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CICS GDS ISSUE PREPARE - Prepare Phase Signaling

CICS GDS ISSUE PREPARE initiates the prepare phase of a two-phase commit protocol in a General Data Stream (GDS) conversation in CICS APPC environments. It signals the readiness to commit a distributed transaction.

What is CICS GDS ISSUE PREPARE?

CICS GDS ISSUE PREPARE initiates the prepare phase of a two-phase commit protocol in a GDS conversation. It signals to the partner system that the local system is ready to commit a distributed transaction and requests confirmation before proceeding with the commit operation.

Command Syntax

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EXEC CICS GDS ISSUE PREPARE
    CONVID(conversation-id)
    RESP(response-code)
    RESP2(response-code-2)
END-EXEC.

Parameters Explained

CONVID Parameter

Specifies the conversation identifier:

  • Must be obtained from GDS ALLOCATE
  • Identifies the active conversation
  • Required for all GDS operations
  • Must be valid and active

RESP Parameters

Response codes returned by the command:

  • RESP: Primary response code
  • RESP2: Secondary response code
  • Always check these codes for command success
  • Handle command failures appropriately

Two-Phase Commit Protocol

1. Prepare Phase

The prepare phase involves:

  • Local transaction preparation
  • Resource locking
  • State validation
  • Readiness confirmation

2. Commit Phase

The commit phase includes:

  • Transaction commitment
  • Resource release
  • State finalization
  • Completion notification

3. Rollback Phase

The rollback phase involves:

  • Transaction rollback
  • Resource cleanup
  • State restoration
  • Error notification

Implementation Example

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WORKING-STORAGE SECTION.
01 WS-CONVERSATION-ID     PIC S9(8) COMP.
01 WS-RESPONSE            PIC S9(8) COMP.
01 WS-RESPONSE2           PIC S9(8) COMP.
 
PROCEDURE DIVISION.
    * Complete local transaction processing
    PERFORM PROCESS-LOCAL-TRANSACTION
    
    * Check if local processing was successful
    IF LOCAL-TRANSACTION-SUCCESSFUL
        EXEC CICS GDS ISSUE PREPARE
            CONVID(WS-CONVERSATION-ID)
            RESP(WS-RESPONSE)
            RESP2(WS-RESPONSE2)
        END-EXEC.
        
        IF WS-RESPONSE NOT EQUAL DFHRESP(NORMAL)
            EXEC CICS WRITE OPERATOR
                TEXT('GDS ISSUE PREPARE failed')
            END-EXEC
            PERFORM ROLLBACK-TRANSACTION
        ELSE
            * Prepare phase initiated successfully
            PERFORM WAIT-FOR-COMMIT-DECISION
        END-IF.
    ELSE
        * Local transaction failed
        PERFORM ROLLBACK-TRANSACTION
    END-IF.

When to Use Prepare Phase

1. Distributed Transactions

Use prepare phase when:

  • Multiple systems are involved
  • Data consistency is critical
  • Transaction coordination is needed
  • Rollback capability is required

2. Critical Business Operations

Use for critical operations:

  • Financial transactions
  • Inventory updates
  • Order processing
  • Account modifications

3. Data Integrity Requirements

Use for integrity requirements:

  • ACID compliance
  • Consistency guarantees
  • Isolation requirements
  • Durability needs

Prepare Phase Processing

1. Local Preparation

Local preparation includes:

  • Transaction validation
  • Resource allocation
  • State verification
  • Readiness assessment

2. Partner Coordination

Partner coordination involves:

  • Prepare signal transmission
  • Response waiting
  • Decision processing
  • Action execution

3. Decision Processing

Decision processing includes:

  • Commit decision
  • Rollback decision
  • Timeout handling
  • Error recovery

Common Response Codes

Success Response Codes

  • NORMAL (0): Prepare phase initiated successfully
  • CONVERSATION (4): Prepare signal sent

Error Response Codes

  • INVREQ (16): Invalid request
  • NOTFND (20): Conversation not found
  • CONVERSATION (24): Conversation error
  • SYSTEM (28): System error

Best Practices

1. Transaction Design

  • Design for atomicity
  • Ensure consistency
  • Maintain isolation
  • Guarantee durability

2. Error Handling

  • Check all response codes
  • Handle prepare failures
  • Implement rollback procedures
  • Log transaction events

3. Performance Optimization

  • Minimize transaction scope
  • Optimize resource usage
  • Reduce lock duration
  • Monitor transaction performance

Explain It Like I'm 5 Years Old

Imagine you and your friend are building something together:

Before you finish building, you want to make sure both of you are ready. You might say "Are you ready to finish this?" and wait for your friend to say "Yes, I'm ready!" before you both put the final pieces in place.

CICS GDS ISSUE PREPARE is like asking "Are you ready to finish this?" to the other computer. It's making sure both computers are ready to complete their work before they actually finish it.

Just like you wouldn't finish building if your friend wasn't ready, the computer won't finish its work until both computers are ready. This way, if something goes wrong, both computers can undo their work together.

Exercises

Exercise 1: Prepare Phase Initiation

Write a CICS GDS ISSUE PREPARE command to initiate the prepare phase of a distributed transaction.

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EXEC CICS GDS ISSUE PREPARE
    CONVID(WS-CONVERSATION-ID)
    RESP(WS-RESPONSE)
    RESP2(WS-RESPONSE2)
END-EXEC.

Exercise 2: Transaction Coordination

Design a two-phase commit strategy for a distributed transaction involving multiple systems. What steps should be taken in each phase?

Answer: Prepare phase: validate local transaction, lock resources, send prepare signal, wait for responses. Commit phase: receive commit decision, execute commit/rollback, release resources, notify completion. Include error handling and timeout management.

Quiz

Question 1

What is the primary purpose of CICS GDS ISSUE PREPARE?

Answer: B) To initiate prepare phase of two-phase commit

Question 2

When should CICS GDS ISSUE PREPARE be used?

Answer: B) For distributed transactions requiring coordination

Related Concepts

CICS GDS Commands

General Data Stream commands for APPC communication

CICS Transaction Management

Transaction management in CICS environments

CICS Distributed Processing

Distributed processing in CICS