What Is Message & Types Of Messages

Message:

What is Message and types of message ?

• In message queuing, a message is a collection of data sent by one program and intended for another program.

WebSphere MQ Predefined message types are:

Data Gram : A simple message for which no reply is expected

• Use a datagram when you do not require a reply from the application that receives the message (that is, gets the message from the queue).
•  An example of an application that could use data grams is one that displays flight information in an airport lounge.A message could contain the data for a whole screen of flight information.Such an application is unlikely to request an acknowledgement for a message because it probably does not matter if a message is not delivered.
• The application sends an update message after a short period of time.

Request : A message for which a reply is expected

• Use a request message when you want a reply from the application that receives the message.
• An example of an application that could use request messages is one that displays the balance of a checking account.The request message could contain the number of the account, and the reply message would contain the account balance.

Reply : A reply to a request message

Report : A message that describes an event such as the occurrence of an error

• A confirmation of arrival (COA) report message. This indicates that the message has reached its target queue. It is generated by the queue manager.
• A confirmation of delivery (COD) report message. This indicates that the message has been retrieved by a receiving application. It is generated by the queue manager.
• Applications can use the first three types of messages to pass information between themselves.
• The fourth type, report, is for applications and queue managers to use to report information about events such as the occurrence of an error.
• You can also define your own types of message

What is Message Persistence ?

• Persistent messages are written to logs and queue data files.
• If a queue manager is restarted after a failure, it recovers these persistent messages as necessary from the logged data.
• Messages that are not persistent are discarded if a queue manager stops, whether the stoppage is as a result of an operator command or because of the failure of some part of your system.
• When you create a message, if you initialize the message descriptor (MQMD) using the defaults, the persistence for the message is taken from the DefPersistence attribute of the queue specified in the MQOPEN command. Alternatively, you can set the persistence of the message using the Persistence field of the MQMD structure to define the message as persistent or nonpersistent.
• The performance of your application is affected when you use persistent messages.
• A persistent message, outside the current unit of work, is written to disk on every put and get operation.
• A persistent message within the current unit of work is logged only when the unit of work is committed.
• Nonpersistent messages can be used for fast messaging.