Affinity

Workload Affinity

Enhanced Definition

In mainframe computing, particularly within a **sysplex** or distributed environment, **affinity** refers to a condition where a subsequent request, transaction, or part of a workload must be processed by the *same specific system* (e.g., LPAR, CICS region, IMS control region) that handled an earlier related request. This requirement typically arises due to the persistence of state, resources, or data in memory on that particular system.

Key Characteristics

- Statefulness: The primary driver for affinity is the need to access stateful information (e.g., control blocks, temporary storage, open files, transaction work areas) that resides only on the original processing system.
- Resource Locality: Resources like open files, database cursors, or specific memory areas might be tied to a particular address space or LPAR, necessitating affinity.
- Load Balancing Challenge: Affinity complicates workload management and load balancing, as requests cannot be freely routed to any available system; they must be directed to the specific system holding the affinity.
- Types: Can exist at various levels: CICS transaction affinity, IMS message affinity, dataset affinity, or even broader LPAR affinity for a persistent session.
- Persistence: The affinity persists for the duration of the logical unit of work, session, or until the state is explicitly released or externalized.
- Impact on Availability: If the system holding the affinity fails, the associated workload or session may be interrupted or lost, impacting high availability.

Use Cases

- CICS Transaction Affinity: A multi-step CICS transaction where intermediate data is stored in a Temporary Storage Queue (TSQ) or COMMAREA within a specific CICS region. Subsequent steps must return to that same region to retrieve the data.
- IMS Conversational Affinity: An IMS conversational transaction where the conversation state is held within the IMS control region. All subsequent messages in that conversation must be routed back to the initiating IMS region.
- Batch Job Dataset Affinity: A batch job that opens a dataset exclusively on a particular LPAR or requires access to a specific Global Resource Serialization (GRS) resource that is locally managed.
- Persistent Session Affinity: A user session maintaining state in a specific application server within a multi-LPAR environment, requiring all requests from that user to be directed to that server.

Related Concepts

Affinity is a critical consideration in sysplex workload management and high availability strategies. It directly impacts how WLM (Workload Manager) routes work, often requiring specialized routing mechanisms like CICSplex routing (e.g., using Dynamic Transaction Routing (DTR) with affinity considerations) or IMS Shared Queues (which aim to reduce affinity by externalizing state). It stands in contrast to stateless architectures, which are inherently easier to scale and load balance across multiple systems.

Best Practices:

Minimize Affinity: Design applications to minimize or eliminate affinity where possible by externalizing state (e.g., to shared DB2, IMS Shared Queues, or coupling facility structures) to improve scalability and availability.
Use Affinity-Aware Routers: Configure workload managers (like WLM, CICSplex DTR, or network dispatchers) to correctly identify and route affinity-bound work to the appropriate system.
Monitor Affinity: Regularly monitor for affinity issues that might lead to workload imbalances or single points of failure, using tools like SMF or RMF.
Plan for Affinity Breakage: Understand the implications of an LPAR or region failure when affinity exists, as it can lead to transaction aborts or data loss if state is not recoverable.
Document Affinity Requirements: Clearly document any application-specific affinity requirements to ensure proper system configuration, operational procedures, and disaster recovery planning.