Bracket - Delimited Conversation

Enhanced Definition

In the context of IBM Systems Network Architecture (SNA) and Advanced Program-to-Program Communication (APPC), a bracket is a logical construct that defines a unit of work or a sequence of related data exchanges between two transaction programs (TPs). It ensures that a series of communications forms a single, atomic transaction, managing the flow and state of the conversation.

Key Characteristics

- Transactional Scope: A bracket encapsulates a logical transaction, ensuring that all operations within it are treated as a single, indivisible unit for commit or rollback purposes.
- Half-Duplex Flip-Flop: Within a bracket, communication is half-duplex, meaning only one program can send data at a time. Control "flips" between the sending and receiving programs.
- State Management: The bracket defines the current state of the conversation (e.g., SEND state, RECEIVE state, PREPARE_TO_RECEIVE state), guiding which program is allowed to transmit.
- Error Recovery: Provides a defined boundary for error detection and recovery, allowing for consistent state management in case of failures.
- APPC/CPI-C Foundation: It is a fundamental concept in APPC and CPI-C (Common Programming Interface for Communications) programming, enabling robust inter-program communication.
- Bracket Initiation/Termination: Brackets are implicitly or explicitly initiated (e.g., by an ALLOCATE verb) and terminated (e.g., by a DEALLOCATE verb or transaction commit/rollback).

Use Cases

- Distributed Transaction Processing (DTP): Coordinating updates across multiple systems, such as a CICS region updating a DB2 database on one system and an IMS database on another, ensuring all updates commit or rollback together.
- Inter-System Communication (ISC): Facilitating reliable communication between different mainframe subsystems or applications, like CICS-to-CICS or CICS-to-IMS transactions.
- Remote Procedure Calls (RPC): Implementing a sequence of requests and responses that collectively form a single logical operation, where the integrity of the entire exchange is critical.
- Data Synchronization: Ensuring that a series of data transfers or updates between two applications maintains data consistency and integrity.

Related Concepts

Brackets are a core feature of SNA (Systems Network Architecture) and are specifically implemented by LU 6.2 (Logical Unit Type 6.2) sessions, which enable peer-to-peer program communication. They are central to APPC (Advanced Program-to-Program Communication) and its programming interface, CPI-C (Common Programming Interface for Communications), where verbs like CM_ALLOCATE, CM_SEND_DATA, and CM_DEALLOCATE manage bracket states. In CICS (Customer Information Control System), brackets are extensively used for Inter-System Communication (ISC) and Distributed Transaction Processing (DTP) to ensure transactional integrity across connected systems.

Best Practices:

Minimize Bracket Duration: Design applications to keep brackets open for the shortest possible time to reduce resource contention, improve throughput, and minimize the impact of potential deadlocks.
Robust Error Handling: Implement comprehensive error handling within the bracketed conversation to ensure proper recovery, rollback of changes, and graceful termination in case of communication or application failures.
Explicit State Management: When programming with APPC/CPI-C, explicitly manage the conversation state using appropriate verbs (e.g., CM_SEND_AND_RECEIVE, CM_PREPARE_TO_RECEIVE) to control the flow and ensure both partners understand the current state.
Resource Cleanup: Always ensure that resources acquired within a bracket (e.g., database locks, file handles) are properly released upon bracket termination, whether successful or due to an error.
Avoid Long-Running Transactions: Structure applications to avoid holding brackets open across user think time or for operations that could take an unpredictable amount of time, as this can severely impact system performance and availability.