IMS Message Queue

Enhanced Definition

The IMS Message Queue is a critical component of the IMS DB/DC (Database/Data Communications) environment, serving as a persistent storage area on DASD (Direct Access Storage Device) for messages awaiting processing or delivery. It temporarily holds input messages from terminals or other applications destined for IMS transactions, and output messages generated by IMS applications for their intended recipients.

Key Characteristics

- Persistent Storage: Messages are stored on DASD, ensuring their persistence across IMS restarts and system failures, crucial for transaction integrity and recovery.
- Input and Output Handling: It manages both incoming messages (e.g., transaction requests from terminals) and outgoing messages (e.g., responses from Message Processing Programs or MPPs).
- FIFO (First-In, First-Out) Processing: Messages are generally processed in the order they are received, though priorities can influence this.
- Managed by IMS Control Region: The IMS Control Region is responsible for managing the message queue, including message enqueuing, dequeuing, and recovery.
- Supports Various Message Types: Handles transaction messages, system commands, and internal IMS messages, facilitating various operational aspects.
- Message Segmentation: Large messages can be segmented and reassembled, allowing for efficient handling of varying message sizes.

Use Cases

- Transaction Request Buffering: Stores incoming transaction requests from end-users (via terminals) or other systems until an available Message Processing Program (MPP) region can process them.
- Output Message Delivery: Holds transaction responses or unsolicited messages generated by IMS applications, awaiting delivery to the originating terminal, another application, or an external system.
- Inter-Program Communication: Facilitates asynchronous communication between different IMS application programs, where one program can place a message on the queue for another program to process later.
- Command Processing: Queues IMS operator commands (/DISPLAY, /START, /STOP) for the IMS Control Region to process.
- Message Recovery: In case of an IMS or system failure, messages on the queue are preserved and can be recovered, ensuring no data loss for in-flight transactions.

Related Concepts

The IMS Message Queue is central to the operation of IMS DB/DC, directly interacting with IMS Transactions and Message Processing Programs (MPPs). An IMS Transaction is defined to process messages from the queue, and MPPs are the application programs that read input messages from the queue and write output messages back to it. Message Format Services (MFS) often works in conjunction with the queue to format messages for terminals, optimizing data transmission. It also plays a role in IMS Connect and OTMA (Open Transaction Manager Access), where external clients or other systems (like CICS) can place messages onto or retrieve messages from the IMS queue.

Best Practices:

Monitor Queue Depth: Regularly monitor the depth of the message queue (/DISPLAY PMSG or DFS000I messages) to identify potential bottlenecks or system overloads before they impact performance.
Proper Queue Dataset Sizing: Ensure the IMS message queue datasets (DDNAME=QBLKS, DDNAME=QBUFS, DDNAME=QMBRS) are adequately sized to accommodate peak message volumes, preventing queue full conditions that can halt transaction processing.
Efficient Transaction Design: Design IMS transactions to process messages quickly and efficiently, minimizing the time messages spend on the queue and reducing resource consumption.
Utilize MFS for Terminal I/O: Employ Message Format Services (MFS) to format messages for terminal devices, reducing the amount of data transmitted and improving overall system efficiency by optimizing queue usage.
Implement Robust Recovery Procedures: Include message queue datasets in regular backup and recovery plans to ensure rapid restoration of service and data integrity in the event of a system failure.