Addressing OLTP Solutions with CICS: The Transaction Server ... - Ibm

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88 CICS for AIX as the Transaction Server A TM assigns a data structure called a transaction identifier (XID). The XID lets the TM track and coordinate all of the work associated with a global transaction. Each RM maps the XID to the recoverable work it did for the transaction. For global uniqueness, the XID should contain atomic action identifiers. Activity among Functional Components: Activity among the three functional components of the X/Open DTP model is dependent on the transaction operation. Transaction Initiation: When an AP instructs its TM to start a global transaction, the TM tells all appropriate RMs to associate the information about the global transaction with any work the AP may request from them. Some RMs are configured so that the TM does not inform them when a global transaction starts. The RM contacts the TM to become associated with a global transaction only after the AP calls it to request actual work. This is called dynamic registration. If static registration is requested, each new transaction in the process generates a call to the RM that tells it that it has joined a new transaction or resumed an old one. Transaction Commitment: When an AP instructs its TM to commit a transaction, the TM and RMs use two-phase-commit presumed rollback to ensure that the transaction′s updates are atomic. In phase 1, the TM asks all RMs to prepare to commit (or prepare) their work. It also asks whether the RM can guarantee its ability to commit the work it did on behalf of a global transaction. If an RM can commit its work, it replies affirmatively. A negative reply reports failure. In phase 2, the TM directs all RMs either to commit or to roll back the work done on behalf of a global transaction, as the case may be. All RMs commit or roll back changes to shared resources and then return status to the TM. When an AP calls its TM to commit a global transaction, the TM reports on whether commitment or rollback was the outcome. This report is based on reports the TM received (directly or through other TMs) from all involved RMs. The XA specification contains two optimizations in the calling sequence between the TM and RM. An RM can withdraw from further participation in a global transaction during phase 1 if it was not asked to update shared resources (the read-only optimization). A TM can use one-phase commit if it is dealing with only one RM that is making changes to shared resources. The XA specification discusses requirements for stable recording of transaction data, including specifying when the TM and RMs are free to discard their knowledge of the global transaction. Transaction Rollback: The TM rolls back the global transaction if any RM responds negatively to the phase 1 request or the AP directs the TM to roll back the global transaction. The TM effects phase 2 by telling RMs to roll back the transaction. The RMs must not let any changes to shared resources become permanent. Heuristic Transaction Completion: In certain, unusual, cases, the RM could experience a long delay between phases 1 and 2 of the two-phase-commit protocol. For example, the TM that issued the prepare-to-commit (phase 1)
equest could block or fail later in the protocol sequence. To free resources, an RM can complete its work heuristically (independent of direction from its TM). The heuristic decision can be prompted by administrative action or by completion of a parametric timeout interval. Heuristic decisions typically cannot depend on the knowledge of results at other RMs that TMs normally coordinate. When any RM makes a heuristic decision, the global transaction may fail to maintain global atomicity; one RM may commit its work while another rolls back its work. Such mixed-heuristic completion can leave shared resources in an inconsistent state. A TM can report a mixed-heuristic condition to its AP. Failures and Recovery: Recovery is a process of restoring shared resources to a consistent state after various types of failure. The X/Open DTP model makes these assumptions: • TMs and RMs have access to stable storage. • TMs initiate and control transaction recovery. • RMs provide for their own restart and recovery as directed by TMs. 7.1.2 CICS for AIX and the X/Open DTP Model As a result of the implementation of the X/Open XA interface within CICS for AIX the system administrator has to provide information about which databases are to be accessed. In this section we explain how that information is supplied and show how the XA interface is activated. 7.1.2.1 Architecture Figure 26 shows the architecture of CICS for AIX Version 2.1.1 in relation to the X/Open DTP model. Figure 26. CICS for AIX and the X/Open DTP Model From Figure 26 you can see that an application program communicates with the transaction manager component through the EXEC CICS commands. The program can also communicate with the resource manager, an RDBMS, using the EXEC SQL commands. Chapter 7. Interactions with the DB2 Family of Products 89
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Addressing OLTP Solutions with CICS
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Take Note! Before using this inform
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iv CICS for AIX as the Transaction
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9.4 Security Issues on the Internet
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viii CICS for AIX as the Transactio
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xii CICS for AIX as the Transaction
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The Team That Wrote This Redbook xi
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xvi CICS for AIX as the Transaction
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2 CICS for AIX as the Transaction S
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1.2 Complex OLTP Business Environme
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Figure 2. OLTP Complex for XYZ Insu
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By using common application program
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1.5 OLTP Market 10 CICS for AIX as
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Figure 5. OLTP Market Segmentation
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14 CICS for AIX as the Transaction
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2.1 IBM Software Servers 2.1.1 Lotu
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2.2.1 CICS Clients V1.1.0 A Transac
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2.2.4 CICS Internet Gateway for AIX
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3.1 Services CICS for AIX is a very
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3.1.2 CICS API 24 CICS for AIX as t
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3.1.3.1 Terminal Users and CICS Reg
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3.2 Implementation 3.2.1 Components
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3.2.2 Processes 3.2.1.3 Application
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3.3.1 DCE RPC 3.3.2 TCP/IP 32 CICS
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Figure 9. Interconnection Using Enc
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connect to a PPC Gateway server mac
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9.3.3 Data Integrity The Web was or
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9.3.5 High Availability The drawbac
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or employee records, eavesdropping
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• Type, size, and quantity of rec
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11.1 The Big Picture 158 CICS for A
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11.2 User Access 160 CICS for AIX a
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Figure 44. Access to Various DB2 Da
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takes place over TCP/IP or SNA comm
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provide greater protection in the u
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How Customers Can Get ITSO Redbooks
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design. The process of composing a
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A year later ISC gave SQL formal in
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designing efficient applications 15
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