CICS Distributed Program Link (DPL)

What is Distributed Program Link (DPL)?

Distributed Program Link (DPL) is a CICS facility that extends the LINK command beyond the local region to enable program execution on remote CICS systems. It provides a seamless way to distribute application processing across multiple systems while maintaining the familiar LINK programming model.

DPL enables distributed transaction processing, workload distribution, and system integration without requiring changes to existing application code. It supports both synchronous and asynchronous execution modes, making it suitable for various distributed application scenarios.

DPL Concepts and Architecture

Understanding DPL requires knowledge of several key concepts and architectural components that work together to enable distributed program execution:

DPL Components

DPL consists of several key components that coordinate distributed program execution:

• DPL Manager: Coordinates DPL operations and manages remote connections
• Remote Program Manager: Handles program execution on target systems
• Communication Layer: Manages data transfer between systems
• Resource Manager: Coordinates resource allocation across systems
• Error Handler: Manages error conditions and recovery

DPL Execution Model

DPL follows a specific execution model that ensures reliable distributed processing:

DPL vs. Traditional LINK

While DPL extends the LINK concept, there are important differences to understand:

DPL Implementation

Implementing DPL requires careful planning and configuration of various components. This section covers the essential implementation steps and considerations.

System Requirements

DPL requires specific system configurations and capabilities:

• CICS TS 3.1 or later for full DPL support
• ISC (Intersystem Communication) enabled
• Proper network connectivity between systems
• Resource definitions for DPL operations
• Security configurations for cross-system access

Resource Definitions

DPL requires several resource definitions to establish and manage distributed operations:

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RDO Commands for DPL Setup:
 
DEFINE GROUP(DPLGROUP) -
       TYPE(DPL) -
       DESCRIPTION('DPL Resources')
 
DEFINE CONNECTION(DPL-CONN1) -
       GROUP(DPLGROUP) -
       NETNAME(DPL-NET1) -
       PROTOCOL(LU62) -
       SECURITY(USERID) -
       TIMEOUT(300) -
       MAXSESS(10)
 
DEFINE SESSION(DPL-SESS1) -
       GROUP(DPLGROUP) -
       CONNECTION(DPL-CONN1) -
       MAXSESS(5) -
       AUTOCONN(YES) -
       STATE(INBOUND)
 
DEFINE PROFILE(DPL-PROF1) -
       GROUP(DPLGROUP) -
       TYPE(DPL) -
       MAXCONN(20) -
       MAXSESS(50) -
       SECURITY(USERID)

System Initialization Table (SIT) Parameters

DPL behavior is controlled through specific SIT parameters:

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SIT Parameters for DPL:
                  
DPL=YES                    # Enable DPL support
DPLINIT=YES               # Enable DPL initialization
DPLINITTO=60              # DPL initialization timeout
DPLINITRET=3              # DPL initialization retry count
DPLINITWAIT=10            # DPL initialization wait time
DPLINITPARM=              # DPL initialization parameters
DPLINITPROC=              # DPL initialization procedure
DPLINITUSER=              # DPL initialization user ID
DPLINITGROUP=             # DPL initialization group
DPLINITCLASS=             # DPL initialization class

Remote Program Execution

Remote program execution is the core functionality of DPL. Understanding how programs execute remotely and how to manage the execution process is essential for successful DPL implementation.

DPL LINK Command

The DPL LINK command extends the traditional LINK command with remote execution capabilities:

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       IDENTIFICATION DIVISION.
       PROGRAM-ID. DPL-CLIENT.
       
       ENVIRONMENT DIVISION.
       DATA DIVISION.
       
       WORKING-STORAGE SECTION.
       01  REMOTE-PROGRAM-NAME    PIC X(8) VALUE 'REMOTEPG'.
       01  REMOTE-SYSTEM-NAME     PIC X(8) VALUE 'REMOTESYS'.
       01  COMMUNICATION-AREA.
           05  REQUEST-TYPE        PIC X(10).
           05  REQUEST-DATA        PIC X(100).
           05  RESPONSE-STATUS     PIC X(4).
       
       PROCEDURE DIVISION.
       
       MAIN-LOGIC.
           EXEC CICS LINK
               PROGRAM(REMOTE-PROGRAM-NAME)
               COMMAREA(COMMUNICATION-AREA)
               LENGTH(LENGTH OF COMMUNICATION-AREA)
               REMOTESYSTEM(REMOTE-SYSTEM-NAME)
           END-EXEC
           
           IF EIBRESP = DFHRESP(NORMAL)
               PERFORM PROCESS-RESPONSE
           ELSE
               PERFORM ERROR-HANDLING
           END-IF
           
           EXEC CICS RETURN END-EXEC
       
       PROCESS-RESPONSE.
           DISPLAY 'Remote program executed successfully'
           DISPLAY 'Response status: ' RESPONSE-STATUS.

Remote Program Requirements

Programs that execute remotely via DPL must meet specific requirements:

• Must be defined in the PPT of the remote system
• Must be loadable and executable on the remote system
• Must handle DPL-specific error conditions
• Must be designed for distributed execution
• Must follow DPL programming guidelines

Data Transfer Considerations

Data transfer between systems requires careful consideration:

• COMMAREA size limitations across systems
• Data format compatibility between systems
• Character encoding and representation
• Performance impact of large data transfers
• Security of data in transit

Synchronous vs. Asynchronous Execution

DPL supports both execution modes for different application requirements:

Error Handling in DPL

Error handling in DPL is more complex than traditional LINK operations due to the distributed nature of execution. Understanding common error conditions and how to handle them is crucial for building robust distributed applications.

Common DPL Error Conditions

DPL operations can encounter various error conditions:

• Connection Failures: Network or system connectivity issues
• Program Not Found: Remote program not available
• Authorization Failures: Security or access control issues
• Resource Exhaustion: Insufficient resources on remote system
• Timeout Conditions: Operation exceeds time limits
• Data Transfer Errors: Communication or format issues

Error Handling Strategies

Effective error handling requires multiple strategies:

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       IDENTIFICATION DIVISION.
       PROGRAM-ID. DPL-ERROR-HANDLER.
       
       ENVIRONMENT DIVISION.
       DATA DIVISION.
       
       WORKING-STORAGE SECTION.
       01  ERROR-STATUS.
           05  ERROR-CODE         PIC S9(8) COMP.
           05  ERROR-TEXT         PIC X(80).
           05  RETRY-COUNT        PIC S9(4) COMP VALUE 0.
           05  MAX-RETRIES        PIC S9(4) COMP VALUE 3.
       
       01  RECOVERY-ACTION        PIC X(20).
       
       PROCEDURE DIVISION.
       
       MAIN-LOGIC.
           EXEC CICS HANDLE CONDITION
               ERROR(ERROR-ROUTINE)
               INVREQ(INVALID-REQUEST)
               NOTAUTH(NOT-AUTHORIZED)
               NOSYS(NOT-AVAILABLE)
           END-EXEC
           
           PERFORM DPL-OPERATION
           
           EXEC CICS RETURN END-EXEC
       
       DPL-OPERATION.
           PERFORM UNTIL RETRY-COUNT >= MAX-RETRIES
               OR ERROR-CODE = 0
               ADD 1 TO RETRY-COUNT
               PERFORM ATTEMPT-DPL-LINK
               IF ERROR-CODE NOT = 0
                   PERFORM ANALYZE-ERROR
                   PERFORM EXECUTE-RECOVERY
               END-IF
           END-PERFORM
           
           IF ERROR-CODE NOT = 0
               PERFORM FINAL-ERROR-HANDLING
           END-IF
       
       ERROR-ROUTINE.
           MOVE EIBRESP TO ERROR-CODE
           PERFORM ANALYZE-ERROR
           PERFORM EXECUTE-RECOVERY
           EXEC CICS RETURN END-EXEC
       
       ANALYZE-ERROR.
           EVALUATE ERROR-CODE
               WHEN 1001
                   MOVE 'RETRY-OPERATION' TO RECOVERY-ACTION
               WHEN 1002
                   MOVE 'FALLBACK-SYSTEM' TO RECOVERY-ACTION
               WHEN 1003
                   MOVE 'ROLLBACK-TRANSACTION' TO RECOVERY-ACTION
               WHEN OTHER
                   MOVE 'ABEND-TRANSACTION' TO RECOVERY-ACTION
           END-EVALUATE.

Recovery Mechanisms

DPL provides several recovery mechanisms for handling errors:

• Automatic Retry: Built-in retry mechanisms for transient failures
• Fallback Systems: Alternative systems for program execution
• Graceful Degradation: Reduced functionality when full operation unavailable
• Transaction Rollback: Automatic rollback on critical failures
• Error Logging: Comprehensive logging for troubleshooting

DPL Performance and Tuning

DPL performance is critical for distributed applications. Understanding performance factors and tuning techniques is essential for optimizing distributed program execution.

Performance Factors

Several factors affect DPL performance:

• Network Latency: Time for data to travel between systems
• Network Bandwidth: Data transfer capacity between systems
• System Load: Workload on source and target systems
• Data Volume: Amount of data transferred between systems
• Connection Pooling: Efficiency of connection management
• Resource Availability: Resources available on target systems

Tuning Strategies

Various strategies can improve DPL performance:

Connection Management

Optimize connection usage for better performance:

• Use connection pooling to reduce establishment overhead
• Maintain persistent connections for frequent operations
• Monitor connection utilization and adjust pool sizes
• Implement connection timeouts to prevent hanging
• Use connection affinity for related operations

Data Transfer Optimization

Minimize data transfer overhead:

• Minimize COMMAREA size to reduce transfer time
• Use efficient data formats and encoding
• Implement data compression for large transfers
• Batch multiple operations when possible
• Use asynchronous operations for non-critical data

System Configuration

Optimize system configuration for DPL operations:

• Configure appropriate timeout values
• Optimize buffer sizes for data transfer
• Implement workload management policies
• Monitor and tune system resources
• Use dedicated connections for critical operations

DPL Security Considerations

Security is a critical aspect of DPL operations. Distributed execution introduces additional security considerations that must be carefully addressed to protect systems and data.

Authentication and Authorization

DPL security begins with proper authentication and authorization:

• User ID validation across systems
• Program access control and authorization
• Cross-system security profiles
• Resource access validation
• Security token management

Data Security

Protecting data during DPL operations is essential:

• Data encryption in transit
• Secure communication protocols
• Data integrity validation
• Access logging and monitoring
• Compliance with security standards

System Security

System-level security measures for DPL:

• Network isolation and segmentation
• Firewall configuration and access control
• Intrusion detection and prevention
• Regular security audits and monitoring
• Security incident response procedures

DPL Monitoring and Troubleshooting

Effective monitoring and troubleshooting are essential for maintaining DPL system health and performance. Understanding monitoring techniques and common issues is crucial for successful DPL operations.

Performance Monitoring

Monitor key DPL performance metrics:

• DPL operation response times
• Connection utilization and availability
• Data transfer rates and volumes
• Error rates and failure patterns
• Resource consumption on both systems

Common Issues and Solutions

Be prepared to address common DPL issues:

• Connection failures and timeouts
• Program not found errors
• Authorization and security failures
• Performance degradation and bottlenecks
• Data transfer and format issues

Diagnostic Tools

Use available diagnostic tools for troubleshooting:

• CICS transaction monitoring (CEMT)
• Performance monitoring tools (RMF, SMF)
• Network diagnostic utilities
• DPL-specific monitoring commands
• System trace and dump analysis

Knowledge Check: CICS Distributed Program Link