The redelivered value as BIT. A value of 1 indicates that the message is being redelivered; 0 indicates that it is not being redelivered.

A message may be redelivered if it was previously received but not acknowledged. For example, the message was received but the application receiving the message did not complete processing the message content before it crashed. In these cases, QAnywhere marks the message as redelivered to alert the receiver that the message might be partly processed.

For example, assume that the receipt of a message occurs in three steps:

If the application fails between steps 1 and 2 or between steps 2 and 3, the message is redelivered when the application restarts.

If the failure occurs between steps 1 and 2, you should process the redelivered message by running steps 2 and 3. If the failure occurs between steps 2 and 3, then the message is already processed and you only need to acknowledge it.

To determine what happened when the application fails, you can have the application call ml_qa_getredelivered to check if the message has been previously redelivered. Only messages that are redelivered need to be looked up in table T1. This is more efficient than having the application access the received message's message ID to check whether the message is in the table T1, because application failures are rare.

You can read this header after a message is received and until a rollback or commit occurs; after that you cannot read it.

• Setting up SQL applications
• ml_qa_createmessage
• ml_qa_getmessage

In the following example, a message is received; if the message was previously delivered but not received, the message ID is output to the database server messages window:

begin
    declare @msgid varchar(128);
    declare @redelivered bit;
    set @msgid = ml_qa_getmessage( 'myaddress' );
    set @redelivered = ml_qa_getredelivered( @msgid );
    if @redelivered = 1 then
        message 'message with message ID ' || @msgid || ' has been redelivered';
    end if;
    commit;
end