A channel provides a communication path between Queue Managers. There are two types of channels - Message Channels and MQI channels (also called Client channels).
Message channels - provide a communication path between two queue managers on the same, or different, platforms. A message channel can transmit messages in one direction only. If two-way communication is required between two queue managers, two message channels are required.
There are six types of message channels
The Sender side of the session is the “transaction coordinator”.
Message channels implement a protocol that includes a commitment protocol. Channels recover from failure by agreement: they must agree on the last committed unit of work [would this be harder if channels were bi- directional??]
MQI channels - connect an MQSeries client to a queue manager on a server machine (where a queue manager is defined). Used for transfer of MQI calls and responses only and is bi-directional.
Sender - Receiver Channel
Requestor - Server Channel
A message any arbitrary data that one program wants to send to another. This data is called the application data. A message needs to include other information, such as its destination and possibly a return address. This type of data is called the message descriptor
There are four types of messages:
A report message is used when something unexpected occurs. For example, if the data in a reply message is not usable, the receiving program might issue a report message. Most useful report messages are generated by the Queue Manager. For example, Delivery confirmation. Messages can have a “time-to-live”, called Expiry. A message that has not been delivered before its expiration is removed (not given to an app) What to do with undeliverable messages? Each queue manager can have a dead-letter queue. Messages, continued Messages can be individually designated persistent or non-persistent (persistent messages are logged to enable recovery) Message Correlator - select which message to get from queue Message Priority - retrieve messages in different order of put Segmented Messages - allows ending of VERY LARGE messages (> 100 MB) A message can contain a “reply to” address (the name of a Queue Manager and Queue). This tells the receiving application where any response should be sent. • Messages are added and removed from queues in Units of Work • The smallest Unit of Work is one message. Units of work are atomic. When an app reads a message from a queue, a message “appears” to have been removed, but in fact, it is still in storage until the app “commits” the unit of work.
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