Queue and Scratch Management Commands › Queue Records

Queue Records

Overview

Queue records are available to all tasks running under DC/UCF, as well as to batch programs. Records in a queue established by one task are available to subsequent tasks running on the same logical terminal, or to concurrent or subsequent tasks running on any other terminal. Queue records are saved across system shutdowns and are recovered across a system crash.

Because queue records are available to concurrent tasks running on other terminals, the records can be used to pass data from one application to another. Additionally, queue records provide a convenient means of storing data for subsequent processing.

Storing a Queue Record

A queue record is stored in the data dictionary as a member occurrence in a set owned by a queue header record. All records associated with a particular queue header are referred to collectively as a queue. The queue is identified by a queue id. Requests to access a queue record can use the queue id to specify the queue in which the object record participates. If a request to store a queue record specifies an unknown queue id, a queue is created with the specified id.

When a queue record is stored, DC/UCF can return a queue record identifier to a specified location in a dialog's record buffers. The queue record identifier can then be used to access the queue record.

Currencies

The CA ADS runtime system maintains currencies for each queue accessed by a task. If concurrently executing tasks access the same queue, each task has its own queue currency. A request for a particular queue record can identify the record by the queue id, by the queue record id, by the position of the record within the queue, or by the relationship of the object record to the record that is current of queue for the requesting task.

Queue records remain in the data dictionary until explicitly deleted or until the retention period specified for the queue has expired. When all records associated with a given queue header have been deleted, the header record is also deleted and the queue no longer exists.

Considerations

• An exclusive lock is placed on a queue record when the record is retrieved or stored, thereby preventing concurrently executing tasks from accessing the same record. Queue record locks are released when the task terminates or when a COMMIT command with the TASK keyword is executed.

  Because no other task can access a locked queue record, a concurrently executing task that attempts to access the record must wait until the lock is released. To minimize such waits, queue access should be as brief as possible.

• Queue currencies are not saved when a task terminates. Each task must establish its own currencies. The following considerations apply:
  • Queue currencies are lost each time a DISPLAY command is executed.
  • Queue currencies are lost across a system shutdown or a system crash.
• All queue management command clauses must be coded in the same order in which they appear in the syntax.
• Queue management commands are allowed in CA ADS Batch only if the application is running under the central version.

More information:

Database Access Commands