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are returned . These three columns are the minimum set of columns necessary to process a message . The message_type_name column indicates what kind of message has been received . You must always know the message type because even if the contract limits a service to a single incoming message type, error and end dialog messages might be received and must be processed appropriately . Using the message_type_id column instead of the name would be a little more efficient because an internal join is eliminated . However, there is no way to control the ID assigned to a message type, so using the ID isn t recommended unless you use only system message types that have stable IDs . The FROM clause specifies which queue to receive messages from . The RECEIVE statement finds the oldest conversation group with messages available on the queue that is not already locked by another transaction . The command then locks the conversation group and uses a DELETE or UPDATE with OUTPUT command to retrieve the messages . Even if there are messages on the queue from multiple conversation groups, only messages from a single conversation group will be returned by a RECEIVE command . This approach ensures that a RECEIVE command will lock only one conversation group at a time, which improves parallelism . It is possible and in many cases desirable for a single transaction to hold multiple conversation group locks, but each RECEIVE statement locks only a single conversation group . If your program has done a lot of work to restore the state for a conversation group, receiving messages from another conversation group may require throwing that state away and retrieving the state for the new conversation group . If there are more messages on the queue for the original conversation group, it will be more efficient to retrieve them while you have the state loaded . To support this, you can specify which conversation group to receive messages from:
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RECEIVE top(1) @message_type = message_type_name, @message_body = message_body, @dialog = conversation_handle FROM dbo.InventoryQueue WHERE conversation_group_id = @CGID;
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If there are no messages from the specified conversation group on the queue, this statement returns no rows . In certain, rare circumstances, it makes sense to send a message and then wait for the response from that message . This is normally a bad thing to do because it negates the advantages of asynchronous messaging . However, in some cases, the application needs to know that an action is complete before continuing . In this case, you can receive messages from a particular conversation:
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RECEIVE top(1) @message_type = message_type_name, @message_body = message_body FROM dbo.InventoryQueue WHERE conversation_handle = @dialog;
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The problem with the RECEIVE statement is that if no messages are available on the queue, it returns immediately just like a SELECT statement that finds no records . In some cases, this is the desired behavior because if the queue is empty, the application can do other things . On the other hand, in many cases polling the queue is a waste of resources . To use resources more efficiently, Service Broker allows you to wrap the RECEIVE statement in a WAITFOR statement . The WAITFOR statement allows the RECEIVE statement to return immediately if messages are in the queue but wait until a message arrives if the queue is empty . The RECEIVE statement now looks like this:
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WAITFOR ( RECEIVE top(1) @message_type = message_type_name, @message_body = message_body, @dialog = conversation_handle FROM dbo.InventoryQueue ), TIMEOUT 2000;
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The TIMEOUT clause defines when the WAITFOR statement should give up and return if no messages arrive . In this case, if there are no messages after 2000 milliseconds, the statement will return no rows . If a message arrives before the timeout expires, the statement returns immediately . If the TIMEOUT clause is omitted, the statement will wait until a message arrives on the queue no matter how long it takes . Important The RECEIVE and SEND keywords must start the RECEIVE and SEND commands .
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Most SQL keywords that start commands such as SELECT, INSERT, and UPDATE are known to the parser as terminal keywords . This means that whenever the parser sees one of these keywords, it knows that a new command is starting . SEND and RECEIVE are not marked as terminal keywords because they are not part of ANSI SQL . To ensure that the parser knows that a new command is starting, the command before SEND or RECEIVE must end with a semicolon (;) .
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