based embodiment, the client application  200  maintains information identifying the latest access for each shared content item stored on the client device  100 . When storage is constrained, the client application  200  selects one or more of the content items that have been least recently accessed (herein, “LRA”). In a host-based embodiment, content management system  110  maintains the access data for every content item; the system  110  updates this information anytime a content item is accessed on any client device  100  with which the content item is shared. LRA selection is only one of a number of possible unattended content item selection methods (herein “UCSM”) each of which can be implemented as either a host-based or client-based system. Any UCSM may consult the vnode reference for the each content item to determine whether it is eligible for removal. The vnode for each content item contains information regarding a number of accesses to the content item as well as other content item status indicators including whether or not the content item is currently in use or open. 
     For succinctness, whenever content items are selected for removal from residency on a client device  100  in response to a storage constraint, the operation is referred to herein as “selecting the unattended content items,” since most of the UCSM operate to identify those content items that are least likely to be accessed by the user. Unattended content item refers to content items selected by any UCSM outlined in the following discussion. 
     Basic LRA Selection: To perform basic LRA selection, the client application  200  maintains a queue of content items ordered by latest local access date with the least recently accessed content item at the top of the queue. The latest access date and time for each content item is maintained in a content access history table. An access to a content item includes the actions of creating, opening, previewing, or modifying a content item. Any number of these actions can be deemed an access, for example, an embodiment might deem an access to be either opening, modifying, or saving a content item but previewing a content item may not be deemed an access. A cumulative sum (e.g., running total) of the storage size is calculated for each content item listed in the queue starting with the least recently accessed content item identified in the queue (i.e., the content item at the top of the queue), and ending with the content item at the end of queue. When storage is constrained, the client application  200  determines an amount of storage space required to store a content item, and so progresses through the queue to identify the index of the content item for which the cumulative storage size exceeds the storage space requirement. The identified index is used to select all content items above and including that index in the queue for removal from the shared content storage directory  120  on the client device  100 . 
     These processes are further explained in Table 1. In this example, 75 MB of storage are required to store a content item. Because content items A and B only total 70 MB, removal of these two content items does not provide a sufficient amount of storage for the item. Accordingly, content items A, B, and C, which have a total cumulative side of 150 MB are selected (as indicated by the designation in the rightmost column), at corresponding indices 00, 01, and 02. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Item 
                   
                 Item 
                 Cumulative 
                   
               
               
                 Index 
                 Name 
                 Local Access Time  
                 Size 
                 size 
                 Selected? 
               
               
                   
               
             
            
               
                 00 
                 Item A 
                 Jan. 3, 2014  
                 10 MB 
                  10 MB 
                 Yes 
               
               
                   
                   
                 4:33 PM  
                   
                   
                   
               
               
                 01 
                 Item B 
                 Mar. 24, 2014  
                 50 MB 
                  60 MB 
                 Yes 
               
               
                   
                   
                 5:12 PM 
                   
                   
                   
               
               
                 02 
                 Item C 
                 Mar. 24, 2014  
                 20 MB 
                  80 MB 
                 Yes 
               
               
                   
                   
                 6:18 PM 
                   
                   
                   
               
               
                 03 
                 Item D 
                 Mar. 30, 2014  
                 80 MB 
                 160 MB 
                 No 
               
               
                   
                   
                 6:22 PM 
                   
                   
                   
               
               
                 04 
                 Item E 
                 May 18, 2014  
                 20 MB 
                 180 MB 
                 No 
               
               
                   
                   
                 7:53 AM 
               
               
                   
               
            
           
         
       
     
     Remote LRA Selection: LRA selection can also be based on remote accesses by other users that have access to the content items through content management system  110 , either directly thereon, or on client devices  100  which have shared versions of the content items. To accomplish this, in one embodiment, each client device  100  synchronizes its own content access history table with content management system  110 , for example, during normal content item synchronization operations, or at other times. This embodiment enables each client device  100  to maintain current access information for every content item that it shares with any other client device. Alternatively, for a host based embodiment, content management system  110  may maintain a content access history table that contains the access history for each content item across all client devices that are designated for synchronization and sharing, so that it has a currently updated list to use for LRA selection. Remote LRA selection then includes the content management system  110  selecting the least recently accessed content items whose cumulative storage size exceeds the required storage space. In this embodiment, this queue is ordered by latest access times from all client devices that are synchronized with respect to the content item. 
     Table 2 is an example of how remote LRA may be implemented. In this example, Content items B and C were last accessed remotely on a different client device on May 24, 2014 and Apr. 5, 2014 respectively but were both last accessed locally on Mar. 24, 2014 (as listed in Table 1). This change in latest access date for Items B and C, due to their remote accesses, moves them farther down in the queue compared to when basic LRA selection is used. As a result, in this example, Items A and D are selected instead of A, B and C. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Last Access 
                   
                   
                   
               
               
                   
                 Item 
                 Time Across all  
                 Item 
                 Cumulative 
                   
               
               
                 Index 
                 Name 
                 Sharing Clients 
                 Size 
                 Sum 
                 Selected? 
               
               
                   
               
             
            
               
                 00 
                 Item A 
                 Jan. 3, 2014 
                 10 MB 
                  10 MB 
                 Yes 
               
               
                   
                   
                 4:33 PM 
                   
                   
                   
               
               
                 01 
                 Item D 
                 Mar. 30, 2014 
                 80 MB 
                  90 MB 
                 Yes 
               
               
                   
                   
                 6:22 PM 
                   
                   
                   
               
               
                 02 
                 Item C 
                 Apr. 5, 2014 
                 20 MB 
                 110 MB 
                 No 
               
               
                   
                   
                 5:57 PM 
                   
                   
                   
               
               
                 03 
                 Item E 
                 May 18, 2014 
                 20 MB 
                 130 MB 
                 No 
               
               
                   
                   
                 7:53 AM 
                   
                   
                   
               
               
                 04 
                 Item B 
                 May 24, 2014 
                 50 MB 
                 180 MB 
                 No 
               
               
                   
                   
                 5:12 PM 
               
               
                   
               
            
           
         
       
     
     Content item Size Selection: Another factor that may be used to select content items for removal is their size. In one embodiment, size is used to minimize the number of content items that are removed and stored remotely from the client device  100 . This can be accomplished by ordering the queue by size (smallest to largest) instead of by access date. Then the required storage space value could be compared to the individual sizes until a content item having a size that exceeds the required storage space is identified. The client application  200  would then select this content item for removal. If no single content item is larger than the required storage space then the largest content item would be selected and its size subtracted from the required storage space value and the process would be repeated from the beginning of the queue. 
     Table 3 is an example of this selection method. For this example, 40 MB of storage are required to store a content item. Item B is the first content item by queue index that exceeds the required storage value of 40 MB and so it is selected for removal from client  100 . 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Index 
                 Item Name 
                 Access Time 
                 Item Size 
                 Selected? 
               
               
                   
               
             
            
               
                 00 
                 Item A 
                 Jan. 3, 2014 
                 10 MB 
                 No 
               
               
                   
                   
                 4:33 PM 
                   
                   
               
               
                 01 
                 Item C 
                 Mar. 24, 2014 
                 20 MB 
                 No 
               
               
                   
                   
                 6:18 PM 
                   
                   
               
               
                 02 
                 Item E 
                 May 18, 2014 
                 20 MB 
                 No 
               
               
                   
                   
                 7:53 AM 
                   
                   
               
               
                 03 
                 Item B 
                 Mar. 24, 2014 
                 50 MB 
                 Yes 
               
               
                   
                   
                 5:12 PM 
                   
                   
               
               
                 04 
                 Item D 
                 Mar. 30, 2014 
                 80 MB 
                 No 
               
               
                   
                   
                 6:22 PM 
               
               
                   
               
            
           
         
       
     
     Content item Size and Access Time Based Selection: The size selection method just described may sometimes select frequently accessed content items for removal. By taking both size and access time into account the content management system can avoid removing content items from a client device  100  that may be requested by the user in the near future. In one embodiment, this dual-variable selection method is accomplished by calculating a weighted score for each content item, based on the amount of storage each content item contributes to reaching the required storage allocation and its last access date. For example
 
Score= w   1   S+w   2   A  
 
where S is a metric representing the content item size, A is a metric representing the time since the last access to the content item, and w 1  and w 2  are the weights. The weights for A and S can based on their relative importance, as determined by the user, by a system administrator, or the based on historical content item access patterns for content items on the particular client device  100 . The queue is then be ordered by Score and the first content item in the queue is selected for removal.
 
     An example implementation of this selection method is illustrated in Table 4 below. For the purposes of this simple example, the access time metric A is the ratio of the difference between the current time and the latest access for the particular content item and the difference between the present time and least recently accessed item access (in this case the date used was Sep. 3, 2014). In this example, size metric is the relationship:
 
For  s≧r:S=r/s  
 
For  s&lt;r:S=s   2   /r   2  
 
where s is the content item size, r is the required storage space, and S is the size metric. This piecewise function has a maximum of 1 when s=r.
 
     In the example displayed in Table 4, the required storage space is 40 MB and the weights w 1  and w 2  are both 1. The size metric and the access time metric are calculated and then used to calculate the total Score, for each content item. In this example, Item B has the highest score and so is selected for removal from the client device  100 . If the content item selected has a size smaller than the required storage space, a new required storage space is calculated as the difference between the old required storage space and the size of the first selected content item, the score is recalculated and a new queue is generated for all content items using the newly calculated required storage space, and the selection process is repeated. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                 Access 
                   
                 Item 
                   
                   
               
               
                 In- 
                 Item 
                 Access 
                 Time 
                 Item 
                 Size 
                 Total 
                   
               
               
                 dex 
                 Name 
                 Time 
                 Score 
                 Size 
                 Score 
                 Score 
                 Selected? 
               
               
                   
               
             
            
               
                 00 
                 Item B 
                 Mar. 24, 2014 
                 0.67 
                 50 MB 
                 0.8  
                 1.47 
                 Yes 
               
               
                   
                   
                 5:12 PM 
                   
                   
                   
                   
                   
               
               
                 01 
                 Item D 
                 Mar. 30, 2014 
                 0.65 
                 80 MB 
                 0.5  
                 1.15 
                 No 
               
               
                   
                   
                 6:22 PM 
                   
                   
                   
                   
                   
               
               
                 02 
                 Item A 
                 Jan. 3, 2014  
                 1    
                 10 MB 
                 0.06 
                 1.06 
                 No 
               
               
                   
                   
                 4:33 PM 
                   
                   
                   
                   
                   
               
               
                 03 
                 Item C 
                 Mar. 24, 2014 
                 0.67 
                 20 MB 
                 0.25 
                 0.92 
                 No 
               
               
                   
                   
                 6:18 PM 
                   
                   
                   
                   
                   
               
               
                 04 
                 Item E 
                 May 18, 2014 
                 0.44 
                 20 MB  
                 0.25 
                 0.69 
                 No 
               
               
                   
                   
                 7:53 AM 
               
               
                   
               
            
           
         
       
     
     Access Frequency and Recency Selection: In order to better select unattended content items, other factors such as frequency can be considered in addition to access time. High frequency-low recency content items are content items that have been frequently selected sometime in the past (e.g., more than 6 months ago) but not recently; low frequency-low recency content items are content items that have never been frequently accessed. Frequency of access can be measured relative to an average frequency on a particular client device, or across any population of client devices, or by type, name space, source domain, or other content item attributes. For example, if a content item has not been accessed on a client device in the last four months but had been accessed 25 times before that time it is likely to be more relevant to the user than a content item of similar recency that was accessed only once in the past. 
     In one embodiment, the number of accesses for each content item is maintained (at either client device  100  or content management system  110 ) in addition to the latest access to each content item. A score is determined for each content item as a weighted combination of metrics representing each variable. For example, a weighted score for each content item, based on a metric for access frequency of a content item and its last access date. For example
 
Score= w   1   F+w   2   A  
 
where F is a metric representing the access frequency, A is a metric representing the time since the last access to the content item, and w 1  and w 2  are the weights. The weights for A and F can based on their relative importance, as determined by the user, by a system administrator, or the based on historical content item access patterns for content items on the particular client device  100 . The queue is then be ordered by score. A cumulative sum is calculated at each index and is compared to the required storage space. When the required storage space is exceeded by the cumulative sum the index and all content items above that index in the queue are selected for removal from the client device  100 .
 
     Table 5 illustrates one example of this selection method. In this example, the required storage space is 40 MB and the weights w 1  and w 2  are both 1. The queue is ordered by the total score and the cumulative sum is compared to the required storage space. This results in the Item C and E being selected for removal from client device  100 . 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                   
                 Access 
                   
                 Access 
                   
                   
                   
                   
               
               
                   
                 Item 
                 Access 
                 Time 
                 Access  
                 Number  
                 Total  
                 Item 
                 Cum. 
                   
               
               
                 Idx  
                 Name 
                 Time 
                 Score 
                 Count 
                 Score 
                 Score 
                 Size 
                 Sum 
                 Selected? 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 00 
                 Item C 
                 Mar. 24, 2014 
                 0.67 
                 50 
                 0.83 
                 1.50 
                 20 MB 
                  20 MB 
                 Yes 
               
               
                   
                   
                 6:18 PM 
                   
                   
                   
                   
                   
                   
                   
               
               
                 01 
                 Item E 
                 May 18, 2014 
                 0.44 
                 60 
                 1 
                 1.44 
                 80 MB 
                 100 MB  
                 Yes 
               
               
                   
                   
                 7:53 AM 
                   
                   
                   
                   
                   
                   
                   
               
               
                 02 
                 Item A 
                 Jan. 3, 2014  
                 1 
                 14 
                 0.23 
                 1.23 
                 10 MB 
                 110 MB 
                 No 
               
               
                   
                   
                 4:33 PM 
                   
                   
                   
                   
                   
                   
                   
               
               
                 03 
                 Item D 
                 Mar. 30, 2014 
                 0.65 
                 32 
                 0.53 
                 1.18 
                 20 MB 
                 130 MB 
                 No 
               
               
                   
                   
                 6:22 PM 
                   
                   
                   
                   
                   
                   
                   
               
               
                 04 
                 Item B 
                 Mar. 24, 2014 
                 0.67 
                 26 
                 0.43 
                 1.10 
                 50 MB 
                 180 MB  
                 No 
               
               
                   
                   
                 5:12 PM 
               
               
                   
               
            
           
         
       
     
     Any of the above UCSM may consider entire folders within a single queue index instead of only individual files. For example, if the LRA UCSM is being used and a folder contains a plurality of files, where the most recently accessed file within the folder has an earlier access date than all other content items in the shared content directory it may be more efficient to select the entire folder as unattended (especially if significant storage space is required). Alternatively, the combined metric for the folder, could be an average, median, or other statistic that generalizes the content items within the folder allowing it to be placed into the queue. 
     In the following description, any of the foregoing methods may be used to select unattended content items for removal from a client device  100 . This process of selecting unattended content items enables the enhanced storage capabilities on the client device, as provided by the constrained content management system  100 . 
     Overview of Content Management System 
     The method of synchronization using content management system  110  between client devices  100 A and  100 B can be explained with reference to the architecture illustrated by  FIG. 2 . The following describes one of a number of possible methods of synchronization that may be used with storage constrained synchronization. 
     Content management system  110  stores content items in data store  218 . Content items are stored in fixed size portions termed a block. The size of a block varies according to the implementation, and in one embodiment, the blocks are 4 megabytes in size. Thus, a small content item is stored as a single block, while a large content item may be split up into dozens, hundreds, or more blocks for storage at content management system  110 . The metadata includes a blocklist that defines the blocks in the content item and an ordering of the blocks in the content item. 
     Pending block table  220  maintains a list of pending blocks expected to be received at content management system. Pending block table  220  stores an association between blocks (identified by block identifiers) and the namespaces to which the blocks belong that clients  100  indicate will be transmitted. 
     Namespace table  222  stores data associating individual content items with namespaces and maintains data associating each namespace with clients. 
     Metadata server  212  is responsible for managing a request from the client to add (“commit”) a new content item to content management system  110 . Metadata server  212  also receives requests to synchronize content items from client device  100 . Metadata server  212  maintains a record of the last time that client device  100  synchronized with content management system  110 . When a request is received from client device  100  to synchronize, metadata server  212  determines any content items that have been committed to namespaces synchronized to that client device  100  since the last synchronization time stamp. In addition, metadata server  212  determines any pending blocks that have been received since the last synchronization time stamp. 
     Notification server  216  is responsible for communicating with clients  100 , and particularly with notifying clients that new data is available. The notification server  216  maintains a list of clients  110  associated with each namespace at namespace table  222 . When the notification server  216  receives an alert from block server  214  or metadata server  212  that a new block is available for a given namespace, notification server  216  identifies clients associated with the namespace from namespace table  212 . Notification server  216  notifies client(s)  100  associated with the namespace to wake client(s)  100  and indicates that new blocks are available for the identified namespace. 
     A typical synchronization between two clients  100 , client device  100 A and client device  100 B occurs as follows. First, client device  100 A adds an additional content item to the shared data. The additional content item is then transmitted to content management system  110 . Content management system  110  notifies client device  100 B that the additional content item is in the shared data, and client device  100 B retrieves the additional content item from content management system  110  as client device  100 B. Content management system  110  maintains a list of content items and pending blocks that are expected to be received at content management system  110  using a pending block table  220 , and notifies client device  100 B to download blocks corresponding to a content item as blocks are received by content management system  110 . Pending blocks are those blocks that correspond to a content item that content management system  110  expects to receive and are used to identify blocks that may be provided to receiving client device  100 B prior to a content item being committed to content management system  110 . 
     To manage in-transit content items, content management system  110  retains a list of pending blocks along with the namespace associated with the pending blocks. When a pending block is received, clients associated with the namespace are notified and can initiate a transfer for the received block. Thus, uploading clients (providing a new content item) and downloading clients (receiving the new content item) may asynchronously transfer blocks to content management system  110 . 
     Overview of Client Device 
     Each client device  100  is a computing device, such as a desktop, laptop, tablet, mobile device, or other system that maintains a local copy of shared data synchronized with content management system  110  and with other clients using the installed client application  200 . The shared data may be synchronized only with clients associated with a single user, or may be synchronized to clients associated with multiple users. Client device  100  includes modules and applications for manipulating and adding data to the shared data, as further described with respect to  FIG. 3 . 
       FIG. 3  shows modules of client application  200 . Client application  200  includes various modules and data stores for synchronizing data with content management system  110 . Client application  200  includes content synchronization module  310 , hashing module  320 , download module  330 , upload module  340 , and storage management module  350 . Additionally, the client application  200  maintains data stores including a file journal  360 , a resident file table  362 , shared data  364 , a remote file table  366 , a configuration file  368 , and a block cache  370 . In addition to client application  200 ,  FIG. 3  also indicates the storage kernel extension  384  present on the operating system of the client device. The configuration of client application  200  and its associated kernel extension using these modules instantiates client application  200  as a particular computer able to perform the functions described herein, which enables the described improvements in the storage capacity and functional performance of the client device. 
     Shared data  364  are data that has been synchronized with content management system  110 , and includes content items received from content management system  110 . When users add, modify, or delete content items in shared data  364 , those changes are synchronized with content management system  110 . The hashing module  320  and the block cache  370  work to identify blocks that comprise content items being uploaded to content management system  110 . The hashing module assigns a block identifier by performing any suitable hashing algorithm, such as MD5 or SHA-1. Content synchronization module  310  then uses these identifiers to compare the resident blocks located in the block cache  370  with the blocks maintained by content management system  110 . These modules are present in the current embodiment but this block implementation is not required for the invention of storage constrained synchronization. 
     When data is modified or added to the shared data  364  on the client device  100  within the client application  200 , the modifications to the shared data  364  are transmitted to content management system  110 . Client device  100  is also configured to receive notifications from content management system  110 . When the client device  100  receives a notification, client device  100  queries content management system  110  for modifications to shared data  364 . When the shared data is modified, the client device  100  requests the modifications from content management system  110  to store shared data on client device  100 . In some cases, the modified data may be associated with a content item represented by a shadow item. In this case, the client device  100  may withhold the request for modified data from content management system  110  until access to the content item represented by the shadow item is requested by an application on the client device  100 . Alternatively, when a shared content item is modified by another client device  100 , content management system  110  may request that the constrained client device  100  restore the content item represented by a shadow item such that the modification can be synchronized at the expense of other content items resident on the constrained client. 
     Within the client application  200 , the file journal  360  stores a table listing metadata for all content items accessible to the account using the client application  200 . Metadata includes revision date and time, namespace, and blocklists corresponding to each content item. Content items that are not resident or not synchronized are still included in the file journal  360 . 
     The resident file table  362  stores a list of files that are always kept resident on the client device  100 , without regard to storage constraints. 
     The remote file table  366  stores a list of files that have been selected to be deleted from the client device and replaced with shadow items. These files are only maintained by content management system  110  and possibly other users with access to the file. 
     The configuration file  368  is a file maintained by the client application  200  and contains the storage allocation  120  for the client device. In some embodiments the storage allocation  120  can be created by the user or computer systems that may have control over the client application  200 . For example an operating system may change the storage allocation  120  so that it can maintain a sufficient amount of storage for use by other applications. 
     The storage kernel extension  384  is configured to monitor requests from applications to the operating system  380  for access to content items, and determine whether or not the requested content items are shadow items, and is one means for performing this function. The storage kernel extension  384  constitutes a direct modification to the structure and function of the operating system that enable the increase in the effective storage capacity on the client device. 
     The kernel extension  384  monitors requests made to open content items managed by the client application  200 . The kernel extension  384  determines when requests are made to open content items managed by the client application  200  by monitoring the file system  382  on the operating system  380 . When a request for a content item is made within the file system  382 , the kernel extension  384  examines the pathname of the content item to determine whether it is within the content items stored within the shared content storage directory  120 . 
     The kernel extension  384  determines whether the requested content item is a shadow item by determining whether its size is under a threshold size. Alternatively, identification of a shadow item can be completed based upon extended file attributes for content items managed by the client application  200 . A file attribute indicating a shadow item could be assigned to shadow items such that the kernel extension could identify a shadow item without examining the requested content item&#39;s size. If the file is determined to be a shadow item by the kernel extension  384 , the kernel extension communicates the identification information to the client application  200 . 
       FIG. 4  is an interaction diagram showing one embodiment of a process for accessing a content item not resident on the client device  100  but included in the file system as if the content item was resident on the client device  100 . The file system  382  receives  400  a request to open a content item within a synchronized folder on the client device  100 . The request may come from any application, such as a file explorer, word processor, document reader, image editor, or the like. The storage kernel extension  384  intercepts  402  such file system requests, and obtains the pathname of the requested content item. The storage kernel extension  384  uses the pathname to determine  404  whether the content item is a shadow item. The storage kernel extension  384  may do this by checking the size of the requested content item to determine if it is below a predetermined threshold, or otherwise consistent with the size of shadow item (4 KB). Alternatively, the storage kernel extension  384  can read a file attribute extension that stores a value indicating whether content item is a shadow item or a regular content item. If the content item is not a shadow item, then the storage kernel extension  384  allows the request to continue as normal and gives the file handle to the file system so that the content item can be opened. 
     Upon determining that the content item is a shadow item, the storage kernel extension  384  sends  406  the request identification number (information about the request including the request type) and the file path to the storage management module  350 , passing in the file name. The storage management system  350  removes  408  the file name from the remote file table  366 . The storage management system  350  then wakes  412  the download thread, which checks content items that require synchronization from content management system  110 . As the requested content item has been removed from the remote file table  408 , the download thread can now request  414  content item information from content management system  110 , including the size of the requested content item in preparation for the download. The storage management module  350  receives  416  the size information from content management system  110 , and determines  418  whether storing the content item on the client device  100  will cause the predetermined storage limit to be exceeded. If the storage limit will be exceeded by the addition of the requested content item, the storage management module  350  selects  422  one or more content items stored on the client device  100  for removal. However, if the storage limit will not be exceeded, the storage management module  350  proceeds to download  430  the content item. 
     In the case that the storage allocation  130  will be exceeded by the addition of the requested content item to the shared content storage directory  120 , the storage management module  350  selects one or more content items to remove, so as to make available sufficient storage space for the requested content item before requesting a download  430  thereby preventing the shared content directory from ever occupying greater than it&#39;s allocated space. The storage management module  350  selects  422  content items for deletion by first determining  420  the unattended content items, using any of the UCSM described above. Where the access history of particular content items or other information pertaining to each selection method are stored on the host system, a request is made to the host system (not shown in  FIG. 4 ) to update the client application&#39;s  300  version of this information. Once the current version of the access history or any other required information for each content item in content management system  110  has been obtained, the storage management module  350  can determine  420  the unattended content items. 
     The storage management module then selects  422  unattended content items for removal from the client device. In this embodiment, to select  422  content items to remove, the storage management module  350  traverses the queue generated by the UCSM in use to create storage space at least as large as the size of the requested content item to be downloaded. The selection of the unattended content items for removal can be conducted using any of the methods described above. 
     The storage management module  350  then adds  424  the names of the selected content items to the remote file table  366 . Once this addition  424  has been confirmed  426 , the storage management module  350  removes  428  the selected content items from shared content storage directory  120  on client device, and then creates, for each removed content item, a corresponding shadow item that has the same metadata and location as the removed content item, but does not contain the content information for the content item. Shadow items may be represented in the user interface of the client as if they are still resident on the client device  100 .  FIG. 8  illustrates an example of how shadow items may be represented in the user interface of the client device  100 . 
     Upon removal of the selected content items, there will be sufficient storage space on the client device  100 , and the requested content item can be downloaded from content management system  110  without exceeding the storage limit for the shared content storage directory  120 . Accordingly, the storage management module  350  sends a download request  430  to the download module  330 . The download module  330  then initiates a download  432  with content management system  110 . Once the content item is downloaded  434  to the download module  330  it is passed  436  to the storage management module  350 , which saves  438  the requested content item to the previously identified location and notifies  440  the storage kernel extension  384  that the download is complete. In one embodiment, the storage management module  350  appends the contents of the downloaded content item to the shadow item metadata, and updates the content item attributes to indicate that content item is now no longer a shadow item. This enables the requesting application to transparently access the requested content item, using the same file handle and identification information it used to initially request access to the content item. The storage kernel extension  384  then passes through the file handle  442  to the file system  382 , which gives the requesting application permission to open the content item  444 . 
       FIG. 5  is an interaction diagram showing one embodiment of a process of saving a content item to shared content storage directory  120  that is approaching its storage allocation  130 . The content item can be a newly created content item in the shared content storage directory  120 , a content item that has been relocated into the shared content storage directory  120 , or a content item that was already in the shared content storage directory  120 , and then modified in such a way to increase its size. The process begins with an application making a request  500  to the operating system&#39;s file system  382  to save a content item within the synchronized folder. The storage kernel extension  384  monitors this request and receives  502  the request-ID, file path, and size from the file system. The storage kernel extension  384  then sends  504  this information to the storage management module  350 . The storage management module determines  506  whether the addition of the new content item will cause the synchronized folder to exceed its storage limit. If the storage limit will not be exceeded, the file system  382  is allowed to save the content item as normal. In the case that the storage limit will be exceeded, the storage management module  350  determines  508  the unattended content items and selects them for removal from the client device. Once the unattended content items are selected their names are added  512  to the remote file table  366  so that their content will not be synchronized by content management system  110 . The storage management module then removes the selected content items from the client device  100  and replaces  514  them with shadow items, which have the same metadata and location as the removed content items but contain no content. When this process is complete there is sufficient storage space in the constrained folder for the storage management module to allow  516  the original content item to be saved. The storage management module then wakes  518  the upload thread, which accesses  520  the metadata so that the contents of the saved content item are uploaded  522  to content management system  110 . 
     In addition to automatically removing content items and creating shadow items, some embodiments also allow for the user to select particular content items to be stored only remotely on content management system  110 . This may be implemented by simply allowing the user to select from a context menu (e.g., “right-click”) on a particular synchronized content item. The client application  200  would then present the user with an option to make the selected content item remote. If the user chooses this option the content item is removed from the client device  100 , the name of the content item is added to the remote file table  366 , and a shadow item with the same metadata and location of the original content item is created to represent the original content. If the user wants to access the content item in the future the same process described in  FIG. 5  may be used to retrieve the content item from content management system  100 . 
     In some embodiments, the client device is configured to enable the user to select particular content items to remain resident on the client device when the storage allocation  130  is reached regardless whether the UCSM in effect would otherwise select them for removal from the client device  100 . This embodiment offers operational improvements that allow the user to maintain quick access to particularly important content items. In this embodiment, the client application  200  enables the user to access a context menu, and then select an option to force a content item to remain resident on the client device  100 . Upon selecting, the name of the content item is added to the resident file table  362 . The resident file table  362  is subsequently accessed during the UCSM used by the storage management module  350  shown in  422  and all content items in the table are excluded from the selection process. For example, when a given content item is selected for removal, the resident file table  362  is examined to determine if the selected content item is listed therein; if so, the selected content item is ignored, and another content item is selected by the UCSM in effect. 
     Because the content associated with shadow items on a client device  100  is not being synchronized it may make content management more complicated. For example, if a user on one client device moves a content item that is represented as a shadow item on a second client device then, if the second client device is not receiving synchronizing data regarding the shadow item, its location may change on the first client device but not on the other. For example, a content item may be deleted from content management system  110  completely by one client device  100  while represented by a shadow item on a different client device  100 . If this situation occurred the user of the second client device  100  may try to access the content item represented by shadow item only to find that it no longer existed. To avoid these confusing circumstances, in some embodiments, the content management system  110  is configured to synchronize shadow items for metadata only; that is, if any of the attributes of a shadow item change, content management system  110  will synchronize the modified attributes to all client devices  100  with access to that content item regardless of whether the content item is represented as a shadow item on any of those client devices. Thus, if a content item is deleted from one client device, the shadow item representing that content item is deleted as well on any other client device  100 . Alternatively in some embodiments, if a content item is modified on another client device such that its size changes so it can fit within the remaining storage in the shared content storage directory  120  on a client device  100  it may be downloaded to the client device  100  even if access to the content item is not requested. 
     Some of the foregoing embodiments represent client-based constrained synchronization systems as the client application  200  is responsible for ensuring that the predetermined storage allocation  130  is not exceeded and for requesting data from the content management system  110 . In a host-based embodiment, illustrated in  FIG. 6 , content management system  110  manages the constrained synchronization process, including maintaining information identifying the remote and resident content item information for each client device  100 . A host-based embodiment may provide the same benefits of increasing effective storage capacity on a client device  100 , while reducing the computation needed from the client device, thereby improving client device  100  performance in comparison to other embodiments. The constrained content management system  600  includes elements of content management system  110  shown in  FIG. 2 , further modified to utilize the storage management module  350  along with the necessary data files required for the storage management module  350  to function properly. Within the constrained content management system the metadata server  212 , block server  214 , notification server  216 , data store  218 , pending block table  220 , and namespace table  222  function in the same manner as implemented in content management system  110 . Additionally, storage management module  350  functions in a manner similar to when it is resident on the client device, where it is responsible for determining when the storage space limit will be exceeded and appropriately creating shadow items. The storage management module  350  is also responsible for receiving information from the client device  100  about requests made by the operating system  380 . When a request is made to open one or more content items information about the request is sent to the content management system  110  to be monitored remotely by the storage management module  350  so that the required downloads are made to provide access to shadow items on the client device  100 . The storage management module  350  uses the client configuration file  610  to provide information regarding the storage configurations on each client device associated with the constrained content management system. The synchronization table  620  is a record of all content items on client devices that require synchronization with the constrained content management system  600 ; the content items included in this table would be a subset of the content items located in the data store  218  since some of the content items are shadow items and require only metadata synchronization. Further, in this embodiment, the synchronization table  620  may be replaced by using both a resident file table  362  and a remote file table  366  configured such that they indicate the client devices  100  on which each content item should be kept remote or resident. For an embodiment using the latter configuration, implementation of metadata synchronization for shadow items is easier as the shadow items are identified directly in the remote file table  366  of each client device  100 . User data  630  is stored on the constrained content management system  600  so that the storage management module  350  can determine the unattended content items. 
       FIG. 7  is an interaction diagram illustrating one embodiment of a process of a host managed constrained storage synchronization. An application on a client device requests  700  for a content item to be saved to the synchronized folders on the client device. The storage kernel extension records  702  the request ID, file path, and content item size and transfers  704  the information to the client application  200 . The client application  200  forwards  706  the content item size information to the storage management module  350  on the constrained content management system  600 . The storage management module  350  requests  708  the storage limitation for the particular client from which it received  706  the content item size information from the client configuration file  610 . The storage management module  350  determines  712  that the storage limit will be exceeded by comparing the size in addition to the other content items resident on client device  100  to the storage allocation received from the client configuration file  610 . The storage management module  350  requests  714  the content data on the client from the synchronization table  620  so that it may select content items to remove from the client from the synchronized content items on the client. The synchronization table responds  716  with the synchronized content data for the particular client. The storage management module  350  requests  718  user access data from user data  630  stored on a host device to use to determine LRA content items. Once this data is received  820  from the user data table  630 . The storage management module  350  can determine  722  the LRA content items and select  724  those that should be removed from the client to provide the required storage space. The storage management module  350  sends requests to remove content items and create shadow items  728  to the client application  200 . It gives  730  permission to the client application  200  to complete the original request  700  to save a content item. Finally the storage management module updates  732  the user data to reflect the first content item access for the saved content item and then requests  734  synchronization of the client device  100  from the metadata server  212  since a new content item is available for upload. 
       FIG. 8  illustrates an example a user interface of client device  100  operating in coordination with a content management system providing constrained synchronization. A synchronized files folder  800  serves as the shared content storage directory  120 . Folder  800  contains a number of content items, each represented by a corresponded icon  810 A, a .m4a music file,  810 B, a .xlsx spreadsheet,  810 C, a .docx word processing file,  810 D, a .mat Matlab file, and  810 E, a .jpg image file. Each icon  810  is overlaid with a status icon  820  that indicates the storage status of the content item. 
     Status icon  820 A (“check icon”) indicates that the content item is currently resident on the client device  100  and is synchronized with the current version of the content item maintained by content management system  110 . 
     Status icon  820 B indicates that the content item will be resident on the client device  100  once synchronization with content management system  110  is complete. 
     Status icon  820 C indicates that the content item is a shadow item and is not currently resident on the client device but still maintained on content management system  110 . 
     Status icon  820 D indicates that the content item is resident on the client device and synchronized with its version maintained by content management system  110 . Additionally, the green circle with the pin icon  840  indicates that the content item has been chosen to remain resident on the client device  800  during a storage constraint. 
       FIG. 9  is a concept diagram illustrating an alternate embodiment of constrained synchronization, which predicts user access to particular content items remote to a client device, and downloads the predicted content items in advance of the access. This approach offers a further improvement in the operation of the client device by eliminating in most cases the time a user may have to wait to retrieve the content item over the network from content management system  110 . A retention score  900  is calculated for each content item  140  within a shared content storage directory  120 . This score is a measure of the predicted importance of a content item and can be calculated as a function of latest access time, or a number of other factors determined to be predictive of a user request, as explained in a later section. Additionally, each content storage directory  120  is configured with a retention score threshold  910 , which may be specified by the user or set at a predetermined value. Whenever the predicted importance of a content item, as measured by the retention score  900  of the same content item, exceeds the retention score threshold  910  of a particular shared content storage directory  120  on a client device  100  with access to the content item, the content item is downloaded to the shared content storage directory when it is remote to the client device and maintained within the shared content directory if it is resident on the client device. 
     Stage  9 . 1  illustrates a typical state of a content management system, which predicts user access to content items. In this illustration, content management system  110  manages two client devices  100 A and  100 B. Shared content storage directories  120 A and  120 B are located within their respective client devices. Shared content storage directory  120 A stores content items  140 A,  140 B, and  140 C while shared content storage directory  120 B stores content item  140 D and a shadow item representation  160 A of content item  140 A. Synchronized versions of all content items  140  are stored on content management system  110 . 
     Additionally, each content item  140  has a corresponding retention score  900 , where  900 A is the retention score for content item  140 A,  900 B is the retention score for content item  140 B and so forth. Each shared content storage directory is also configured with a retention score threshold  910 , where  910 A is the retention score threshold for shared content storage directory  120 A and  910 B is the retention score threshold for shared content storage directory  120 B. 
     In stage  9 . 1 , content item  140 A is not maintained in shared content storage directory  120 B. Though in this case there are no content items resident within a shared content storage directory  120  that have a retention score  900  lower than the retention score threshold  910 , this scenario is possible if traits from other embodiments, described previously or in a following section, are used in addition to those from this embodiment. For example a storage allocation may still be in affect and so if the storage allocation is sufficiently large it may not be necessary to keep a file remote even if it has a retention score  900  lower than the retention score threshold  910 . 
     In stage  9 . 2 , a user of client device  100 A performs a user action  920  on content item  140 A that is considered an access to content item  140 A. Because, in this example, retention scores  900  are calculated as a function of latest access time, the retention score  900 A of content item  140 A increases from 20 to 60 (The magnitude of this change is arbitrary for the purpose of this example. Details on retention score calculation are provided later and may not result in the same score change). 
     In stage  9 . 3 , the content management system  110  or, in some embodiments, the client application on client  100 B, determines that the retention score  900 A of content item  140 A is greater than or equal to the retention score threshold  910 B of the shared content storage directory  120 B where content item  140 A is remote. Because the retention score  900 A exceeds the retention score threshold  910 B, the content item  140 A is downloaded to client device  100 B and stored in shared content storage directory  120 B. 
     Similar to the UCSMs there are a number of retention score calculation methods. Generally retention scores can be normalized against user behavioral attributes, resulting in retention scores for the same content item that are different for each client device, or global so that scores are the same for each client device. The advantage of normalized retention scores is that they level out differences in user behavior. For example, if the retention score is a function of the latest access time of a content item where the score increases as the time between the present time and the latest access time decreases, a more active user would drive up the retention scores of content items shared with that user when compared to content items shared with a less active user. If the retention scores are not normalized for a third user, sharing with both the active user and the less active user, the retention scores would lose their predictive quality as only the items from the active user would have the highest retention scores even though a recent access by the active user is less predictive of an access by the third user than is a recent access by the less active user. Whenever a retention score is normalized it can be normalized to an attribute of a particular user or a particular content item. 
     The following methods are examples of methods for determining a retention score, or a score predicting a user access of a content item. Additionally, a retention score may use a combination of the following methods to create the most predictive measure of predicted importance. Typically the retention score increases as the predicted importance of a content item increases, however the opposite can be true if convenient for an embodiment. In this case the corresponding retention score threshold would be a minimum value where if the retention score of content item was less than or equal to the retention score threshold it would be downloaded to the corresponding shared content storage directory. For the purposes of this discussion the default case of an increasing retention score will be assumed. 
     Latest Access Scoring: For latest access scoring the retention score of a content item is a function of the latest access time of that content item. The retention score could simply be the inverse of the difference between the current time and the latest access time in seconds:
 
 RS= 1/( t   C   −t   A )
 
where RS is the retention score, t C  is the current time, and t A  is the latest access time.
 
     If normalization is needed for the particular embodiment a variety of user attributes may be used such as a user&#39;s or client device&#39;s access frequency defined as the number of accesses of any shared content item by a particular user or on a particular client device within a predetermined time period. Alternatively, the average latest access time of content items shared with a particular user or client device may be used. 
     Access Frequency Scoring: For access frequency scoring, the retention score of a content item increases with an increase in the number of accesses to the same content item within a predetermined time period. To normalize access frequency scoring the access frequency for a given content item could be divided or otherwise scaled by the average access frequency for all content items on a client device or shared with a user. 
     Location Related Access Scoring: For location related access scoring, the retention score of a first content item is a weighted combination of the latest access time, access frequency, or any other characteristic of the content item itself and the same characteristic of additional content items stored in the same folder as the first content item. This implies that accesses to content items within a folder are predictive of accesses to other content items within the same folder. 
     Similar Access Scoring: For similar access scoring, the retention score of a first content item is a weighted combination of the latest access time, access frequency of the content item itself and the same characteristic of additional content items with similar attributes as the first content item. Attributes may include content item type, size, location, users with access to the content item, etc. This implies that accesses to similar content items are predictive of future accesses to a content item. 
     Criteria Based Retention Scoring: For criteria based retention scoring, the retention score of a content item is based on the number of previously identified predictive criteria satisfied by the content item. For example, access to a content item by another user within 24 hours, an access frequency greater than 5 accesses in the last week, and accesses to sufficiently similar content items within the last 3 days may all be criteria predetermined to be predictive of an attempt to access a remote content item within the next 6 hours. Therefore, the retention score of a content item may increase by a predetermined magnitude for each of the criteria satisfied by the content item. The magnitude of the increase for a particular satisfied criterion may be proportional to the predictive strength of the particular criterion. 
       FIG. 10  illustrates a system environment for a content management system using predicted content item importance for constrained synchronization. Most of the modules of the constrained content management system  600  that are present in  FIG. 10  perform similar or identical functions to those described with reference to  FIG. 6  except where noted in the foregoing section. Therefore, the functions of all modules within content management system  1000  are not explained in detail in this section. 
     Content management system  1000  includes metadata server  212 , block server  214 , notification server  216 , data store  218 , pending block table  220 , namespace table  222 , storage management module  350 , client configuration file  610 , synchronization table  620 , user data  630 , retention score table  1010 , and retention score module  1020 . Client configuration file  610  and user data  630  have significant changes over previous versions described in  FIG. 6 . Client configuration file  610  is modified to include the retention score threshold for each shared content storage directory of each client device, while user data is modified to include user data relevant to the retention scoring method being used. The retention score module  1020  takes in user data  630  and data from the data store  218  to generate the retention score table  1010 . The retention score table is a table enumerating the retention score of each content item managed by the content management system  1000 . A separate retention score table may exist for each client device if normalization is being used to calculate retention scores. Whenever the retention score of a content item is updated, the retention score module  1020  consults the client configuration file  610  and the synchronization table  620  to determine if the corresponding content item to the recently changed retention score is remote on any client devices and if it exceeds any of the retention score thresholds of those client devices. If a retention score threshold is exceeded the retention score module requests that the storage management module  350  perform the necessary download and replacement of the representing shadow item. 
       FIG. 11  illustrates the software architecture of the client application  1100  for another embodiment of constrained synchronization. This embodiment conducts all downloading of remote content items, removal of unattended content items, and creation of shadow files while the client device is determined to be idle by the client application. This change in timing of the constrained synchronization process improves the client device by offering a functional improvement over the previously described embodiments while providing a similar increase in effective storage capacity. To perform these functions the idle state triggered embodiment modifies the system architecture illustrated in  FIG. 3 . In this embodiment, client application  1100  is comprised of content synchronization module  310 , retention state module  1110 , file journal  360 , resident file table  362 , shared data  364 , remote file table  366 , configuration file  368 , and block cache  370 . The content synchronization module  310  is further comprised of hashing module  320 , download module  330 , upload module  340 , and storage management module  350 . The retention state module  1110  is further comprised of state calculation module  1120 , state comparison module  1130 , action module  1140 , and system status module  1150 . Unless otherwise specified all previously mentioned modules and data tables have the same function as previously described slightly modified as one skilled in the art would recognize to accommodate the new modules. Any major modifications are explained below. 
     System status module  1150  uses storage kernel extension  382  to measure system activity on operating system  380 . System activity can be measured using metrics for processor activity including but not limited to the number of non-idle processor cycles as a ratio of processor frequency or another CPU utilization metric (with or without adjustment for multiple processor cores), the number of threads, or the number of processes of a client device  100 . Network activity metrics may also be used including network utilization, defined in bits per second or packets per second, as a ratio of the maximum speed for a particular port or connection. Additionally, memory usage metrics including the amount of available or free random access memory (RAM) may be used to measure system activity. The system status module  1150  may use the activity metrics mentioned above or any other suitable activity metrics, individually or in combination to measure overall system activity. 
     When the measure of system activity is below a predetermined activity threshold, the system status module  1150  reports to the retention score module  1110  that the client device is currently idle. This activity threshold may be defined as a percentage of the total computational resources of the client device, as defined by an activity metric, or the activity threshold may be defined as a particular value of an activity metric. For example, an activity threshold may be defined as the state of the client device  100  using less than 25% of available processing resources. Alternatively, the activity threshold may be defined as the state when the other processes of the client device  100  are, in total, using less than 2 GB of memory, or that there is at least 4 GB of total memory available on the client device. 
     When the client device  100  has been determined as being in an idle state by the system status module  1150 , state calculation module  1120  determines the retention state of the shared content storage directory  120 . Generally the retention state consists of the content items resident on the client device and a set of attributes corresponding to those content items. These attributes may include content item size, latest access time, access frequency, directory location, or any other suitable attribute that would be indicative of a content item&#39;s importance for retention on a client device. Additionally, the retention state could be represented by a set of statistics calculated using at least one of the attributes listed above. 
     Comparison module  1130  receives the retention state from the state calculation module  1120 , it then compares the current retention state of the shared content storage directory  120  with a predetermined threshold retention state, defined in the configuration file  368 , that may be specified by the user. The threshold retention state is a set of criteria pertaining to the attributes or calculated statistics of the client device included in the retention state. The comparison module  1130  determines whether the current retention state satisfies the criteria of the threshold retention state. If these criteria are violated (e.g. not satisfied), the comparison module  1130  reports the content items corresponding to the attributes, or the calculated statistics based on those attributes, that violate the threshold retention state criteria to the action module  1140 . 
     Action module  1140  receives the report from the comparison module  1130 . It then determines what actions will bring the retention state back to within the threshold retention state criteria. These actions may include removing content items from the shared content storage directory  120  and replacing them with shadow items, or replacing shadow items representing remote content items with the content items themselves. Once these actions have been determined, the action module  1140  requests that content synchronization module  310  complete the required actions. 
     Alternatively, idle state triggered constrained synchronization could be conducted by the content management system itself further reducing the computation burden on the client device and increasing device availability for other uses.  FIG. 12  illustrates a system environment that completes this task. Constrained content management system  1200  is comprised of metadata server  212 , block server  214 , notification server  216 , data store  218 , pending block table  220 , namespace table  222 , storage management module  350 , client configuration file  610 , synchronization table  620 , user data  630 , retention state table  1210 , retention state module  1220 . Unless otherwise specified all previously mentioned modules and data tables have the same function as previously described slightly modified as one skilled in the art would recognize to accommodate the new modules. Any major modifications are explained below. 
     In this version of the embodiment, client application  200  on a client device connected to the content management system  1200  reports to the content management system  1200  on the status of the client device. When the client device is idle the content management system  1200  uses the retention state module  1220  to determine the retention state of the shared content storage directory  120  on the idle client device. The retention state module then updates the retention state table  1210 , which contains the current retention state of all client devices connected to the content management system  1200 . The retention state module  1220  then conducts steps similar to retention state module  1110  using potentially similar submodules, as described during the discussion of  FIG. 11 . 
     The retention state of a shared content storage directory can be determined using a variety of methods. Generally, the retention state is criteria based and is maintained periodically whenever the client application determines that the client device is idle. However, it is also possible to implement the retention state and threshold retention state numerically such that each state is represented by a statistic calculated using the attributes of the content items resident on the client device. If the retention state is criteria based, the threshold retention state is a set of criteria that the content items within the shared content storage directory must satisfy. Additionally, in the case of a criteria based retention state, the user may be given an option to choose the retention state criteria thereby allowing customization of the categories of content items resident on a client device  100 . 
     The period used to check each client device can be a predetermined value of the content management system, set by the user, or determined based on usage patterns of the particular client device. For example, if a user accesses content items on their client device on average every 24 hours the period could be set to ensure that the shared content storage directory is maintained before 24 hours passes. 
     As an alternative to checking a shared content directory periodically, another embodiment could maintain a shared content directory only when the shared content directory satisfies a second set of criteria that indicate urgency, for example, nearing a hardware storage limit. 
     Storage Space Criteria: One possible set of criteria is to have a storage allocation criteria. For example, a storage allocation could be set at 20 GB but instead of behaving like the previous embodiments, the content management system would allow the content items stored on the shared content storage directory to exceed the criteria value (in this example 20 GB) until the device was idle. Then a similar process of determining unattended content items could be used to remove the appropriate content items and satisfy the storage space criteria for the shared content storage directory. 
     Access Time Criteria: A second criterion could be an access time criterion. For example, the criterion could state that no content item with a latest access time earlier than a predetermined time interval in the past can be resident within the shared content storage directory. These content items would be allowed to remain resident within the shared content storage directory until the client device was idle. At that point the retention state module would simply request the removal of all content items with a latest access time earlier than the predetermined time interval. 
     Content Item Size Criteria: Another set of criteria is the content item size criterion. For this method, a threshold on the individual content item&#39;s size is set. Therefore, whenever the device is idle any content item over or under that threshold is removed from residence on the client device. 
     Access Frequency Criteria: Finally an access frequency criterion is used to set a minimum number of accesses within a predetermined time interval required to remain resident on a client device. If a particular content item is not accessed frequently enough it is removed from the client device whenever it is idle. 
     Note that this list of retention criteria is not exhaustive. Additionally, these criteria may be used in conjunction with each other resulting in more complex rules. 
       FIG. 13  is a flow diagram illustrating the function of idle state triggered constrained content management. First the system checks  1300  to determine whether a particular client device is idle. This step is completed either periodically or in response to the content storage directory reaching a predetermined threshold. If the device is idle, the system determines  1310 , the retention state of the client device. Then the system compares the current retention state of the shared content storage directory to the retention state criteria for the shared content storage directory. If the criteria are satisfied by the current retention state of the shared content storage directory the system resumes checking  1300  to determine whether the client device is idle. If the retention state criteria are violated the system identifies  1330  actions to perform on the shared content storage directory that are required for the shared content storage directory to meet the retention state criteria. The system then performs  1340  those actions on the shared content storage directory to conform to the predetermined retention state criteria. 
     The foregoing description of the embodiments of the invention has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. 
     Some portions of this description describe the embodiments of the invention in terms of algorithms and symbolic representations of operations on information. These algorithmic descriptions and representations are commonly used by those skilled in the data processing arts to convey the substance of their work effectively to others skilled in the art. These operations, while described functionally, computationally, or logically, are understood to be implemented by computer programs or equivalent electrical circuits, microcode, or the like. Furthermore, it has also proven convenient at times, to refer to these arrangements of operations as modules, without loss of generality. The described operations and their associated modules may be embodied in software, firmware, hardware, or any combinations thereof. 
     Any of the steps, operations, or processes described herein may be performed or implemented with one or more hardware or software modules, alone or in combination with other devices. In one embodiment, a software module is implemented with a computer program product comprising a computer-readable medium containing computer program code, which can be executed by a computer processor for performing any or all of the steps, operations, or processes described. 
     Embodiments of the invention may also relate to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, and/or it may comprise a general-purpose computing device selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a non-transitory, tangible computer readable storage medium, or any type of media suitable for storing electronic instructions, which may be coupled to a computer system bus. Furthermore, any computing systems referred to in the specification may include a single processor or may be architectures employing multiple processor designs for increased computing capability. 
     Embodiments of the invention may also relate to a product that is produced by a computing process described herein. Such a product may comprise information resulting from a computing process, where the information is stored on a non-transitory, tangible computer readable storage medium and may include any embodiment of a computer program product or other data combination described herein. 
     Finally, the language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.