Patent Publication Number: US-7904592-B2

Title: Distributed, fault-tolerant message store

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Application Ser. No. 60/418,728 filed on Oct. 17, 2002, incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The embodiments of the present invention generally relate to the storage and retrieval of messages destined for mobile devices. More particularly, the embodiments relate to systems and methods of managing messages that provide improved scalability and fault-tolerance. 
     2. Discussion 
     In the highly competitive wireless networking industry, there is a well-documented trend toward enhanced functionality. While the above trend is desirable to the consumer, it presents significant challenges to network designers as well as service providers. One area of particular concern is the increasing demand for messaging services. 
     Providing messaging services such as the short message service (SMS) described in “Technical Realization of the Short Message Service (SMS) Point-to-Point (PP),” European Telecommunications Standards Institute, ETSI TS 100.901 v7.5.0, December 2001, to mobile devices such as mobile telephones, personal digital assistants (PDAs) and pagers enables subscribers to communicate in environments in which traditional voice and other nodes of communication are impractical, inconvenient, and often unavailable. Messaging services such as the multimedia messaging service (MMS) described in “WAP MMS Architecture Overview”, Version 25 Apr. 2001, WAP-205-MMSArchOverview-20010425-a, WAP Forum, (a copy of which can be obtained from a website maintained by the WAP Forum); “WAP MMS Client Transactions”, Version 15 Jan. 2002, WAP-206-MMSCTR-20020115-a, WAP Forum, (a copy of which can be obtained from a website maintained by the WAP Forum); and “WAP MMS Encapsulation Protocol”, WAP-209-MMSEncapsulation-20020105-a, WAP Forum, (a copy of which can be obtained from a website maintained by the WAP Forum), are presented with similar challenges in the mobile environment. 
     As the demand for messaging services increases, the burden on servers to require large storage capacity for messages as well as high-performance message storage and retrieval also increases. Single servers have been proposed, but are typically unable to scale to increased capacity and performance requirements. Furthermore, frequent backup operations are often required in order to maintain data in the event of a server failure. Alternative approaches involve the use of multiple, distributed servers, but they require human intervention in order to reconfigure the wireless network in the event of a server failure. Conventional distributed server techniques are therefore typically unable to handle individual server failures reliably. There is therefore a need to provide an approach that is scalable as well as fault-tolerant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various advantages of the embodiments of the present invention will become apparent to one skilled in the art by reading the following specification and appended claims, and by referencing the following drawings, in which: 
         FIG. 1  is a flowchart of an example of a method of storing information according to one embodiment of the invention; 
         FIG. 2  is a flowchart of an example of a method of retrieving information in accordance with one embodiment of the invention; 
         FIG. 3  is a block diagram of an example of a message arrival process in accordance with one embodiment of the invention; 
         FIG. 4  is a block diagram of an example of a message retrieval process in accordance with one embodiment of the invention; and 
         FIG. 5  is a block diagram of an example of a message retrieval process in accordance with an alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Systems and methods of managing a network provide for the use of addressing functions to store information to and retrieve information from the network. Each addressing function corresponds to a topology of the network, and as nodes are added to and removed from the network new addressing functions are established. If a message is received at a node for storage, an actual destination node is calculated based on a subscriber identifier contained in the message and the current addressing function. The message is sent to the actual destination node for storage. If a message retrieval request is received at a node, a plurality of destination nodes is calculated based on a subscriber identifier contained in the request and all valid addressing functions. The calculated destination nodes are queried for messages. 
       FIG. 1  shows a method  60  of storing information to a network. In the illustrated example, the information to be stored is a message destined for a wireless handset in a wireless network. It should be noted, however, that other types of information can also be stored such as image files, sound files, etc. Illustrated method  60  provides for receiving a message at an initial storage node at processing block  62 , where the message includes a subscriber identifier. Block  64  provides for calculating an actual destination node based on the subscriber identifier and a current addressing function  15 , which corresponds to a current topology of the network. The message is sent to the actual destination node for storage at block  66 . As will be discussed in greater detail, method  60  can also provide for storing the message to an internal queue of the initial storage node and removing the message from the internal queue if a confirmation of receipt is received from the actual destination node. Such an approach increases the fault-tolerance of the network. 
       FIG. 2  shows a method  10  of retrieving messages in a wireless network. Method  10  can be implemented in any node of the network, where the node functions, at least in part, as a messaging server. While methods  10  and  60  ( FIG. 1 ) will primarily be discussed in reference to text messaging applications, the embodiments are not so limited. Indeed, voice and other messaging systems as well as other information storage systems can benefit from the principles described herein. Notwithstanding, there are a number of aspects of text messaging for which methods  10  and  60  ( FIG. 1 ) are well suited. 
     Processing block  12  provides for receiving a message retrieval request at a first node of the network. A plurality of destination nodes is identified based on the retrieval request and a plurality of addressing functions  16  at block  14 . Each addressing function  16  corresponds to a topology of the network at a particular moment in time. The topology of the network changes whenever a node is added or removed due to communication failure, bandwidth allocation, storage allocation, etc. The result is a query list  20 , which contains the appropriate destination nodes for the given retrieval request. Block  18  provides for querying the plurality of destination nodes for messages. 
     Thus, embodiments of the present invention provide for messaging systems that store messages on a message store that is maintained by a network node. The network node adds, updates, and deletes messages to the store, and retrieves messages from the store to the user device. The network node is also responsible for user authentication to prevent unauthorized access to messages. With messages stored on a networked message store, a user can potentially access his or her messages from multiple locations and multiple user devices. A networked message store, in comparison to a destination user device, also provides high capacity, low cost, and reliable storage. 
     Thus, a cluster of multiple, distributed nodes is used to store messages when a message to be stored is received by an arbitrary “initial node”. Any destination node can be the initial node, which computes an addressing function such as a “destination hash function” to determine the server node to store the message. The hash function may also point to the initial node. If necessary, the initial node forwards the message to the destination node for storage. To protect against communication failure between the initial node and the destination node, the message is queued on the initial node. If the queue becomes full, future messages are rejected. Rejected messages (when the queue is full) are not accepted from the sender and remain on the sender&#39;s mail store. 
     Assuming n nodes numbered from 0 to n−1, a simple hash function H( ) can be to use a numeric unique ID of the user (e.g., Mobile Subscriber Identification Number (MSISDN)) modulo n: H(user)=userID % n. 
     When the number of nodes changes by the addition or removal of nodes, the destination hash function may no longer work correctly since (a) it will not be able to store messages to an added node, and (b) it will not be able to retrieve messages from a removed node. To address this problem, multiple hash functions may be used. In such an approach, for example, a hash function is considered active if it corresponds to a network topology no older than the message validity period (e.g., 30 days). All active hash functions are maintained. Active hash functions eventually become older than the message validity period and are then deleted. A node may be permanently removed from the cluster only after no active hash functions have the node as a possible value. 
     When a message retrieval request is received by an arbitrary “initial node”, all active hash functions are used to compute the set of destination nodes. Each destination node is queried, and the initial node concatenates the results received. For storing messages, the hash function corresponding to the current network topology is used. 
     For the purpose of potentially reducing overall messaging traffic between nodes, active hash functions can be expired prior to the message validity period at the expense of some additional bookkeeping overhead. If a node can guarantee that no messages contained in its local storage map to one or more of the active hash functions, the node can mark those hash functions as “locally expired.” A node can “lazily” report the status of locally expired hash functions back to a calling node. In other words, the node can limit reporting to instances in which other information is also being sent. A calling node can then avoid subsequent (and unnecessary) messaging traffic by simply ignoring requests that compute to a node in the “locally expired” list of an active hash function. 
     Although not required by the illustrated method, hash functions can be ordered and accessed in terms of a unique, increasing time stamp that is kept consistent across the entire set of nodes. By choosing this approach, a hash function can be scheduled and delivered to all nodes in the cluster prior to its activation. The access order also follows the natural ordering of messages in a mailbox. The time stamp therefore enables easy indexing and maintenance of the hash functions. 
     Although the hash functions described up to this point have produced a single result (i.e., destination node address), other types of hash functions may be used. For example, each hash function can alternatively produce multiple destination node addresses. In such a case, safety can be enhanced through redundant data storage. The same message could therefore be stored in multiple nodes. If one of the nodes has a failure, the message can still be retrieved from the other nodes. 
     Generating multiple destination node addresses also has advantages with regard to message retrieval. For example, retrieval can be implemented by generating a plurality of queries in parallel, where with the message is taken from the first node to respond. Parallel retrieval can also be used to retrieve parts of a message from multiple destination nodes. For example, if a message is stored redundantly on three nodes, one third of the message can be retrieved from each node. 
     Alternatively, retrieval can be implemented sequentially over a plurality of nodes in a predefined order. For example, a node may be designated as a backup or secondary node that is queried only if the primary node is unavailable. 
     In addition to above-described storage and retrieval operations, moving subscriber mailboxes can be done in a unique way that uses multiple addressing functions. Moving mailboxes is often done for load-balancing reasons as the system expands and shrinks. The conventional approach to moving mailboxes has been to copy the mailboxes to their new location and to delete the old mailboxes. A shortcoming with this approach is that the mailboxes typically cannot be accessed until the entire move is complete. Since the volume of data being moved is often quite large, the system can have an extended amount of time in which mailboxes cannot be accessed. One approach to dealing with this concern is to redirect new messages associated with each mailbox to a new node and leave old messages associated with the mailboxes at the old node. The approach involves creating new mailboxes on the new node, or, if desired, creating the new mailboxes individually and on demand when new mail arrives for storage. The addressing functions are then updated so that each subscriber identifier calculates to the new node. The old set of addressing functions, however, is maintained. 
     At the time of retrieval, both sets of addressing functions can be used. Such an approach enables messages to be retrieved from both the old node and the new node. Furthermore, the expiration policy of the addressing functions can be linked to the expiration policy of old messages. For example, if messages expire thirty days after they are stored, then the old addressing function set should be maintained for thirty days as well. Similarly, the deletion policy of old mailboxes can be linked to the expiration policy of old messages. An individual mailbox that contains old messages can be deleted at the end of the thirty day period or sooner if all of the messages in the mailbox have been retrieved and deleted. 
     To reduce the amount of time for which the old addressing function set is maintained, the old messages can be moved to the new node in reduced segments such as individual mailboxes or smaller sets of mailboxes. Once all of the messages have been moved for the old addressing function set, the old addressing function set can be removed. 
     It should be noted, however, that if the chosen addressing function matches well with actual messaging traffic, the addition and the deletion of new mailbox nodes automatically re-balances the load across the entire set of new nodes. Explicit mailbox transfer may therefore only be needed in emergency situations if new nodes are added preemptively in response to increased traffic. 
     In one embodiment, a mobile messaging gateway (MMG) uses a temporary message store (TMS) system to store received short messages and mail digests. A handset retrieves a short message or a digest directly from the MMG with no need to access the message storage system (MSS) or SMS Center (SMSC). This method reduces the load on the MSS and SMSC systems. Other benefits include a simplification of the handset design and potentially faster message retrievals. Handsets can be simpler because they only need to use a single protocol to a single MMG to retrieve different types of messages (SMS, Internet email, picture mail, etc.). 
     A short message can be defined as a message whose body size is under a system-specified threshold (e.g., 300 bytes). A mail digest can be defined as a listing summary of an individual user&#39;s long mail messages. Optionally, the mail digest may also include short messages in the listing summary. A long mail summary is used to build the digest and can contain the From address, Subject, the first part of the text message and a reference to the location of a long mail message stored at a particular MSS node. 
     The following discussion, directed to one approach to an MMG TMS system, is merely exemplary in nature and is in no way intended to limit the embodiments of the invention or their application or uses. 
     Guidelines 
       FIGS. 3 and 4  demonstrate one example of a network topology that can be used to implement the techniques described herein. For example, the method  60  ( FIG. 1 ) of storing information can be implemented in the message arrival diagram of  FIG. 3  and the method  10  ( FIG. 2 ) of retrieving messages can be implemented in the message arrival diagram of  FIG. 4 . 
     The illustrated network  21  has a plurality of MMG nodes  24 ,  26 , and  48 , an MSS node  28  and an SMSC  30 . The MMG i  node  24  has a TMS  32 , where the MMG j  node  26  has a TMS  34 . The MMG, node  48  has a TMS  56 . Messages are maintained in a given TMS for a predetermined period such as 14 days from receipt. At any time, each user may be limited to a maximum number of messages such as 500 messages. Messages can be recovered from the TMS in the event of an MMG node failure. With regard to scalability, MMG nodes that are added to the network scale linearly with the number of users serviced. Furthermore, with regard to load-balancing/fail-over, the removal or addition of an MMG node can be done without interrupting service. Linear scaling is possible because only very limited state information is used internally by each MMG node. For the same reason, load balancing the requests among the multiple MMG nodes is possible. Each MMG node can send a MWI (message-waiting indicator) message to the user&#39;s handset to indicate a new message in compliance with the Third Generation Point-to-Point (3GPP) standard, 3GPP-TS100.901. From the user&#39;s point of view, there is no difference between using one MMG node or another MMG node. 
     Assumptions 
     The average size of a short message in one example is 300 bytes of body and 500 bytes of header, so that the total is 800 bytes. The size of a message digest is 300 bytes of body and 500 bytes of header, so that the total is 800 bytes. Thus, short messages and message digests are assumed to have the same size. 
     With specific reference to  FIG. 3 , it can be seen that at arrow  22  a mail or SMS message arrives at an arbitrary MMG node, the MMG i  node  24 . Typically, the MMG nodes in the network are hosted by the cellular carrier/service provider. The MMG nodes also are behind a load-balancer switch, and any MMG node may receive a message. In the case of a simple mail transfer protocol (SMTP) message, the MMG i  node  24  determines a subscriber identifier for the message and whether the message can be converted into a short mail. The MMG i  node  24  uses the subscriber identifier and the hash function corresponding to the current topology to determine the actual destination MMG node (e.g., MMG j  node  26 ). Temporary messages might be one of the following types: converted short mails, SMS messages, or long mail summaries. A long mail summary is used to build the digest and contains the From address, Subject, the first part of the text message, and a reference to the location of a long mail message stored at a particular MSS node. In the case of a long mail, the MMG i  node  24  sends the long mail at arrow  36  to the MSS node  28  for storage to persistent memory or disk  38  and constructs the digest. At arrow  40  the MMG i  node  24  forwards the temporary message to the destination MMG node, MMG j  node  26 . It is possible that the MMG i  node  24  is the destination for the temporary message, and therefore no forwarding is necessary. The MMG j    26  stores the temporary message on TMS  34 . Prior to storing the temporary message, a check is done to determine whether the user has exceeded his or her maximum message limit. If the maximum message limit is exceeded, the new message is discarded and a deletion request is sent to the MSS node  28 . At arrow  42  the MMG j  node  26  sends a MWI message to a device associated with the subscriber identifier such as wireless handset  44  via an SMSC  30 . The MWI message does not need to contain an IP address, URI, or any other type of data. 
     Turning now to  FIG. 4 , it can be seen that at arrow  46  the handset  44  sends a message retrieval request to an arbitrary MMG, node  48 . As already noted, the MMG nodes are behind a load-balancer switch. Accordingly, any MMG node may receive a message retrieval request. The MMG, node  48  uses the destination hash functions to determine the destination MMG nodes as discussed above. At arrow  50  MMG i  node  48  retrieves the message from destination nodes such as MMG j  node  26 . It is possible that the MMG, node  48  is the destination for the message, and therefore no forwarding is necessary. At arrow  52  the MMG, node  48  builds and sends to the handset  44  a short message or a formatted mail digest based on the user&#39;s temporary messages. After the handset retrieval transaction has been confirmed to be complete, the MMG, node  43  sends a “message-sent” confirmation message to the MMG j  node at arrow  54 . Upon receiving confirmation, the MMG j  node  26  deletes the message. It should be noted that confirmation and deletion as described may not be necessary if retrieved messages are retained until they expire. There is also a possibility that short messages will involve both a retrieval of the message digest and the message body. 
       FIG. 5  shows a scenario in which a long mail message is retrieved from the MSS node  28 . It should be noted that the network can also include other MSS nodes. At arrow  68  the handset  44  sends a message retrieval request to an arbitrary MMG m  node  70 . As already discussed, any MMG node may receive a message retrieval request. The MMG m  node  70  uses the destination hash functions to determine the destination MMG nodes. At arrow  78  MMG m  node retrieves the message from destination nodes such as MMG j  node  26 . The retrieved message includes a reference to the location of the long mail messages stored at the MSS node  28 . At arrow  72  MMG m  node  70  retrieves one or more long mail messages from the MSS node  28 . At arrow  74  the MMG m  node  70  builds and sends the long mail messages to the handset  44 . After the handset retrieval transaction has been confirmed to be complete, the MMG m  node  70  sends a “message-sent” confirmation message to the MMG j  node  26  at arrow  80  and sends a “mail-delete” request message to the MSS node  28  at arrow  76 . On receiving confirmation, the MMG j  node  26  and the MSS node  28  delete the retrieved messages. Confirmation and deletion may not be necessary if retrieved messages are retained until they expire. 
     Destination Hash Function 
     Messages are approximately evenly distributed among nodes and the hash function minimizes the need to redistribute messages if a node is added or removed. The hash function is also computable by each MMG node independently and is quick to compute. As already discussed, assuming n MMG nodes numbered from 0 to n−1, a simple hash function H( ) is to use the user&#39;s MSISDN modulo the number of MMG nodes: H(MSISDN)=MSISDN % n. Such hash functions are well known to persons skilled in the art, and the present invention is not intended to be limited to any particular hash function. 
     The MMG node for a particular user then has the index H(MSISDN). The MMG nodes can be identified by IP address using a configuration table that maps an index [0, n−1] to the node IP address. Each geographical region can have an independent set of MMG nodes that are selected among by the hash function. 
     When the number of MMG nodes changes by adding or removing MMG nodes, the hash function may no longer work correctly since it will not be able to store messages to an added node, or it will not be able to retrieve messages from a removed node. To address this problem, a plurality of hash functions is used, where each hash function corresponds to a topology of the network at a particular moment in time. A hash function is considered active if it corresponds to an MMG node network topology no older than the message validity period (such as 30 days). All active hash functions are maintained. Active hash functions eventually become older than the message validity period and are then deleted. For storing messages, the hash function corresponding to the current network topology is used. For retrieving messages, however, all active hash functions are used to compute the destination MMG node, and all destination nodes are queried to retrieve the user&#39;s temporary messages. The above approach and method of calculating destination MMG nodes can also be applied, if necessary, to calculating destination MSS nodes for long message storage and retrieval. 
     Message Store, Caching and Maintenance Thread 
     The indexing and storage system of the MMG message queue is used for the TMS. The number of messages per user is maintained to check the maximum per-user message limit. An indexing method is used to access all the user&#39;s temporary messages on a node. Each MMG node maintains a cache of the temporary messages that are stored locally. Shared memory protected by a mutual-exclusion lock prevents conflicts by the multiple MMG processes running simultaneously on the same node. Each MMG process runs a maintenance thread that executes periodic tasks such as deleting expired temporary messages and removing obsolete hash functions. 
     Communication Failure 
     To protect against temporary communication failure between MMG nodes, the source MMG queues the messages. When the queue is full, new temporary messages are rejected. When a new MMG node is added to the set of MMG nodes, a new destination hash function is created and added to the active list for all MMG nodes. 
     Those skilled in the art can appreciate from the foregoing description that the broad techniques of the embodiments of the present invention can be implemented in a variety of forms. Therefore, while the embodiments of this invention have been described in connection with particular examples thereof, the true scope of the embodiments of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification, and following claims.