Abstract:
A server system includes a processor and a data structure having an entry for a resource, the entry including a first sequence number. The server has communication procedures for receiving a request from a client to access the resource, where the request includes a second sequence number obtained from a service, and a resource request handling program. Upon receiving the request, the resource request handling program determines whether the server has any record of having previously received a request to access the resource. If not, the server returns a provisional rejection to the client, requiring the client to verify that it holds a lock on the specified resource. A provisional bit in the entry is initially set to indicate that the resource has not been accessed since the system was last initialized. The provisional bit is reset when a request to access the resource is granted.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 10/957,550, filed Sep. 30, 2004, which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates generally to accessing data across a computer network. More specifically, the invention relates to a system and method for enforcing resource locking across a computer network. 
         [0003]    In general, distributed networks contain at least one server and multiple remote clients that access one or more resources (e.g., data files) on the server. Locking facilities are typically provided in such distributed networks to control the use of the resources by the multiple clients. By acquiring a lock on a record, or a file, or other resource (typically located on a server), a client indicates its intention to make use of the record, file or other resource. In practice there are various kinds of locks, including locks for enforcing exclusive access, locks for enforcing shared access, locks on portions of a data file, and locks on an entire data file. 
         [0004]    Typically, such distributed networks contain a global locking service for distributing locks to each of the multiple clients requiring access to the resources. For example, each time a client requires accessing a resource on the server, the client requests and obtains a lock from the global locking service. The lock is used to protect access to the resource on the server, i.e., only those clients that hold a valid lock are allowed to access to the resource. The client then transmits the lock to the server together with a request to access the resource, and the server determines if it should grant access to the resource. Typically, the server queries the global locking service to determine whether the lock is valid. If the lock is valid the client is allowed access to the resource. If the lock is not valid the request to access the resource is rejected. 
         [0005]    The above-described method requires constant communication between the server and the global locking service. However, this constant communication is inefficient, as it consumes bandwidth and places an unnecessary high load on the global locking server. In addition, extra programming care is required to allow the server to continue responding to client requests even when the global locking service becomes unavailable. Examples of requests the server may respond to are requests for resources not protected by locks, and requests for resources that are protected by locks managed by a different global locking service (i.e., there may be multiple, independent global locking services, and different resources may be protected by locks managed by different locking services). 
         [0006]    Another existing system for controlling locks requires synchronization between the clocks on the clients, server, and global locking service. If the clocks are synchronized, then leases that expire after a set time can be used by the system. For example, when the client obtains a lock, it also receives a lease. The lease includes an expiration time, which declares that the client holds the lock until at least the expiration time. The client transmits the lease to the server along with its request to access the file. The server checks its clock, and accepts the request to access the file if the lease has not yet expired. However, this method either requires 1) communication between the server and the global locking service to synchronize times, or 2) communication between the sever and a remote clock, and communication between the global locking service and the remote clock. Again such constant communication is inefficient. Moreover, this method is also subject to clock skew caused by propagation delays or the like. 
         [0007]    To reduce communication between the server and the lock service, some systems store state information about lock distribution and validation. The server consults this state rather than contacting the locking service. To avoid requiring the server to contact the locking service upon restart, this state must be made persistent. However, maintaining a persistent state requires non-volatile memory, and also requires procedures for re-establishing the state of the system when recovering from a crash or power failure or the like, which adds expense to the system. 
         [0008]    In light of the above, it would be highly desirable to provide a system and method for enforcing a locking regime at a server without requiring a local persistent state and without requiring direct constant communication between the server and a global locking service. 
       SUMMARY OF DESCRIBED EMBODIMENTS 
       [0009]    A server system includes a processor and a data structure having an entry for a particular resource, the entry including a first sequence number. The server has communication procedures for receiving a request from a client to access the particular resource, where the request includes a second sequence number obtained from a service. Upon receiving the request, a resource request handling program determines whether the server has any record of having previously received a request to access the particular resource. If not, the server returns a provisional rejection to the client, requiring the client to verify that it holds a lock on the specified resource. 
         [0010]    In some embodiments, a provisional bit in the entry is initially set to indicate that the particular resource has not been accessed since the system was last initialized. The provisional bit is reset when a request to access the particular resource is granted. When a request to access the particular resource contains a sequence number that is not equal to the sequence number in the entry, the state of the provisional bit determines how the request is processed. In some embodiments, if the provisional bit is set, the server responds with a provisional rejection of the request. If the provisional bit is not set, and the sequence number in the request is higher than or equal to the sequence number in the entry, the request is granted and the sequence number in the entry is set equal to the sequence number in the request. Further, if the sequence number in the request is lower than the sequence number in the entry, the request is rejected. 
         [0011]    In another embodiment of the invention, a server system includes a processor and a data structure having an entry for a particular message thread, the entry including a first sequence number. The server has communication procedures for receiving a message from a client, where the message includes a second sequence number obtained from a service. The message also is identified as belonging to the particular message thread. Upon receiving the message, a message handling program determines whether the server has any record of having previously received a message with respect to the particular message thread. If not, the server returns a provisional rejection to the message, requiring the client to verify the second sequence number and to resubmit the message. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0012]      FIG. 1  is a block diagram of an exemplary locking system; 
           [0013]      FIG. 2A  is a block diagram of the server shown in  FIG. 1 ; 
           [0014]      FIG. 2B  is a block diagram of the resource table shown in  FIG. 2A ; 
           [0015]      FIG. 2C  is a block diagram of the client shown in  FIG. 1 ; and 
           [0016]      FIGS. 3A-3B  are flow charts of a method for enforcing access to resources on a server. 
       
    
    
       [0017]    Like reference numerals refer to corresponding parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0018]    In the embodiments described below, the clients of the system are either assumed to not be malicious, or the clients of the system are assumed to have been authenticated in a manner outside the scope of the lock management system and method described here. From another viewpoint, any client request (to access a resource) that reaches the point of an accept/reject decision by the appropriate server is assumed to come from a client that is in full compliance with the resource management rules of the system. In addition, the communications mechanisms used between the components of the system are assumed to recognize and discard duplicate messages that may be introduced by the network hardware or software. Thus, if a message is received that is substantially similar to one received previously, it is because the sender has sent a substantially similar message according to the methods described below. Building on these assumptions, the embodiments described below avoid the aforementioned limitations of prior art systems. 
         [0019]      FIG. 1  is a block diagram of a locking system  100 . The locking system  100  enforces limits on access to each protected resource in the system. For example, write access to a resource may be granted to only one client at a time to guarantee consistency. The locking system  100  includes at least one locking service server  102 , at least one client  104  and at least one server  106 , all coupled to one another via one or more communication networks  108 . The locking service server  102  may be a standard locking service server as is well understood in the art, and as further explained below with reference to  FIGS. 3A and 3B . The locking service server  102  supplies locks to the client  104  when the client  104  requests a lock to access a resource  110  on the server  106 . Of course, if a resource is already locked, the lock request may be denied by the locking service server  102 . Each lock enforces limits on access to the resource on the server. Such locks may include: advisory locks, where the client acquires the lock before accessing the corresponding resource; mandatory locks, where attempting unauthorized access to a locked resource will force an exception in the entity attempting to make the access; a semaphore lock, where no distinction is made between shared (read only) or exclusive (read and write) modes; shared locks, where several clients can acquire a shared lock for read-only access to the resource; exclusive locks; and the like. 
         [0020]    The network  108  comprises a series of points or nodes interconnected by communication paths. The network  108  may interconnect with other networks, may contain subnetworks, and may be characterized in terms of its spatial distance as either a local area network (LAN), a metropolitan area network (MAN), a wide area network (WAN), or a global network, such as the Internet. The network may further be characterized by the type of data transmission technology used, such as, without limitation, a TCP/IP (Transmission Control Protocol/Internet Protocol) network, an SNA (Systems Network Architecture) network, or an ATM (Asynchronous Transfer Mode) network. The client  104  and servers  102  and  106  connect to the network via communication links, such as coaxial cable, copper wire, optical fiber, radio waves, microwave, satellite links, or other wired or wireless links. 
         [0021]    The servers  102  and  106  and client  104  may be any suitable computing devices that are capable of connecting to the network  118 , such as personal computers, rack mounted computers running server software, laptop computers, or the like. The server  106  contains at least one resource  110 , a sequence number  112 , and a provisional bit  114 , as explained in further detail below in relation to  FIGS. 2A ,  2 B,  3 A, and  3 B. 
         [0022]      FIG. 2A  is a more detailed block diagram of the server  106  shown in  FIG. 1 . The server  106  include at least one data processor or central processing unit (CPU)  200 ; communication circuitry  202  for communicating with the network  108 , locking service server  102 , and client  104  (all of  FIG. 1 ); a memory  204 ; and at least one bus  224  that interconnects the aforementioned components. The server  106  may also optionally include one or more user interface devices, such as a monitor  206  and a keyboard and mouse  208 . The communications circuitry may include one or more Network Interface Cards (NICs) for communicating with the network  118  using one or more communication protocols, such TCP/IP, ATM, Ethernet, or the like. 
         [0023]    The memory  204  may include volatile (non-persistent) memory, such as high-speed random access memory, and may also include non-volatile memory, such as one or more magnetic disk storage devices. The memory  204  stores an operating system  212 , such as LINUX, UNIX or WINDOWS that includes procedures for the direct control and management of hardware and basic system operations, as well as running application software. The memory  204  may also store communication procedures  214  used for communicating with the network  108 , locking service server  102 , and client  104  (all of  FIG. 1 ). 
         [0024]    The memory  204  also may include: an initialization module  216 , resource(s)  110 , a resource table  218 , a resource request handling program  220 , and other application program(s)  222 . The initialization module  216  initializes the server  106 , i.e., sets-up the server when the server is initially switched-on or when the server is power-cycled, such as after a system crash. The resource(s)  110  may be any resources, such as data files or the like. The resource table  218  contains lock information for a list of resource entries, as described below in relation to  FIG. 2B . The resource request handling program  220  handles all requests for resources  110 , as described below with reference to  FIGS. 3A and 3B . 
         [0025]      FIG. 2B  is a block diagram of an exemplary resource table  218 . In some embodiments, the resource table  218  is stored in volatile (non-persistent) memory  226  that forms part of the memory  204  ( FIG. 2A ). For this reason, the resource table  218  is assembled on-the-fly as requests for resources are received by the server  106  ( FIG. 1 ). In other words, when the server  106  ( FIG. 1 ) is shutdown, the resource table  218  ( FIG. 2B ) is lost, and will be reassembled as requests for resources are received by the server, as explained below with reference to  FIGS. 3A and 3B . 
         [0026]    The resource table  218  contains a resource entry  226  for each resource  110  or for a subset of the resources  110  ( FIG. 2A ). A default entry  228  as used as a template for a new resource entry that is assembled the first time that a request is received for a particular resource, as explained below with reference to  FIGS. 3A and 3B . In some embodiments, the default entry is part of the resource request handling program  220  ( FIG. 2A ) and is not stored in the resource table  218 . Each resource entry  226  includes a resource identifier (ID)  230  that identifies a particular resource  218  ( FIG. 2A ). For example, the resource ID  230  may be the address of the resource  218  ( FIG. 2A ) or a range of addresses associated with the resource. Each resource entry  226  also includes the latest received sequence number  232  (Sequence No. 1) for that resource. Finally, each resource entry  226  also includes a provisional bit  234  that can be set to true (1) or false (0). Use of the resource ID  230 , sequence number  232  and the provisional bit  234  are described in detail below in relation to  FIGS. 3A and 3B . 
         [0027]    In some embodiments, the server may at any time discard entries from the resource table  218 . It may do so to reclaim memory, for example. 
         [0028]      FIG. 2C  is a more detailed block diagram of the client  104  shown in  FIG. 1 . In some embodiments, the client  104  includes at least one data processor or central processing unit (CPU)  240 ; communication circuitry  242  for communicating with the network  118 , locking service server  102 , and server  106  (all of  FIG. 1 ); a memory  246 ; and at least one bus  252  that interconnects the aforementioned components. The client  104  may also optionally include one or more user interface devices, such as a monitor  251  and a keyboard and mouse  250  also coupled to the bus  252 . In some embodiments, the communications circuitry includes one or more Network Interface Cards (NICs) for communicating with the network  108  ( FIG. 1 ) using one or more communication protocols, such TCP/IP, ATM, Ethernet, or the like. 
         [0029]    The memory  246  may include volatile (non-persistent) memory, such as high-speed random access memory, and may include non-volatile memory, such as one or more magnetic disk storage devices. The memory  246  stores an operating system  254 , such as LINUX, UNIX or WINDOWS that includes procedures for the direct control and management of hardware and basic system operations, as well as running application software. The memory  246  may also store communication procedures  256  used for communicating with the network  118 , locking service server  102 , and server  106  (all of  FIG. 1 ). 
         [0030]    The memory  246  also may include resource request/lock procedures  258 , resource handling procedures  260 , and other application program(s)  264 . The resource request/lock procedures  258  request locks from the locking service server  102  ( FIG. 1 ), obtain a lock from the locking service server, and request access to resource(s)  110  ( FIG. 2B ) from the server  106  ( FIG. 1 ). The resource handling procedures  260  access and use the resource(s)  110  ( FIG. 2A ). 
         [0031]      FIGS. 3A-3B  are flow charts of a method for enforcing access to resources on the server  106  ( FIG. 1 ). Whenever the server  106  ( FIG. 1 ) is initialized, i.e., switched-on or booted, the initialization module  216  ( FIG. 2A ) initializes the resource table  218  ( FIG. 2A ), at  314 . In some embodiments, initialization of the resource table is necessary as the resource table is stored in volatile (non-persistent) memory  226  ( FIG. 2B ), and, therefore, cannot keep any persistent state information. Also in some embodiments, initialization of the resource table is necessary as the server cannot communicate with the locking server to discover the current sequence number for a particular resource. 
         [0032]    In some embodiments, initialization of the resource table comprises creating a single default entry  228  ( FIG. 2B ). Alternatively, an entry may be generated for each resource at initialization of the server, where each entry contains the same information as the default entry, described below. 
         [0033]    The default entry includes a first sequence number  232  (sometimes herein called “Seq. No. 1”) ( FIG. 2B ) that is set to a value lower than any possibly valid received sequence number. For example, the first sequence number  232  (Seq. No. 1) in the default entry is set to zero (0). The provisional bit in the default entry is initialized to true (1). 
         [0034]    Whenever the client  104  ( FIG. 1 ) wants to access a resource R of the resource(s)  110  ( FIG. 1  and  FIG. 2A ), the client requests a lock for the resource R from the locking service server  102  ( FIG. 1 ) at  302 . The locking service server receives the request for the lock, at  304 , grants a lock on resource R by assigning the next available sequence number (sometimes herein called “Seq. No. 2”) for that resource R to the client making the request, at  306 . A sequence number is a monotonically increasing logical number or timestamp. For example, the next available sequence number, otherwise referred to as the second sequence number (Seq. No. 2), may be 0004 where the previous sequence number was 0003. The lock and second sequence number (Seq. No. 2) is then transmitted toward the client at  308 . 
         [0035]    It is noted here that  FIG. 3A  does not explicitly address the situation in which a client&#39;s lock request is denied by the locking service server. When this happens, the client may retry the request until the lock is granted at  306 , or it may perhaps abandon the effort. The present discussion only concerns what happens when a client request for a lock is granted. 
         [0036]    The client receives the lock and second sequence number, at  310 , and requests access to the resource R from the server  106  ( FIG. 1 ), at  312 . The request for access includes the lock, which itself includes the second sequence number (Seq. No. 2). Alternatively, the request for access only includes the second sequence number (Seq. No. 2). In some embodiments, the request for access includes authentication information, such that the server can authenticate that the client is authorized to access the server. In such embodiments, the server authenticates the client or the request at  316  prior to further processing of the request. However, in some embodiments, it is assumed that the client  104  ( FIG. 1 ) is not malicious, and, therefore, no authentication is required. 
         [0037]    The server  106  ( FIG. 1 ) receives the request for access to the resource R at  316 . The resource request handling program  220  ( FIG. 2A ) on the server then determines whether there is an entry  226  ( FIG. 2B ) for the resource R, for which access is being requested, at  318 . If there is no entry for resource R ( 318 —No), i.e., this request is the first request for resource R since the server was initialized, then the resource request handling program creates a new entry for the resource in the resource table at  336  ( FIG. 3B ). The default entry  228  ( FIG. 2B ) may be used as the template for the new entry for the resource R. As a result, the new entry for resource R includes a provisional bit set to true (1). Optionally, the first sequence number (Seq. No. 1) of the new entry may be set to a low value, such as zero (0), which is lower than any possibly valid received sequence number, and a provisional bit set to true (1). 
         [0038]    The resource request handling program  220  ( FIG. 2A ) then sets the first sequence number (Seq. No. 1) of the entry for resource R to the second sequence number (Seq. No. 2) (i.e., the sequence number in the client request) at  338 . In the event that the second sequence number is an invalid sequence number, the first sequence number is set to the default value. Alternately, the first sequence number is left unchanged because it was previously initialized to the default value. Assuming the second sequence number was valid, the new entry for resource R, which is stored in the resource table, stores the second sequence number received from the client instead of the default first sequence number. After creating and setting the values in the new entry for resource R, the resource request handling program rejects the request to access the resource R, at  340 . In some embodiments, the resource request handling program rejects the request to access the resource R with a “verify-and-retry” error. This manner of rejection instructs the client to retry accessing the resource. 
         [0039]    The rejection is received by the client  104  ( FIG. 1 ), at  342 . The resource request procedures  258  ( FIG. 2C ) on the client  104  ( FIG. 1 ) then check whether the lock is valid and/or held, at  344 . This may involve communicating with the locking service server  102  ( FIG. 1 ), which checks if the lock is valid and/or held, at  346 . Alternatively, the resource request procedures may check locally whether the lock is valid and/or held, at  344 . 
         [0040]    If the lock is not held ( 348 —No), then the process repeats itself, by the client requesting a lock from the locking service server, at  302  ( FIG. 3A ). However, if the lock is held ( 348 —Yes), then the client again requests access to the Resource R at  312  ( FIG. 3A ). The request is then received by the server, at  316 , as described above. The resource request handling program  220  ( FIG. 2A ) on the server then determines whether there is an entry  226  ( FIG. 2B ) for the resource R, for which access is being requested, at  318 . This time, as a new entry for the resource was previously created (as described above), there is now an entry for resource R ( 318 —Yes). The resource request handling program then determines whether the second sequence number (Seq. No. 2) is smaller than the first sequence number (Seq. No. 1), i.e., whether Seq. No. 2&lt;Seq. No. 1, at  320 . If the second sequence number (Seq. No. 2) is smaller than the first sequence number (Seq. No. 1) ( 320 —Yes), then the server rejects the request to access the resource R, at  322 . The client receives the rejection and requests a new lock, at  302 . In other words, where the sequence number accompanying the request to access the resource is lower than the current sequence number associated with a particular resource R, then the request to access the resource is rejected, as a request having a higher sequence number was previously received for the particular resource R. 
         [0041]    If the second sequence number (Seq. No. 2) is not smaller than the first sequence number (Seq. No. 1) ( 320 —No), then the request handling program  220  ( FIG. 2A ) on the server determines whether the second sequence number (Seq. No. 2) is equal to the first sequence number (Seq. No. 1), i.e., whether Seq. No. 2=Seq. No. 1, at step  324 . If the second sequence number (Seq. No. 2) is equal to the first sequence number (Seq. No. 1) ( 324 —Yes), then the client is allowed to access the resource R, at  326  and  328 . The provisional bit in the entry for the resource R is set to zero (0, False) at step  326 . In other words, where the client has requested access to a resource R for the first time since the server was initialized, and the server has instructed the client to check whether the lock is valid and/or held, the client is given access to the resource on a subsequent request for access, if the lock is valid and/or held. 
         [0042]    If the second sequence number (Seq. No. 2) is not equal to the first sequence number (Seq. No. 1) ( 324 —No), then the request handling program  220  ( FIG. 2A ) on the server determines whether the provisional bit is set to false or zero (0), at  330 . In other words, if the second sequence number (Seq. No. 2) is larger the first sequence number (Seq. No. 1), then the request handling program determines whether the provisional bit is set to false or zero (0). The provisional bit is only set to false or zero (0) when the resource R has been accessed for the first time after server initialization at step  326 . 
         [0043]    If the provisional bit is not false or zero ( 330 —No), i.e., the resource has not yet been accessed for the first time since server initialization, then the first sequence number is set to the second sequence number, at step  338  ( FIG. 3B ), and the request is rejected with a “verify-and-retry” error at step  340 , as described above. If, however, the provisional bit is false or zero ( 330 —Yes), i.e., the resource has been accessed for the first time since server initialization, then the client is allowed to access the resource R, at  332  and  334 . The first sequence number (Seq. No. 1) is set to the second sequence number (Seq. No. 2), at  332 , and stored in the entry  226  ( FIG. 2B ) for that resource R. 
         [0044]    Accordingly, the above described embodiment provides a system and method for enforcing a locking regime at a server without requiring a local persistent state or direct constant communication between the server and a global locking service. 
         [0045]    It should be noted that the server  106  may at any time discard entries from the resource table  218 , for instance, to reclaim memory. A set of oldest, or least recently used entries may be discarded. Discarding entries from the resource table  218  causes no adverse consequences other than requiring the generation of new entries when locks on the associated resources are requested. 
         [0046]    The system and method described above may be used more generally for enforcing a global ordering of messages. In such embodiments, the locking service server  102  becomes a distributor of global ordering sequence numbers or timestamps, and resource access requests become messages. Messages from clients include sequence numbers obtained from a service. Messages from clients are accepted and rejected by the server in accordance with the sequence numbers of the messages, using the system and method as described above. Multiple message sequences are supported by assigning each distinct message sequence a respective distinct message thread identifier, which replaces the resource identifier in the above described embodiments. 
         [0047]    The foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. Furthermore, the order of steps in the method are not necessarily intended to occur in the sequence laid out. It is intended that the scope of the invention be defined by the following claims and their equivalents.