ECDT - External CICS Data Table
An External CICS Data Table (ECDT) is a type of CICS data table that resides in a dedicated CICS region, known as the *data-owning region* (DOR), separate from the application regions (AORs) that access it. Its primary purpose is to enable multiple CICS application regions to share a common set of data, promoting data consistency and efficient resource utilization across the CICSplex. An External CICS Data Table (ECDT) is a CICS facility that allows multiple CICS regions to share an in-memory copy of a VSAM KSDS file, managed by a dedicated file-owning CICS region. Its primary purpose is to improve application performance by reducing physical I/O to disk for frequently accessed data.
Key Characteristics
-
- Cross-Region Sharing: Allows multiple CICS AORs to access and potentially update the same data table, ensuring a single source of truth for shared data.
- Dedicated Data-Owning Region (DOR): The data table is defined and managed within a specific CICS region (the DOR), which is responsible for its integrity and availability.
- Interregion Communication (MRO/IRC): Access from an AOR to an ECDT in a DOR is facilitated through CICS Multi-Region Operation (MRO) or Interregion Communication (IRC).
- DFHDTCT Macro Definition: The ECDT itself is defined in the DOR using the
DFHDTCTmacro, specifying the table's structure, size, and loading mechanism. - File Control Table (FCT) Entry: In the AOR, the ECDT is defined as a
FILEentry in the FCT, pointing to the remote DOR where the table resides. - Performance Considerations: While centralizing data, remote access introduces network latency, which can impact performance compared to an Internal CICS Data Table (ICDT) if not carefully managed.
Use Cases
-
- Common Reference Data: Storing frequently accessed, relatively static reference data (e.g., product codes, currency conversion rates, geographical data) that needs to be consistent across various CICS applications running in different AORs.
- Shared Configuration Parameters: Maintaining system-wide configuration settings or application parameters that need to be accessible and consistent for all applications within a CICSplex.
- Reduced Storage Footprint: Consolidating data into a single table in the DOR eliminates the need to replicate the data in each AOR, reducing overall storage consumption.
- Decoupling Data Management: Separating data table management into a dedicated region can simplify application design and maintenance by centralizing data access logic.
Related Concepts
ECDTs are an extension of the CICS Data Table (CDT) concept, which includes both Internal CICS Data Tables (ICDTs) and External CICS Data Tables. While ICDTs reside in the same region as the accessing application, ECDTs leverage CICS Multi-Region Operation (MRO) or Interregion Communication (IRC) to enable remote access. They are defined as FILE entries in the File Control Table (FCT) of the AOR, pointing to the remote resource, and are built and managed in the DOR using the DFHDTCT macro.
- Data Volatility Assessment: ECDTs are best suited for read-mostly or relatively static data. Frequent updates across multiple AORs can lead to increased interregion communication overhead and potential contention.
- DOR Sizing and Tuning: Ensure the data-owning region (DOR) is adequately resourced (CPU, memory) and tuned to handle the combined workload from all accessing AORs.
- Network Latency Management: Position the DOR and AORs to minimize network latency, as this directly impacts the response time for ECDT access.
- Robust Error Handling: Implement comprehensive error handling in application programs to gracefully manage potential communication failures or unavailability of the DOR.
- Performance Monitoring: Continuously monitor the performance metrics of both the AORs and the DOR, including transaction response times, CPU utilization, and MRO/IRC activity, to identify and address bottlenecks.