Abstract:
Database updates to primary and secondary database structures are performed by writing information from a client to both primary and secondary database services and building the database structure at the primary database service while delaying application of data written to the secondary database service until notification at the secondary database service from the primary database service. The primary database service provides notification upon completing the database structure so that the secondary database service can proceed with building the database structure from the information provided by the client or with information provided by the primary database service.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to the field of database storage of information, and more particularly to an opportunistic database duplex operations. 
     2. Description of the Related Art 
     Redundancy for storage of information in a network cluster database is provided by duplexing information of a client for storage at a primary and a secondary database service. The pureScale database available from IBM has an application managed duplexing that writes a pureScale database structure from a client to two different database service provides. The client database application uses Remote Direct Memory Access (RDMA) to send and receive application data directly through a network interface with direct memory accesses that avoid delays introduced by operating system manipulation of data. One RDMA model issues a send/receive by a client to a server and another RDMA model uses server polling with a server responding to a request from a client pulling data from the client. For example, with one type of pureScale database, an appliance of a service provider node responds to a client request to modify the database structure by reading information to be written from the client node with RDMA and then transferring the information to the database service structure. The server and client interface may use standard socket protocols when transaction times are not critical. 
     Two conventional schemes exist for duplexing information in a database: “parallel write” duplexing and “store and forward” duplexing. Parallel write duplexing issues two write requests, one to each of two database services, so that the writes occur in parallel. Store and forward writes complete the write to the primary service and then use the primary service to write the secondary service. An advantage of parallel write duplexing is that a write in the parallel model finishes at about the same time for both database services that are receiving the information from the client. An advantage of store and forward writes is that duplexing does not depend upon all clients being able to reach the secondary service at all times. A difficulty with parallel write duplexing is that meta data between the two services must be synchronized if any issues arise during the processing of the information on either service, such as a failure to receive the data at a service because of a network failure. If the secondary service gets partitioned and not reachable by a single client, duplexing is lost and the secondary database structure must be re-built from the primary database structure. The status of a write to a database service and the duplexed state depend on both write streams. If one write fails and one succeeds, duplexing is broken and data redundancy fails until both service database structures are built. To rebuild the failed structure, a determination has to be made of whether the primary structure should remain as the primary to rebuild the secondary structure or the secondary structure should promote to be the primary structure to re-build the intended primary structure. A difficulty with store and forward duplexing is that write time doubles for a synchronous write, once to the primary service and once to the secondary service. 
     SUMMARY OF THE INVENTION 
     Therefore, a system and method is provided which reduces the time needed to complete duplexing of information between database services without full accessibility to secondary nodes through the duplexing operation. 
     In accordance with the present invention, a system and method are provided which substantially reduce the disadvantages and problems associated with previous methods and systems for duplexing information writes between database services. A client writes information to both a primary and secondary database service structure, however, the secondary database service structure delays action on the information copied from the client until a predetermined condition. The primary database service applies the information to implant the information in a database structure and notifies the secondary database service to complete the database structure at the secondary database service. Each database service might contain multiple structures where the primary server may be different for each structure. 
     More specifically, a cluster of computing nodes includes clients that generate and use information and a database that stores the information with plural database services. A client node has a processor and memory that executes a database agent for interacting with a primary and secondary database service through a network. The database agent issues a write of information to the primary and secondary database service to copy the information to the primary and secondary database services. The primary database service applies the information to build a database structure and then issues a write of the information from the primary database service to the secondary database service. The secondary database service delays action on information copied from the client until the write is received from the primary database service. In response to the write from the primary database service, the secondary database service determines if the information copied correctly from the client and, if so, applies the information to build the database structure. If the information did not copy correctly from the client, the secondary database service obtains a copy of the information from the primary database service to build the database structure. The primary and secondary database services determine their status with meta data of the information or with a transaction identifier included with the information. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention may be better understood, and its numerous objects, features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference number throughout the several figures designates a like or similar element. 
         FIG. 1  depicts a block diagram of computing nodes interfaced through a network to save information in a database having primary and secondary services; and 
         FIG. 2  depicts a flow diagram of a process for saving information of a client node to primary and secondary database services. 
     
    
    
     DETAILED DESCRIPTION 
     A system and method provides database services to clients using a parallel write model for more rapid copying of information from a client to primary and secondary database services, and a store and forward model to implant a database structure at the secondary database service using information copied from the client. A client agent application programming interface writes information to both the primary and secondary database services at the same time. The primary database service responds to the write by pulling the information from the client and then applying the information to build a database structure for implantation to the primary database. The secondary database service responds to the write by pulling the information from the client and storing the information without building the database structure. When the primary database service completes local processing of a write, it issues a write of the information to the secondary database service. If the secondary database service has already pulled the information from the client, then the secondary database service responds to the write from the primary database service by applying the information copied from the client to build the database structure. If the secondary database service does not have the information from the client, then it copies the information from the primary database service to build the database structure. This approach takes advantage of the speed provided by a parallel write algorithm when clients are connected to both services but maintains duplexing when the connection between some clients and the secondary database service are down. Since the secondary database service waits for a write from the primary database service, no advanced logic is needed to synchronize data or meta data should one of the services fail part of the operation because the write operation fails if the primary write fails. However, duplexing is not broken unless the communication connection between the primary and secondary database services is broken. 
     As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon. Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device. 
     A computer readable signal medium may include a propagated data signal with computer readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device. Program code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing. 
     Computer program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). 
     Aspects of the present invention are described below with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks. The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     Referring now to  FIG. 1 , a block diagram depicts computing nodes interfaced through a network to save information in a database having primary and secondary services. Each of plural client computing nodes  10  processes information with a processor  12  and memory  14 , such as by executing instructions of an application  16  that generates information stored in a storage device  18 . Client nodes  10  have a database agent  20  with an application programming interface (API)  22  that supports database storage of information through a network  24  at one or more of plural server nodes  26 . Database agent  20  issues “writes” of information by using RDMA to send and receive data with database services supported by server nodes  26 . A database service receives a “write” from a client node  10  to modify a database structure and, in response, the database service reads the information to be written from the client node  10  and transfers the information to the database service structure. 
     In order to maintain redundancy of information, database agent  20  simultaneously issues write information to both a primary database service  28  and a secondary database service  30 . Database tools  32  having instructions that run on a processor  12  and memory  14  of the server nodes  26  respond to write requests from database agents  20  of client nodes  10  to copy information from the client nodes. Database agent  20  of a client node  10  includes an indicator with information to be written so that database tool  32  can determine its status as a primary database service  28  or secondary database service  30 . For example, API  22  issues a primary write to the primary database service  28  and a secondary write to the secondary database service  30  so that each database service retrieves the information to be written from the client node  10 . In one embodiment, the writes include a transaction identifier generated by API  22  that the primary and secondary database services use to coordinate the writes. The transaction identifier is only valid for a single request between duplexed primary and secondary database services and aids in each database service assuming appropriate primary and secondary roles. The transaction identifier supports implantation of data into the primary or secondary database service with the same meta data or with a primary meta data included in information provided the primary database service  28  and secondary meta data included in information provided to secondary database service  30 . The transaction identifier or other indicators provide the secondary database service  30  with notification of its status as secondary service so that the secondary database service  30  can operate with the same meta data as the primary database service  28  or with different meta data. 
     Once a write from a client node  10  initiates a copy of information to both primary database service  28  and secondary database service  30 , primary database service  28  responds to the write by retrieving the information from client node  10  and applies the information to implant the information in database structure  34 . Secondary database service  30  also retrieves the information from client node  10 , however, secondary database service  30  saves the information without building a database structure to implant in database structure  34 . Once primary database service  28  completes local processing to update primary database structure  34 , database tool  32  of primary database service  28  issues a write to secondary database service  30 , similar to a write that would issue in the store and forward model of duplexing. Database tool  32  of secondary database service  30  responds to the write from primary database service  28  by first attempting to create the database structure from the information already copied from client node  10 . If the information copied from client node  10  is complete and successfully implants in data structure  34 , then secondary database service  30  does not have to retrieve information from primary database service  28  to complete the write. If implantation of information copied from client node  10  is unsuccessful, then secondary database service  30  retrieves the information from primary database service  28  to update database structure  34 . In one embodiment, database tool  32  of secondary database service  30  will delay retrieving information from primary database service  28  if additional time is needed to complete copying of information from client node  10 . If the write to primary database service  28  fails, then the implantation of the information at secondary database service  30  is not performed, thus eliminating the need for advanced logic that synchronizes information and meta data in the event of a failure at primary data service  28 . 
     Referring now to  FIG. 2 , a flow diagram depicts a process for saving information of a client node to primary and secondary database services. The process begins at step  36  with issuance of a write command from a client to a primary database service and a secondary database service. The primary database service receives the write command at step  38  to retrieve information from the client and apply the information to build a database structure for implantation at the primary database service. At step  40 , the primary database service completes local processing to update a primary database with the information from the client and determines if the update is complete. If the implantation of the client information to the primary database structure does not complete, the process continues to step  42  to end without attempting to implant the information at the secondary database service. If at step  40  a determination is made that the primary database has correctly updated the database structure from the client, then at step  44  the primary database service issues a write command to write the information from the primary database service to the secondary database service. 
     At step  36 , the write from the client to the secondary database service initiates copying of the information from the client to the secondary database service. At step  46 , the information from the client is stored at the secondary database service without action taken to implant the information to the secondary database. The secondary database service waits to apply the information to build a database structure until the write issued from the primary database service at step  44  arrives at the secondary database service and is detected at step  48 . At step  50 , the secondary database service responds to the write command from the primary database service by determining if the information copied from the client to the secondary database service is complete and correct. If the information is not complete and correct, the process continues to step  52  to determine if the information is still in the process of copying to the secondary database service from the client. If copying is continuing at step  52  the process returns to step  50  to allow some additional time to complete copying of information from the client. If at step  52  a determination is made that the client is not copying information or has copied information incorrectly, then the process continues to step  54  to copy the information from the primary database to the secondary database. At step  56 , the secondary database builds the database structure for the information to update the secondary database. The update to the database structure is performed with the information copied from the client as determined by step  50  or with the information copied from the primary database service at step  54  if the information did not correctly and completely copy from the client to the secondary database service. 
     In one alternative embodiment, the secondary database service receives a copy of information from the client and then checks with the primary database service to determine if a write command will arrive. If not, the secondary database service can discard the copy of information or, alternatively, can send a copy of the data to the primary database service to allow the primary database service to build and implant the database structure and then issue the write command. Similarly, the secondary database service can instruct the primary database service regarding the client write so that the primary database service contacts the client to obtain a copy of the information from the client. As another alternative, the secondary database service can elect itself to be the primary database service. 
     Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.