8.3 The Dispatcher

The Dispatcher is a multithreaded dispatching agent that permits multiple multithreaded server processes to share responsibility for SMTP connection services. When using the Dispatcher, it is possible to have several multithreaded SMTP server processes running concurrently, all handling connections to the same port. In addition, each server may have one or more active connections.

The Dispatcher acts as a central receiver for the TCP ports listed in its configuration. For each defined service, the Dispatcher may create one or more SMTP server processes to handle the connections after they are established.

In general, when the Dispatcher receives a connection for a defined TCP port, it checks its pool of available worker processes for the service on that port and chooses the best candidate for the new connection. If no suitable candidate is available and the configuration permits it, the Dispatcher may create a new worker process to handle this and subsequent connections. The Dispatcher may also create a new worker process in expectation of future incoming connections. There are several configuration options which may be used to tune the Dispatcher's control of its various services, and in particular, to control the number of worker processes and the number of connections each worker process handles.

See 10.4.4 Dispatcher Configuration File for more information.

8.3.1 Creation and Expiration of Server Processes

Automatic housekeeping facilities within the Dispatcher control the creation of new and expiration of old or idle server processes. The basic options that control the Dispatcher's behavior are MIN_PROCS and MAX_PROCS. MIN_PROCS provides a guaranteed level of service by having a number of server processes ready and waiting for incoming connections. MAX_PROCS, on the other hand, sets an upper limit on how many server processes may be concurrently active for the given service.

It is possible that a currently running server process might not be able to accept any connections because it is already handling the maximum number of connections of which it is capable, or because the process has been scheduled for termination. The Dispatcher may create additional processes to assist with future connections.

The MIN_CONNS and MAX_CONNS options provide a mechanism to help you distribute the connections among your server processes. MIN_CONNS specifies the number of connections that flags a server process as “busy enough” while MAX_CONNS specifies the “busiest” that a server process can be.

In general, the Dispatcher creates a new server process when the current number of server processes is less than MIN_PROCS or when all existing server processes are “busy enough” (the number of currently active connections each has is at least MIN_CONNS).

If a server process is killed unexpectedly, for example, by the UNIX system kill command, the Dispatcher still creates new server processes as new connections come in.

For information about configuring the Dispatcher, see 10.4.4 Dispatcher Configuration File

8.3.2 To Start and Stop the Dispatcher

To start the Dispatcher, execute the command:

start-msg dispatcher

This command subsumes and makes obsolete any other start-msg command that was used previously to start up a component of the MTA that the Dispatcher has been configured to manage. Specifically, you should no longer use imsimta start smtp. An attempt to execute any of the obsoleted commands causes the MTA to issue a warning.

To shut down the Dispatcher, execute the command:

stop-msg dispatcher

What happens with the server processes when the Dispatcher is shut down depends upon the underlying TCP/IP package. If you modify your MTA configuration or options that apply to the Dispatcher, you must restart the Dispatcher so that the new configuration or options take effect.

To restart the Dispatcher, execute the command:

imsimta restart dispatcher

Restarting the Dispatcher has the effect of shutting down the currently running Dispatcher, then immediately starting a new one.