Each server has a monitor process. The monitor process is created by the topology process.

If the network connection is working, then the monitor process reports this and the topology process starts the connection pool. Per server we get 1 + pool size connections to each server.

After waiting for heartbeat_frequency_ms milliseconds, the monitor process calls hello command and reports the result to the topology process.

The result of the hello command is mapped the ServerDescription structure and sent to the topology process, which updates it internal data structure.

see https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-discovery-and-monitoring.rst#monitoring