Coordinator

Distributed Transaction

Enhanced Definition

In the context of z/OS, a **coordinator** is a component, typically within a transaction manager like CICS or IMS, responsible for managing the integrity and atomicity of a **distributed transaction** across multiple participating **resource managers** (e.g., DB2, IMS DB, MQ) or systems. It implements the **two-phase commit (2PC)** protocol to ensure that all participants either commit or roll back the transaction together, maintaining data consistency.

Key Characteristics

- Two-Phase Commit (2PC) Implementation: Orchestrates the prepare and commit/rollback phases across all participating resource managers to guarantee atomicity.
- Transaction State Management: Tracks the state of the distributed transaction (e.g., active, prepared, committed, rolled back, in-doubt) and records it in a log for recovery.
- Participant Management: Identifies and communicates with all resource managers involved in a specific logical unit of work (LUW).
- Failure Recovery: Provides mechanisms to resolve in-doubt transactions following system failures, ensuring that consistency is restored once the coordinator or resource manager recovers.
- Inter-System Communication: Utilizes protocols like APPC/SNA or TCP/IP (e.g., DRDA for DB2) to communicate with participants residing on different z/OS LPARs or even other platforms.

Use Cases

- CICS-DB2 Distributed Updates: A CICS transaction updating a DB2 database on one z/OS system and another DB2 database on a different z/OS system, where the CICS region acts as the coordinator.
- IMS TM and External Resource Managers: An IMS transaction manager coordinating updates to IMS DB and an external DB2 database, ensuring all changes are committed or rolled back synchronously.
- Cross-Platform Data Synchronization: A z/OS application initiating a transaction that updates data on z/OS (e.g., DB2) and also on a distributed system (e.g., a Linux database), with the z/OS transaction manager acting as the coordinator.
- Message Queue Integration: A transaction involving an update to a database and placing a message on an IBM MQ queue, where the coordinator ensures both operations succeed or fail together.

Related Concepts

The coordinator is central to the two-phase commit (2PC) protocol, which it uses to interact with resource managers (like DB2, IMS DB, MQ) that manage the actual data. It is typically a function or component of a transaction manager (e.g., CICS Transaction Server, IMS Transaction Manager) that defines the boundaries of a logical unit of work (LUW). For DB2 distributed transactions, the coordinator often leverages Distributed Relational Database Architecture (DRDA) for communication.

Best Practices:

Robust Logging: Ensure that the coordinator's transaction logs and the resource managers' recovery logs are properly configured, mirrored, and regularly backed up to facilitate efficient recovery.
Monitor in-doubt Transactions: Regularly monitor for and resolve in-doubt transactions, which can occur after failures and prevent resources from being released. Tools like CICS DSNC DISPLAY INDOUBT or DB2 DISPLAY THREAD can help.
Network Reliability: Design and maintain a highly reliable network infrastructure between the coordinator and its participants to minimize communication failures that can lead to in-doubt situations.
Application Design for Idempotency: Design distributed applications to be idempotent where possible, allowing safe re-execution in case of partial failures or in-doubt resolution.
Transaction Timeout Configuration: Configure appropriate transaction timeouts for the coordinator and resource managers to prevent long-running in-doubt transactions from holding resources indefinitely.