[MQueue] Message Group and Message Segmentation

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. IBM WebSphere MQ v6.0
. Chapter 3.1 - IBM WebSphere Message Group and Message Segmentation
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=== 關於 Message and Correlation Identifiers ===
Two fields in the Message Descriptor :
1. MsgID (message identifier ) :
　If an application specifies MQMI_NONE on an MQPUT call, the Queue Manager
　generates a unique message identifier.

2. CorrelID (correlation identifier) :
　In a reply or report message, it is normally copies from the MsgID of the original message.

※ Both fields are treated as bit strings by the Queue Manager.


=== 關於 Retrieving Messages ===
1. With selection criteria :
　- Set MsgID and/or CorrelID prior to an MQGET call
　- Also ensure that MatchOptions in get message options is set to
　　MQMO_MATCH_MSG_ID + MQMO_MATCH_CORREL_ID

2. Without selection criteria :
　- Reset MsgID and CorrelID to MQMI_NONE and MQCI_NONE respectively
　　before each MQGET call
　- Or, set MatchOptions in get message options to MQMO_NONE

3. On return from an MQGET call, MsgID and CorrelID are set to the values
　　for the message retrieved.


=== Order of Retrieving Messages 取出訊息順序 ===
1. Messages on a Queue can be retrieved by an application in the same order
　　they were put by another application, provided :
　- The messages all have the same priority.
　- The messages were all put within the same unit of work, or all put outside of a unit of work.
　- No other application is getting messages from the Queue.
　- The Queue is Local to the putting application.
　- But they may be interspersed with messages put by other applications.

2. If the Queue is not Local to the putting application, the order of retrieval is still
　　preserved provided :
　- The first three conditions above still apply.
　- Only a single path is configured for the messages.
　- No message is put on a Dead Letter Queue
　- No nonpersistent messages are transmitted over a fast message channel.


=== 關於 Message Group (Guaranteed 機制) ===
[ Message Group ] <= (Logical message 1) + (Logical message 2) + (Logical message 3)

※ A Message Group : Consists of one or more logical messages.

※ A Logical message is :
　- A physical message (unless it is split into segments).
　- Identified by the GroupID and MsgSeqNumber fields in the message desccriptor.

說明：
1. All logical messages belonging to the same group have the same value for the GroupID field.

2. The MsgSeqNumber field has the value 1 for the first logical message, 2 for the second,
　and so on.There is, therefore, an implied ordering to the logical messages within a group.

3. A physical message which does not belong to any group has the value MQGI_NONE in
　the GroupID field, and the value 1 in the MsgSeqNumber field.


=== 關於Message Group 2 個主要用途 ===
1. To ensure ordering on retrieval in circumstances where this is not already guaranteed.
　- An application is able to put a sequence of messages constituting a Message Group on a Queue by specifying the put message option MQPMO_LOGICAL_ORDER. The Queue Manager generates a unique group identifier (GroupID) and assigns a message sequence number (MsgSeqNumber) to each message as it it put on the Queue.
　- Another application is then able to get the messages constituting the group from the Queue, in the same order they were put, by specifying the get message option MQGMO_LOGICAL_ORDER.

2. To allow an application to group together related messages.
　- This may be useful, for example, if it is important for a group of related messages to be processed by the same server instance, or by a particular server instance. By setting the value MQMO_MATCH_GROUP_ID in the MatchOptions filed in get message options, an application can retrieve only those messages with a specified group Identifier.

※ Needed : (Version 5 and 6 Queue Managers only)
　- To ensure ordering on retrieval (Where it is not already guraranteed).
　- To allow an application to group together related messages.


=== 關於 Message Segmentation ===
為了解決以前的程式，Message buffer固定且小的問題。
※ A segment is :
　- A physical message.
　- Identified by the GroupID, MsgSeqNumber, Offset fields in the message descriptor.

※ Segmentation is needed when a message is too large for an application, a Queue, or a Queue Manager.

※ A message can be segmented and reassembled :
　- By the Queue Manager.
　- By an application.

1. To ask the Queue Manager to segment a message if necessary, the putting application simply sets the value MQMF_SEQMENTATION_ALLOWED in the MsgFlags field of the message descriptor and issues one MQPUT call.

Similarly, the getting application simply specifies the get message option MQGMO_COMPLETE_MSG in order to request the Queue Manager only to return a complete logical message on an MQGET call. And if the logical message is segmented, the Queue Manager reassembles it before returning it to the application.

2. If a message is too large for an application to handle in a songle buffer, the application may perform the segmentation itself by issuing an MQPUT call for each segment. Similarly, an application may issue an MQGET call for each segment of a logical message.