Patent Publication Number: US-2023135875-A1

Title: System and method for maintaining links and revisions

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/407,737, filed Aug. 20, 2021, now U.S. Pat. No. 11,544,451, which is a continuation of U.S. patent application Ser. No. 16/994,944, filed Aug. 17, 2020, now U.S. Pat. No. 11,100,281. This application is also related to U.S. patent application Ser. No. 16/292,701, filed Mar. 5, 2019, now U.S. Pat. No. 10,733,369, which is a continuation of U.S. patent application Ser. No. 16/008,295, filed Jun. 14, 2018, now U.S. Pat. No. 10,275,441, which is a divisional of U.S. patent application Ser. No. 15/922,424, filed Mar. 15, 2018, now U.S. Pat. No. 10,255,263, which is a continuation-in-part of U.S. patent application Ser. No. 15/188,200, filed Jun. 21, 2016, now U.S. Pat. No. 10,019,433, which is a continuation of U.S. patent application Ser. No. 14/850,156, filed Sep. 10, 2015, now U.S. Pat. No. 9,378,269, which is a continuation of U.S. patent application Ser. No. 14/714,845, filed May 18, 2015, now U.S. Pat. No. 9,158,832. This application is also related to U.S. patent application Ser. No. 16/871,512, filed May 11, 2020, which is a continuation-in-part of U.S. patent application Ser. No. 16/293,998, filed Mar. 6, 2019, now U.S. Pat. No. 11,048,861, which is a continuation of U.S. application Ser. No. 16/191,821, filed Nov. 15, 2018, now U.S. Pat. No. 10,325,014, which is a continuation-in-part of U.S. patent application Ser. No. 15/411,237, filed Jan. 20, 2017, now U.S. Pat. No. 10,331,776, which is a continuation of U.S. patent application Ser. No. 15/049,221, filed Feb. 22, 2016, now U.S. Pat. No. 9,552,343, which is a continuation of U.S. patent application Ser. No. 14/808,029, filed Jul. 24, 2015, now U.S. Pat. No. 9,292,482, which claims the priority benefit of U.S. Provisional Patent Application No. 62/155,000, filed on Apr. 30, 2015. The disclosures of each of the above-listed applications are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to electronic document management and, more particularly, to a data storage and retrieval system and method for maintaining links and revisions in a plurality of documents. 
     BACKGROUND 
     Keeping track of different types of data entries and interdependencies among the different entries is a task for which computers are ideally suited, and modern society depends heavily on this capability. From social networking platforms to financial analysis applications, computers, along with robust communication networks, are able to propagate a change in one data item (e.g., a change in a cell of a spreadsheet or a change in a user&#39;s status on a social network) to other data items (e.g., a recalculation of a formula in a spreadsheet or an update of an emoticon on the devices of the user&#39;s friends). 
     One problem that arises with propagating changes among many interdependent data entries is that it can be very slow when the number of entries and interdependencies is high and when the entries are stored across different documents, databases, servers and different geographical locations of the servers. For example, those who work with large spreadsheets are familiar with the experience in which, when a change is made to one cell of a spreadsheet, the spreadsheet program spends a long time updating itself repeatedly as the formulas depending on the changed cell get recalculated, the formulas depending on those formulas get recalculated, and so on. Dependencies that cross documents or servers create similar delays. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       While the appended claims set forth the features of the present techniques with particularity, these techniques, together with their objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings of which: 
         FIG.  1    is an example of a networking environment in which various embodiments of the disclosure may be implemented, according to an embodiment; 
         FIG.  2    is a block diagram of a computing device, according to an embodiment. 
         FIG.  3    is a block diagram of an example database configured to store workspaces with separate revision counters using the computing device of  FIG.  2   , according to an embodiment; 
         FIGS.  4 A to  4 E  and  FIGS.  5 A to  5 E  are diagrams showing a sequence of timeslices of revisions to spreadsheets with cells having formula dependencies and linking dependencies using document revision counters, according to an embodiment; 
         FIG.  6    is a flow diagram showing a sequence of revisions to documents using a workspace revision counter and document revision counters, according to an embodiment; 
         FIG.  7    is a flow diagram showing a sequence of revisions to documents having separate branches using a workspace revision counter and document revision counters, according to an embodiment; 
         FIG.  8    is a flow diagram showing a sequence of revisions to documents and integration of those revisions into other branches, according to an embodiment; 
         FIG.  9    is a flow diagram showing a sequence of revisions to documents using a workspace revision counter and a workspace revision queue where temporary revisions are displayed; 
         FIG.  10    is a flowchart illustrating an example method, implemented on a server, for rendering linked content in a first document having a table with a plurality of cells, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In systems configured to maintain multiple documents with various dependencies on each other, and particularly those with dozens of documents of different types, the accuracy of a report or displayed output that purports to capture a “snapshot” or “time slice” of the content of the documents may depend upon whether a change in one document has propagated to another document. In some scenarios, a user viewing several documents at the same time, but where those documents are only a subset of the entire set of documents, may not be able to view an accurate snapshot until the changes have been propagated across the entire set. As one example, when a cell in a spreadsheet is used as a “source” for content displayed in a “destination” 10-K financial document and also used in a destination Exhibit document, a change made to the source spreadsheet may be propagated to the 10-K document first (i.e., before the change has propagated to the Exhibit), so at a certain time slice, the 10-K document has been updated, but the Exhibit document has not yet been updated, and a user viewing both the 10-K document and the Exhibit document at the same time may become confused when entries between the destination documents, with purportedly the same values, do not match each other. 
     Disclosed herein is a system for maintaining links and revisions for a plurality of documents. Various embodiments of the disclosure are implemented in a computer networking environment. The system is configured to receive requests that indicate revisions to be carried out on the plurality of documents where at least one of the requests corresponds to revisions for different documents of the plurality of documents. The plurality of documents may be referred to herein as a “workspace,” for example, a shared repository of a group of documents for a corporation or business unit. For each of the received requests, a workspace revision counter that is shared by the plurality of documents is incremented. The workspace revision counter indicates a revision state of the plurality of documents. In other words, the workspace revision counter indicates a revision state of the documents as an integral data unit, as opposed to separate data units for each document with respective document revision counters. A revision indicated by a request is caused to be performed on one or more documents that correspond to the request. In some scenarios, a single request indicates changes to multiple documents, for example, a request to update a link between a source element and a destination element. 
     Turning to  FIG.  1   , an example of a computer networking environment in which various embodiments of the disclosure may be implemented is shown. A first computing device  100  is communicatively linked to a network  102 . Possible implementations of the network  102  include a local-area network, a wide-area network, a private network, a public network (e.g., the Internet), or any combination of these. The network  102  may include both wired and wireless components. Also communicatively linked to the network  102  are a second computing device  104   a , a third computing device  104   b , a fourth computing device  104   c , and a fifth computing device  106 . The fifth computing device  106  is communicatively linked to a media storage device  108  (e.g., a redundant array of independent disks). For the sake of example, it is assumed that a first user  120  operates the second computing device  104   a , a second user  122  operates the third computing device  104   b , and a third user  124  operates the fourth computing device  104   c . Each of the computing devices  104   a ,  104   b , and  104   c  executes client software (reference numerals  105   a ,  105   b , and  105   c , respectively). One possible implementation of the client software is a web browser. 
     Residing within the media storage device  108  is a database  108   a  containing multiple documents, three of which are depicted in  FIG.  1   : a first document  114 , a second document  116 , and a third document  118 . The first computing device  100  and the fifth computing device  106  are depicted as rack-mounted servers, while the second, third, and fourth computing devices  104   a ,  104   b , and  104   c  are depicted as a notebook computers. However, the computing devices depicted in  FIG.  1    are merely representative. Other possible implementations of a computing device include a desktop computer, a tablet computing, and a smartphone. Furthermore, although the first, second, and third documents  114 ,  116 , and  118  are depicted as being stored in a single device, they may, in fact, be stored on multiple storage devices (e.g., sharded into multiple physical chunks) of a cloud storage service. Finally, there may be more than or fewer than the first, second, and third documents  114 ,  116 , and  118  residing on the media storage device  108 . 
     In various embodiments, at least some documents are stored using a suitable data structure configured to maintain links and references between cells, tables, paragraphs, sections, or other suitable portions of a document. In an embodiment, documents are stored using an RTree data structure. In another embodiment, documents are stored using a causal tree data structure. 
     In an embodiment, the system includes a computing device that configures the computer memory according to a causal tree (a type of logic tree) representing a structure of a document. The computer memory may be internal to or external to the computing device. Causal tree structures are useful representations of how content and metadata associated with the content are organized. For example, a document may be represented by a single causal tree structure or a bounded set of causal tree structures. The causal tree structure is useful in efficiently tracking and storing changes made in the document. A typical causal tree structure includes nodes of the editing instructions in the document, and each editing instruction has a unique identifier or ID. The editing instructions include, for example, text characters, insertion of text characters, deletion of text characters, formatting instructions, copy and paste, cut and paste, etc. In other words, a causal tree structure is a representation of all the instructions (regardless of type) that compose a document. The causal tree structure starts with a root node and a collection of observation instances, from which all other instruction nodes branch. Except for the root node and observations, each editing instruction in the document is caused by whichever editing instruction that came before it. Every editing instruction is aware of the ID of its parent instruction, i.e., the instruction that “caused” it. In an embodiment, each instruction (other than the root node and observations) in the document may be represented as a 3-tuple: ID (ID of the instruction), CauseID (ID of the parent instruction), and Value (value of the instruction). Observations have a 3-tuple: ID (ID of the instruction), Start ID (ID of the first character in a range), and Stop ID (ID of character immediately after the last character in a range unless the same as the Start ID which indicates only a single character is to be observed). Additional instructions may be added to an observation to provide additional information or to modify the range being observed. Examples of observations are discussed in U.S. patent application Ser. No. 16/871,512. 
     In an embodiment, the system includes a computing device that configures the computer memory according to an RTree (a type of logic tree) representing a structure of a spreadsheet or other document. The computer memory may be internal to or external to the computing device. In an embodiment, the RTree has a plurality of nodes, at least some of which contain one or more minimum bounding rectangles. Each minimum bounding rectangle (“MBR”) encompasses cells of the spreadsheet from a different one of a plurality of columns of the spreadsheet, but does not encompass cells of any of the other columns of the plurality of columns. A node of the RTree may hold multiple MBRs or a single MBR. 
     For convenient reference, the first computing device  100  will also be referred to as a “productivity server  100 ” and the fifth computing device  106  will be also be referred to as a “database server  106 .” Although depicted in  FIG.  1    as separate devices, in some embodiments, the functionality of the productivity server  100  and the database server  106  are on the same device. The productivity server  100  executes productivity software  101  to provide document collaboration services. The database server  106  executes Software-as-a-Service (“SaaS”) platform software  107  to provide database services to the productivity software  101 , such as maintaining the contents of the database  108   a  and providing a programming platform for various processes launched by the productivity software (e.g., to manipulate, store, and retrieve documents and other information from the database  108   a ). Under the control of the productivity software  101 , the productivity server  100  interacts with the database server  106  (which operates under the control of the SaaS platform software  107 ) and the computing devices  104   a ,  104   b , and  104   c  (also referred to as “client devices”) to allow the computing devices to access the first document  114 , the second document  116 , and the third document  118  so that the first user  120 , the second user  122 , and the third user  124  can collaborate in editing the documents (e.g., moving sections around in a particular document). 
     In an embodiment, documents maintained on the media storage device  108  may be organized into sections, with each section (e.g., the contents of the section) being maintained in its own separate data structure referred to as a “section entity.” For example, the first document  114  in  FIG.  1    has a first section represented by a first section entity  130 , a second section represented by a second section entity  132 , and a third section represented by a third section entity  134 . The productivity software  101  uses an outline entity  136  (also stored on the media storage device) to determine how the sections are organized. 
       FIG.  2    is a block diagram of a computing device  200 , according to an embodiment. One or more of the computing devices of  FIG.  1    (including the media storage device  108 ) have the general architecture shown in  FIG.  2   , in various embodiments. The device depicted in  FIG.  2    includes a processor  152  (e.g., a microprocessor, controller, or application-specific integrated circuit), a primary memory  154  (e.g., volatile memory, random-access memory), a secondary memory  156  (e.g., non-volatile memory, solid state drive, hard disk drive), user input devices  158  (e.g., a keyboard, mouse, or touchscreen), a display  160  (e.g., an organic, light-emitting diode display), and a network interface  162  (which may be wired or wireless). The memories  154  and  156  store instructions and data. The processor  152  executes the instructions and uses the data to carry out various procedures including, in some embodiments, the methods described herein. 
     Each of the elements of  FIG.  2    is communicatively linked to one or more other elements via one or more data pathways  163 . Possible implementations of the data pathways  163  include wires, conductive pathways on a microchip, and wireless connections. In an embodiment, the processor  152  is one of multiple processors in the computing device, each of which is capable of executing one or more separate threads. In an embodiment, the processor  152  communicates with other processors external to the computing device in order to initiate the execution of different threads on those other processors. 
     The term “local memory” as used herein refers to one or both of the memories  154  and  156  (i.e., memory accessible by the processor  152  within the computing device). In some embodiments, the secondary memory  156  is implemented as, or supplemented by an external memory  156 A. The media storage device  108  is a possible implementation of the external memory  156 A. The processor  152  executes the instructions and uses the data to carry out various procedures including, in some embodiments, the methods described herein, including displaying a graphical user interface  169 . The graphical user interface  169  is, according to one embodiment, software that the processor  152  executes to display a report on the display device  160 , and which permits a user to make inputs into the report via the user input devices  168 . 
     The computing devices of  FIG.  1    (i.e., the processor  152  of each of the computing devices) are able to communicate with other devices of  FIG.  1    via the network interface  162  over the network  152 . In an embodiment, this communication takes place via a user interface that the productivity server  150  provides to the computing devices  154   a ,  154   b , and  154   c . The specific nature of the user interface and what the user interface shows at any given time may vary depending on what the user has chosen to view. Also, multiple users may interact with different instances of the user interface on different devices. In some embodiments, the productivity server  150  carries out calculations to determine how content is to be rendered on a computing device, generates rendering instructions based on those calculations, and transmits those rendering instructions to the computing device. Using the received instructions, the computing device renders the content on a display. In other embodiments, the productivity server  150  transmits instructions regarding an asset to a computing device. In carrying out the received instructions, the computing device performs the appropriate calculations locally to render the content of the asset on a display. 
       FIG.  3    is a block diagram of an example database  300  configured to store workspaces with separate revision counters using the computing device of  FIG.  2   . In the embodiment shown in  FIG.  3   , the database  300  generally corresponds to the database  108   a  and includes the first document  114 , the second document  116 , and the third document  118 . In other embodiments, the database  300  includes one, two, four, or more documents. 
     In various embodiments, the database  300  includes a first workspace  310  having a document table  320 , a workspace revision queue  330 , and a workspace revision counter  340 . The first workspace  310  represents a shared repository of a plurality of documents. In some scenarios, the repository is associated with a corporation, business unit, user group, or other entity. The plurality of documents may be of the same or different types in various embodiments, for example, spreadsheet documents, text documents, presentation documents, or other suitable document types. In an embodiment, the workspace  310  is configured to store the plurality of documents (i.e., documents  114 ,  116 , and  118 ), or suitable data structures associated with the documents, in the document table  320 . 
     The workspace revision counter  340  (or “workspace level revision counter”) is configured to be shared by the plurality of documents and indicates a revision state of the plurality of documents at any given point in time. In other words, the workspace revision counter  340  indicates a revision state of the plurality of documents as an integral data unit, as opposed to separate document revision counters for individual documents (“document level revision counters”). The workspace revision counter  340  is a workspace level revision for grouping the revision of all workspace content at any given point in time within a workspace. By sharing the workspace revision counter  340  among the plurality of documents, a change or revision to any single document causes an increment to the workspace revision counter  340 . As an example, when a first change to a first document in the workspace  310  increments the workspace revision counter from 7 to 8, then a second change to a second document in the workspace  310  occurring after the first change increments the workspace revision counter  340  from 8 to 9. In a further example, the workspace revision counter  340  is incremented from 9 to 10 when a third change to the first document is requested. 
     The workspace revision queue  330  is configured to store revisions to the plurality of documents, more specifically, requests for revisions. The workspace revision queue  330  is shared by the plurality of documents and stores revisions to different documents of the plurality of documents. In various embodiments, the workspace revision queue  330  is a queue for ordering requests for revisions in an linear fashion across the entire workspace. In the embodiment shown in  FIG.  3   , using the above example, the first change to the first document, the second change to the second document, and the third change to the first document are queued as revisions  332 ,  334 , and  336 . In an embodiment, the computing device  200  processes or performs the revisions in the workspace revision queue  330  in a first in, first out (FIFO) manner. In other embodiments, the computing device  200  prioritizes at least some of the revisions, for example, based on a priority level of the corresponding document to be revised, a priority level of a user that requested the revision, or other suitable criteria. In some embodiments, the computing device  200  groups at least some of the revisions in the workspace revision queue  330 , for example, according to whether the revisions can be performed in parallel. 
     In the embodiment shown in  FIG.  3   , the database  300  also includes a second workspace  350  having a document table  370 , a workspace revision queue  380 , and a workspace revision counter  390  (analogous to the document table  320 , the workspace revision queue  330 , and the workspace revision counter  340 ). In some embodiments, the database  300  is configured to provide a separate workspace for different pluralities of documents, for example, for different corporations, business units, user groups, or other entities. 
     In some embodiments, the database  300  includes a document revision queue for one or more of the plurality of documents. The document revision queue is configured to store temporary copies of revision and is not shared among the plurality of documents, but is instead specific to a particular document. In an embodiment, for example, the first document  114  includes a document revision queue  314 . The document revision queue allows for separate versions or branches of a document to be maintained concurrently, as described herein. In an embodiment, the document revision queue is specific to a locked section of a document where the locked section is a section of the document that is restricted from editing by users outside of an editing group. 
       FIGS.  4 A to  4 E  and  FIGS.  5 A to  5 E  are diagrams showing a sequence of timeslices for revisions to spreadsheets with cells having formula dependencies and linking dependencies using document revision counters. In the embodiment shown, the sequence shows revisions to a first spreadsheet document (referred to herein as “Sheet1”) and a second spreadsheet document (“Sheet2”) with versions indicated as “v1”, “v2”, and so on. Notably, the version numbers of the documents are independent of each other. For ease of description, only two columns (“A” and “B”) and two rows (“1” and “2”) are shown in  FIGS.  4 A to  4 E  and  FIGS.  5 A to  5 E . 
       FIG.  4 A  shows an initial state of the documents with both the first document and the second document at version 1 (“Sheet1_v1” and “Sheet2 v1”) with empty cells. At  FIG.  4 B , Sheet2 has been modified and advances to revision 2 (“v2”) to include a formula in cell B2, specifically, a summation of the values in column A (“=SUM(A)=0”). Since cells A1 and A2 are empty, the summation of cell B1 of Sheet2 in  FIG.  4 B  is zero. At  FIG.  4 C , Sheet2 has been modified and advances to revision 2, where cell A2 of Sheet2 contains a link to cell B1 of Sheet2 (the link is represented by “S1B1”) and cell A2 contains a formula that relies upon cell A1 (“=A1*3=0”). The link indicates that cell B1 of Sheet1 is a source element for cell A2 of Sheet2, which is a destination element. At  FIG.  4 D , Sheet1 has been modified and advances to version 3 (“v3”), where cell B2 contains a link to cell A2 of Sheet2 (the link is represented by “S2A2”). In other words, cell A2 of Sheet2 is the source of the link, and cell B2 of Sheet1 the destination of the link. 
     As used herein, a link is a reference, pointer, or data structure that refers to linked content (or the location of the linked content), while linked content is a set of content, for example, a set of one or more characters or numbers, a set of one or more sentences, a set of one or more paragraphs, a set of one or more cells within a spreadsheet, a set of one or more images, or various combinations thereof. For example, in  FIG.  4 C , the value 0 in cell A1 of Sheet2 is the linked content, and “S1B1” is a representation that indicate that cell A1 of Sheet2 contains a link. Although “S1B1” and “52A2” are used to represent links in  FIGS.  4 C to  4 E and  5 A to  4 E , the user interface may not display these representations. In various implementations, no visual indicator or different visual indicators (e.g., icons, underlining, different font color or font face, different background color, a box that surrounds the link, etc.) may be used to indicate the existence of a link, the source of a link, or the destination of a link. In other embodiments, a user may need to perform another gesture on the user interface (e.g., hover, right click, double click, etc.) to trigger the display of the source(s) or destination(s) of a link (e.g., via a pop-up panel or side panel). In an embodiment, the linked set of content contains a plurality of elements (i.e., characters, cells, paragraphs, etc.) that appear consecutively within a document, for example, cells A4 through A7 of a spreadsheet or sentences one through five of a text document. In another embodiment, the linked set of content contains a plurality of elements that do not appear consecutively, for example, cells B18:C20 of a spreadsheet (i.e., cells B18, B19, B20, C18, C19, and C20). 
     At  FIG.  4 E , Sheet 1 has been modified and advances to version 4 (“v4”), where cell A1 has a value of 1 and cell B1, based on its formula, has its displayed value changed to 1. In some scenarios, the link of cell A1 in Sheet2 is not immediately updated, for example, due to processing delays associated with identifying when a source element has changed. Accordingly, at the timeslice shown in  FIG.  4 E , Sheet2 has not yet been updated to a new version. 
     At  FIG.  5 A , the link of cell A1 in Sheet2 has been updated to include the appropriate value from source element B1 of Sheet1 (“1”), cell A2 in Sheet2 is being processed to calculate its formula, and Sheet2 advanced to version 3. In some scenarios, the formula in cell A2 is relatively complex and may have a long processing time (e.g., several minutes or more) before its value has been determined. In other scenarios, the formula may refer to an external source (e.g., a document outside of the workspace  310 ) that may have reduced availability or delayed updates, for example, by being stored on a remote computer. In still other scenarios, the formula may include a link to another “busy” document that is being used by many other users so that access to its data is delayed. 
     At  FIG.  5 B , the formula in cell A2 of Sheet2 has been calculated, a value of “2” has been inserted in cell A2 of Sheet1, the formula in cell B1 of Sheet1 is updated to a value of 3, and Sheet1 has advanced to version 4 (“v5”), but the link in cell B2 of Sheet1 has not yet been updated with the result of the formula in cell A2 of Sheet2. At this timeslice, Sheet1 is inconsistent with itself because the value of cell A2 in Sheet2 has not propagated to cell B2 of Sheet1. Moreover, Sheet2 is not consistent with Sheet1 because cell A1 of Sheet2 has not been updated with the updated value (“3”) of cell B1 of Sheet1. 
     At  FIG.  5 C , cell B2 of Sheet1 has been updated to the most recent confirmed value of its link to cell A2 of Sheet2 and Sheet1 advances to version 6 (“v6”). Additionally, cell A1 of Sheet2 is updated to the most recent value of source cell B1 and Sheet2 advances to version 4 (“v4”). At  FIG.  5 D , cell A2 of Sheet2 has been calculated, but the value is not propagated to cell B2 of Sheet1 until  FIG.  5 E . 
     One solution to the problem of propagating values, either through formulas or links, is to utilize the workspace revision counter  340 . Although the workspace revision counter  340  may be incremented more often and more quickly than individual document revision counters, the workspace revision counter  340  provides a single value that can be referenced to refer to a single timeslice for all documents in the workspace  310  where all values have been propagated. 
       FIG.  6    is a flow diagram showing a sequence  600  of revisions to documents using a workspace revision counter, for example, the workspace revision counter  340 , according to an embodiment. In the embodiment shown in  FIG.  6   , first and second documents (“Doc1” and “Doc2”) are provided for editing to various clients (including Users 1, 2, 3, and 4) by a frontend user interface (“frontend”). In some embodiments, the frontend user interface is provided by the first computing device  100 , the fifth computing device  106 , or another suitable computing device. In some embodiments, the clients utilize respective ones of the computing devices  104   a ,  104   b ,  104   c . In the embodiment shown in  FIG.  6   , User1 and User3 modify the first document, while User2 and User4 modify the second document, via respective user interfaces. Although only two documents and four clients are shown, in other embodiments, the frontend may provide hundreds of documents to hundreds of clients concurrently. 
     During block  610 , User1 sends a request for a revision to the first document (“EditDoc(doc1, . . . )”) and the request is received by the frontend. In some scenarios, the request includes one, two, three, or more revisions. The frontend causes the revision to be performed on the first document, for example, by updating the first document within the database  108   a , and increments a document revision counter (“Doc1.revision+1”). The frontend provides the updated document revision counter (“2”) to the User1. 
     During block  615 , the frontend increments the workspace revision counter  340 , resulting in a new value of “75”. Although the most recent revision incremented the document revision counter of the first document to “2”, the workspace revision counter  340  is utilized for each document in the workspace  310 , so its value is higher than the document revision counter. 
     During block  620 , User2 sends a request for a revision to the second document (“EditDoc(doc2, . . . )”) and the request is received by the frontend. The frontend causes the revision to be performed on the second document, for example, by updating the first document within the database  108   a , and increments a document revision counter (“Doc2.revision+1”). The frontend provides the updated document revision counter (“12”) to the User2. 
     During block  625 , the frontend increments the workspace revision counter  340 , resulting in a new value of “76”. Notably, revisions to both the first document and the second document result in updates to the same counter, specifically, the workspace revision counter  340 . Subsequent revisions to the first document at block  630  and to the second document at block  640  include increments to the respective document revision counters and are also followed by updates to the workspace revision counter  340  at blocks  635  and  645 . 
     In another embodiment, if a first document contains the source element of a link and a second document contains the destination element of the link, then when a user sends a request to edit the source element of the link (e.g., linked content or other properties of the link) in the first document, the request will also trigger a request to edit the destination element of the link in the second document. In other words, when a user makes a revision to the source element of the link in the first document, the revision is propagated to the destination element of the link in the second document. In this instance, the document revision counter of the first document will increment by 1, the document revision counter of the second document will increment by 1, and the workspace level counter will also increment by 1. 
     Cloud-based document collaboration platforms tend to be fully open and collaborative. That is, all users who are invited to edit a document (e.g., text document, graphics-based document, spreadsheet, or a hybrid of one or more of the foregoing) are able to see one another&#39;s edits in real time or nearly real time. However, there are many scenarios in which one or more users would prefer not to share their draft work product with other collaborators. In these scenarios, the user (or group of users) may create a branch of the document, or a branch of a portion thereof (e.g., a section of a document), where read and/or write access to the branch is limited to themselves only (a “private user”) or to themselves and any additional users (a “private group”). Once a section becomes private, users other than the private user or those not within the private group will not be able to see additional edits being made but will only see the state of the section as it was just prior to being taken private. The private user or a user within the private group (assuming they have sufficient permission) can choose to make the edits public, which unlocks the private section and allows the rest of the collaborators to view the changes and to make their own edits to the section if desired. 
     In an embodiment, edits to the document are managed through the use of a causal tree or causal graph, and when a section of the document is taken private, the document collaboration system creates a copy of the relevant segment or segments of the causal tree or causal graph, uses the segment or segments to keep track of the edits and, when the section is subsequently made public, merges the segment or segments into the original causal graph. 
     In another embodiment, edits to the document are managed through the use of an Rtree (also referred to herein as “R-Tree”), and when a section of the document is taken private, the document collaboration system creates a copy of the relevant segment or segments of the Rtree, uses the segment or segments to keep track of the edits and, when the section is subsequently made public, merges the segment or segments into the original Rtree. 
       FIG.  7    is a flow diagram showing a sequence  700  of revisions to documents having separate branches using a workspace revision counter and document revision counters, for example, the workspace revision counter  340 . The embodiment shown in  FIG.  7    is similar to that of  FIG.  6   , where first and second documents (“Doc1” and “Doc2”) are provided for editing to various clients (including Users 1, 2, 3, and 4) by a frontend user interface (“frontend”). 
     In the embodiment of  FIG.  7   , the revisions to the first and second documents are initially stored in a separate branch that may be combined with a main branch at a later time, discarded, or maintained separately from one or more other branches. As an example, a secondary branch of the first document  114  may be edited and reviewed by a user and changes by the user may be stored and managed in the document revision queue  314  without affecting a main branch of the first document  114 . When the changes from the user are to be finalized and incorporated into the main branch (e.g., to publish an update to a publicly available document), the changes to the document may be incorporated into the main branch, for example, by merging or rebasing. In various embodiments, the main branch and any secondary branches are identified by respective branch identifiers (“branch IDs”), for example, a unique identifier, that allow revisions in a secondary branch to be incorporated into a main branch, revisions in a main branch to be incorporated into a secondary branch, etc. 
     Merging generally corresponds to a process of comparing a secondary branch to a main branch and making any needed changes to the main branch to be consistent with the secondary branch. Rebasing generally corresponds to a process of making the changes that were made on the secondary branch (relative to a common earlier base), but instead using a “sibling” branch as the new base to be modified. In other words, rebasing effectively “replays” changes from the secondary branch (e.g., stored in the document revision queue  314 ) onto another branch sequentially in the order they were introduced, whereas merging takes the endpoints of the branches and simply merges them together. 
     In the embodiment shown in  FIG.  7   , the first document and the second document have their own respective secondary branches (“Doc1 Draft Branch” and “Doc2 Draft Branch”). However, in other embodiments, two or more documents within a workspace are part of a same branch. In some embodiments, a branch for an entire workspace is created and later merged or rebased with another branch, or maintained separately. 
     At block  710  and block  730 , respectively, User2 and User2 request revisions to the first document, analogously to blocks  610  and  630 . Similarly, at blocks  720  and  740 , User2 and User4 request revisions to the second document, analogously to blocks  620  and  640 . The revisions corresponding to the first document are stored in the document revision queue  314 , in an embodiment, and the revisions corresponding to the second document are stored in a corresponding document revision queue (not shown). In some other embodiments, the document revisions for the first document and the second document are stored in a same database or central repository, but are flagged as being limited to a particular branch, for example, using a branch identifier that uniquely identifies the branch. 
     At block  750 , User1 requests a merge of the secondary branch of the first document with the main branch and the revisions stored in the document revision queue  314  are merged or rebased with those in the main branch. At block  755 , the frontend increments the workspace revision counter  340 . In this embodiment, the separate revisions of the first document at blocks  710  and  730  are combined into a same request for a revision and correspond to a same revision number (“75”) for the workspace  310 . Similarly, the separate revisions of the second document at blocks  720  and  740  are combined into a same request (block  760 ) for a revision and correspond to a same revision number (“76”, block  765 ) for the workspace  310 . The requests at blocks  750  and  760  identify the revisions to be incorporated into the main branch by using a branch identifier that corresponds to the branch. 
       FIG.  8    is a flow diagram showing a sequence  800  of revisions to documents and integration of those revisions into other branches using the workspace revision counter  340 , according to an embodiment. In the embodiment shown in  FIG.  8   , a first document (“Doc1”) is provided for editing to various clients (including Users 1 and 2) by a frontend user interface (“frontend”). In some embodiments, the frontend user interface is provided by the first computing device  100 , the fifth computing device  106 , or another suitable computing device. In some embodiments, the clients utilize respective ones of the fourth computing devices  104   a . In the embodiment shown in  FIG.  8   , User1 and User2 modify the first document via respective user interfaces. Although only one documents and two clients are shown, in other embodiments, the frontend may provide hundreds of documents to hundreds of clients concurrently. 
     At block  810 , the first user (User1) makes revisions to a secondary branch of the first document (e.g., a “private” branch) that are stored separately from other revisions by the second user (User2), which are performed at block  820 . At block  830 , the first user requests that the changes from their secondary branch be incorporated into the main branch in a manner similar to that described above with respect to block  750 . At block  840 , the frontend increments the workspace revision counter  340 . 
     In contrast to the merging of a secondary branch into the main branch (e.g., a “fan-in” action), at block  850 , the revisions to the main branch that were fanned in are “fanned out” to the secondary draft of the second user. In various embodiments, the fanning out process is a merge process or a rebase process, as described above. 
     At block  860 , the second user (User2) makes revisions to a secondary branch of the first document that are stored separately from the revisions by the first user. At block  870 , the second user incorporates the changes from their secondary branch into the main branch in a manner similar to that described above with respect to block  830 . At block  880 , the frontend increments the workspace revision counter  340 . 
       FIG.  9    is a flow diagram showing a sequence  900  of revisions to documents using a workspace revision counter and workspace revision queue where temporary revisions are displayed, according to an embodiment. In some scenarios, utilization of the workspace revision queue  330  reduces performance (e.g., longer processing times, longer queue times before a revision is performed) due to higher memory requirements for data structures associated with the workspace  310 . In an embodiment, for example, a single RTree or causal tree is shared for the plurality of documents in the workspace  310  and has a larger size than separate RTrees for the documents. Additionally, contention for access to the RTree by different documents being revised at the same time may increase the queue times for a revision to be processed. 
     In the embodiment shown in  FIG.  9   , the computing device  200  is configured to perform “optimistic” revisions at the document level, but identify those revisions as being “inconsistent” within the user interface until the revision has been processed and determined to be consistent at the workspace level. The optimistic revisions are revisions that are received from a user for a displayed document (e.g., a secondary branch displayed on a user interface  104   a ), performed for the displayed document and updated on the user interface  104   a , but without fully updating formulas or links in the displayed document that refer to other documents, other sections of documents, or external sources. Optimistic revisions provide improved feedback to the user (i.e., near real-time, without having to wait for changes to propagate through the workspace revision queue), but may be incorrect if they rely on the results of a formula calculation or link that has not completed. 
     As one example, a cell B1 in a first sheet (S1B1) and a cell B3 of a second sheet (S2B3) contains formulas as follows: 
       S1B1=SUM(S1A1, S1A2, S2B3) 
       S2B3 =S1A1*3 
     where S1A1 corresponds to a cell A1 of the first sheet with an initial value of “2”, S1A2 corresponds to a cell A2 of the first sheet having an initial value of “5”. In this example, the cell S2B3 has an initial value of “6” (2*3) and the cell S1B1 has an initial value of “13” (2+5+6). When the user revises cell S1A1 to a value of “4”, an optimistic revision indicates a new value of “15” (4+5+6), using the updated value of cell S2A1 but without an update to the value referenced in the second sheet (S2B3). In this example, the value of “15” is shown, but with a temporary identification on the displayed document that indicates that the value is a temporary revision, not a final revision (i.e., with an updated value from cell S2B3). Once the final revision has been propagated, where S2B3 is updated to “12” (4*3) and S1B1 is updated to 21 (4+5+12), the temporary identification is removed. Examples of a temporary identification include a different font color or font face, a different background color, a box that surrounds the value, underlining, or other suitable visual indication. 
     At blocks  910 ,  920 ,  930 , and  940 , various users revise first and second documents and send requests for the revisions to the frontend, in a manner similarly to that described above with respect to blocks  710 ,  720 ,  730 , and  740 . In the embodiment of  FIG.  9   , however, the revisions at blocks  910 ,  920 ,  930 , and  940  are optimistic or temporary until the computing device  200  has finalized the revisions, for example, by updating formulas and links contained within an RTree for the workspace  310 . At blocks  910 ,  920 ,  930 , and  940 , the temporary revisions are marked as “inconsistent,” as discussed above. Moreover, updates to the workspace revision queue  330  are marked as inconsistent until the revisions have been finalized. 
     In some embodiments, a separate process is performed for finalizing the revisions using the workspace revision queue, for example, a write-behind consistency process. The write-behind consistency process traverses the entirety of the RTree for the workspace  310  and updates formulas, links, or both formulas and links. In an embodiment, the frontend is provided by the productivity server  100  and the write-behind consistency process is performed by the database server  106 . When the write-behind process is complete, the database server  106  marks the workspace revision queue  330 , or a particular revision therein, as being consistent. In the embodiment shown in  FIG.  9   , the write-behind consistency process is shown performing separate final revisions for blocks  910 ,  920 ,  930 , and  940  at blocks  950 ,  960 ,  970 , and  980 , respectively. 
     In some embodiments, causing the revision to be performed includes queuing a temporary copy of the revision in a document revision queue that is specific to the document corresponding to the revision. In an embodiment, for example, the document revision queue corresponds to the document revision queue  314 . A temporary revision is performed on a computing device that displays a secondary branch of the document corresponding to the revision, without performing a revision on a corresponding main branch of the document. In an embodiment, for example, the productivity server  100  performs the temporary revision on a branch of the first document at block  910 , without performing a final revision at block  950  (i.e., before the final revision has been performed). In other embodiments, the temporary revision corresponds to the blocks  920 ,  930 , or  940  of  FIG.  9   . The revision is queued as a final revision in the workspace revision queue  330  and performed on the main branch, for example, corresponding to blocks  950 ,  960 ,  970 , or  980  of  FIG.  9   . 
     In some embodiments, a received request for a revision indicates a revision to two or more documents. In an embodiment, for example, the request is for a revision to a link where the revision corresponds to a source element within a first document and a destination element within a second document. The link revision is initially queued in the first document revision queue that is specific to the document containing the source element of the link (e.g., the document being edited by the user that makes the request). In an embodiment, this document revision queue is processed by the frontend provided by the productivity server  100 . The link revision is initially identified as being “inconsistent” until the write-behind consistency process, performed by the database server  106 , further processes the revision and determines that the revision is consistent with other revisions, links, and/or formulas. In an embodiment, the link revision is queued in the workspace revision queue, the write-behind consistency process traverses the RTree for the workspace  310  for the link revision, and queues the link revision in a document revision queue that is specific to the second document containing the destination element. 
     In some embodiments, revisions or updates to the workspace  310  that originate outside of the workspace  310  are also handled using the write-behind consistency process. In this way, an update to an external document (e.g., outside of the workspace  310 ) that is relied upon by a document within the workspace  310  is associated with a final revision and reference number for the workspace revision counter  340 . In various embodiments, the external document is located on a remote server, cloud service, in a different workspace (e.g., in the workspace  350 ), or other suitable location. 
     As discussed above, in some embodiments, the computing device  200  utilizes an RTree as a data structure to store electronic documents of the workspace  310 . In an embodiment, the computing device  200  utilizes the RTree for maintaining formulas that reference different cells. In another embodiment, the computing device  200  utilizes the RTree for maintaining both formulas and links to different cells. In this embodiment, a single RTree is utilized for maintaining formulas and links throughout the plurality of documents of the workspace  410 . This approach improves detection of circular references across all documents within the workspace  310  and also improves the flow of values from one document to another document over links and formulas. In some embodiments, the computing device  200  maintains separate RTrees (e.g., one or more RTrees per document), but links the RTrees by utilizing a common reference time. 
       FIG.  10    is a flowchart illustrating an example method, implemented on a server, for maintaining links and revisions for a plurality of documents, according to an embodiment. In some embodiments, the method  1000  is implemented by the productivity server  100  of  FIG.  1   , which interacts with the database server  106  and the client devices  104 .  FIG.  10    is described with reference to  FIG.  1    for explanatory purposes. In other embodiments, however, the method  1000  is implemented by another suitable computing device. 
     At block  1002 , requests are received that indicate revisions to be carried out on the plurality of documents. In an embodiment, the plurality of documents corresponds to the plurality of documents in the document table  320  ( FIG.  3   ). In some embodiments, at least one of the requests correspond to revisions for different documents of the plurality of documents, for example, the first document  114  and the second document  116 . In various embodiments, the requests correspond to blocks  610 ,  620 ,  630 , or  640  of  FIG.  6   , blocks  710 ,  720 ,  730 ,  740 ,  750 , or  760  of  FIG.  7   , blocks  810 ,  820 ,  830 ,  850 ,  860 , or  870  of  FIG.  8   , or blocks  910 ,  920 ,  930 , or  940  of  FIG.  9   . 
     At block  1004 , a workspace revision counter that is shared by the plurality of documents is incremented. In an embodiment, the workspace revision counter indicates a revision state of the plurality of documents. In some embodiments, the workspace revision counter corresponds to the workspace revision counter  340 . In various embodiments, incrementing the workspace revision counter  340  corresponds to blocks  615 ,  625 ,  635 , or  645  of  FIG.  6   , blocks  755  or  765  of  FIG.  7   , blocks  840  or  880  of  FIG.  8   , or blocks  915 ,  925 ,  935 , or  945  of  FIG.  9   . 
     At block  1006 , the revision is queued in a workspace revision queue that is shared by the plurality of documents. In an embodiment, the workspace revision queue corresponds to the workspace revision queue  330 . 
     At block  1008 , the revision indicated by the request is caused to be performed on one or more documents of the plurality of documents that correspond to the request. 
     In some embodiments, the method  1000  further includes displaying a temporary identification that corresponds to the temporary revision on the displayed document and indicates that the temporary revision is not the final revision. The temporary identification is removed from the displayed document when the final revision has been performed. In an embodiment, for example, a temporary revision is shown on a computing device using a different font color or font face, a different background color, a box that surrounds the value, underlining, or other suitable visual indication as the temporary identification at block  910 , and the temporary identification is removed at block  950 . In some embodiments, at least some user interface features of a user interface on which the document is displayed are disabled while at least some temporary identifications are displayed. In an embodiment, for example, user interface features such as generating a report based on the plurality of documents, exporting the plurality of documents, or other actions are temporarily disabled until the revisions have been finalized. 
     In an embodiment, the method  1000  further includes receiving a revision for data that is external to the plurality of documents and linked from at least one of the plurality of documents. In an embodiment, the external data corresponds to data from an external workspace, for example, the workspace  350 . In another embodiment, the external data corresponds to data from a remote server, cloud service, or other suitable location. The workspace revision counter is incremented based on the revision for the external data. The revision for the external data is queued in the workspace revision queue, i.e., the workspace revision queue  330 . 
     All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     For the purposes of promoting an understanding of the principles of the disclosure, reference has been made to the embodiments illustrated in the drawings, and specific language has been used to describe these embodiments. However, no limitation of the scope of the disclosure is intended by this specific language, and the disclosure should be construed to encompass all embodiments that would normally occur to one of ordinary skill in the art. The terminology used herein is for the purpose of describing the particular embodiments and is not intended to be limiting of exemplary embodiments of the disclosure. In the description of the embodiments, certain detailed explanations of related art are omitted when it is deemed that they may unnecessarily obscure the essence of the disclosure. 
     The apparatus described herein may comprise a processor, a memory for storing program data to be executed by the processor, a permanent storage such as a disk drive, a communications port for handling communications with external devices, and user interface devices, including a display, touch panel, keys, buttons, etc. When software modules are involved, these software modules may be stored as program instructions or computer readable code executable by the processor on a non-transitory computer-readable media such as magnetic storage media (e.g., magnetic tapes, hard disks, floppy disks), optical recording media (e.g., CD-ROMs, Digital Versatile Discs (DVDs), etc.), and solid state memory (e.g., random-access memory (RAM), read-only memory (ROM), static random-access memory (SRAM), electrically erasable programmable read-only memory (EEPROM), flash memory, thumb drives, solid state drives, etc.). The computer readable recording media may also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. This computer readable recording media may be read by the computer, stored in the memory, and executed by the processor. 
     Also, using the disclosure herein, programmers of ordinary skill in the art to which the disclosure pertains may easily implement functional programs, codes, and code segments for making and using the disclosure. 
     The disclosure may be described in terms of functional block components and various processing steps. Such functional blocks may be realized by any number of hardware and/or software components configured to perform the specified functions. For example, the disclosure may employ various integrated circuit components, e.g., memory elements, processing elements, logic elements, look-up tables, and the like, which may carry out a variety of functions under the control of one or more microprocessors or other control devices. Similarly, where the elements of the disclosure are implemented using software programming or software elements, the disclosure may be implemented with any programming or scripting language such as C, C++, JAVA®, assembler, or the like, with the various algorithms being implemented with any combination of data structures, objects, processes, routines or other programming elements. Functional aspects may be implemented in algorithms that execute on one or more processors. Furthermore, the disclosure may employ any number of conventional techniques for electronics configuration, signal processing and/or control, data processing and the like. Finally, the steps of all methods described herein may be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
     For the sake of brevity, conventional electronics, control systems, software development and other functional aspects of the systems (and components of the individual operating components of the systems) may not be described in detail. Furthermore, the connecting lines, or connectors shown in the various figures presented are intended to represent exemplary functional relationships and/or physical or logical couplings between the various elements. It should be noted that many alternative or additional functional relationships, physical connections or logical connections may be present in a practical device. The words “mechanism”, “element”, “unit”, “structure”, “means”, and “construction” are used broadly and are not limited to mechanical or physical embodiments, but may include software routines in conjunction with processors, etc. 
     The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the disclosure and does not pose a limitation on the scope of the disclosure unless otherwise claimed. Numerous modifications and adaptations will be readily apparent to those of ordinary skill in this art without departing from the spirit and scope of the disclosure as defined by the following claims. Therefore, the scope of the disclosure is defined not by the detailed description of the disclosure but by the following claims, and all differences within the scope will be construed as being included in the disclosure. 
     No item or component is essential to the practice of the disclosure unless the element is specifically described as “essential” or “critical”. It will also be recognized that the terms “comprises”, “comprising”, “includes”, “including”, “has”, and “having”, as used herein, are specifically intended to be read as open-ended terms of art. The use of the terms “a” and “an” and “the” and similar referents in the context of describing the disclosure (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless the context clearly indicates otherwise. In addition, it should be understood that although the terms “first”, “second”, etc. may be used herein to describe various elements, these elements should not be limited by these terms, which are only used to distinguish one element from another. Furthermore, recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein.