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Method and apparatus for implementing a shared message queue using a list structure
| Details |
Inventors: Elko, David A.; Dievendorff, Richard; Flaherty, Dermot J.; Nick, Jeffrey M.; Surman, David H.; Warnes, James H.; Westcott, Douglas W.;
Assignee: International Business Machines Corporation (Armonk, NY)
Primary Examiner: Robinson; Greta
Assistant Examiner: Pannala; Sathyanarayan
Attorney, Agent or Firm: Kinnaman, Jr.; William A.
A method and apparatus for implementing a shared message queue using a list structure. A put list is defined comprising a sequence of list entries, each of which corresponds to a message in the queue and has an associated list entry key. Each list entry key corresponding to an uncommitted message falls within an uncommitted key range defining an uncommitted portion of the put list, while each list entry key corresponding to a committed message falls within a committed key range defining a committed portion of the put list. To write a message to the queue, a list entry is added to the put list having a list entry key within the uncommitted key range. |
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DETAILED DESCRIPTION In general, the present invention contemplates a method and apparatus for implementing a shared message queue using a list structure.
A put list is defined comprising a sequence of list entries, each of which corresponds to a message in the queue and has an associated list entry key.
Each list entry key corresponding to an uncommitted message falls within an uncommitted key range defining an uncommitted portion of the put list, while each list entry key corresponding to a committed message falls within a committed key range defining a committed portion of the put list.
To write a message to the queue, a list entry is added to the put list having a list entry key within the uncommitted key range.
Putting messages initially into the uncommitted portion of the put list in this manner hides uncommitted puts from other units of work (which only look at the committed list portion) without requiring explicit locking protocols.
List entry keys in the uncommitted key range are assigned in order of message priority and in order of arrival time for messages of a given priority.
Each list entry key has a more significant portion indicating the list portion to which the corresponding list entry belongs and a less significant portion indicating the order of the list entry in the list portion.
To commit messages to the queue, the list entry keys associated with the list entries are modified to fall within the committed key range to move the list entries to the committed portion of the put list while preserving their relative order as determined by the list entry keys.
To read a message from the queue, the list entry whose list entry key has the lowest value in the committed key range is moved from the committed portion of the put list to a get list defined for the queue manager.
This insures that only the highest-priority committed message is returned in response to a read request without knowing the key of the entry to be read.
Also, this insures no other unit of work can read a message that another unit of work has already read
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