SQL Remote User's Guide
Administering SQL Remote for Adaptive Server Enterprise
Running the Message Agent
The three phases of Message Agent operation are described in the section Message Agent operation phases. To summarize, these phases are:
Receiving messages.
Scanning the transaction log.
Sending messages.
You may wish to run separate copies of the Message Agent to carry out these different phases. You can specify which phases a given Message Agent is to execute on the Message Agent command line.
The command-line options are as follows:
Receive The -r command-line option instructs the Message Agent to receive messages while it is running. To cause the Message Agent to shut down after receiving available messages, use the -b option in addition to -r.
Scan log The -i command-line option instructs the Message Agent to scan the transaction log into the stable queue while it is running.
Send The -s command-line option instructs the Message Agent to send messages while it is running.
Multiple phases If none of -r, -i, or -s is specified, the Message Agent executes all three phases. Otherwise, only the indicated phases are executed.
There are several circumstances where you may wish to run multiple Message Agents.
Ensuring the transaction log does not run out of space It is important that the transaction log not be allowed to become full. For this reason, you must scan the transaction log frequently enough to ensure that all entries required by SQL Remote are placed in the stable queue. Therefore, you may want to run a Message Agent that scans the transaction log continuously, even if you are only receiving and sending messages in batch mode.
Mixing operating systems If you wish to use a message link supported under one operating system, you must use a Message Agent on that platform to send and receive messages. You can do this, while running the log scanning on a UNIX machine, by running two copies of the Message Agent.
The operations of two or more Message Agents are synchronized by a table called sr_marker. This table has a single column called marker, of data type datetime.
When the Message Agent wants to wait for transactions to be scanned into the stable queue, it updates sr_marker and waits for it to work its way through the system. The column in sr_queue_state is also called marker, and contains the most recent marker to be scanned from the transaction log.