An ismaster is sent for (1) periodic SDAM monitoring and (2) during the initial handshake when opening a new socket to a server. It is not clear to me for both cases whether the ismaster should always be sent with OP_QUERY, or if we should use OP_MSG if the current server description indicates it supports OP_MSG. I think we should clarify in both cases which wire protocol to use.

OP_MSG spec says:

OP_MSG is only available in MongoDB 3.6 (maxWireVersion >= 6) and later. MongoDB drivers MUST continue to perform the MongoDB Handshake using OP_QUERY to determine if the node supports OP_MSG.

If the node supports OP_MSG, any and all messages MUST use OP_MSG, optionally compressed with OP_COMPRESSED.

I'm not sure if that means we should always use OP_QUERY for ismaster, or if we should use OP_MSG for ismaster if the current server description indicates that the server supports OP_MSG.

SDAM spec says that the reply to an initial handshake ismaster should also used to update the server/topology description.

If we're using the reply to the initial handshake to update our server/topology description, then the initial handshake is a part of monitoring as well. That indicates to me that if we decide to always use OP_QUERY for monitoring, we should always do so for the handshake.

FWIW, libmongoc always uses OP_QUERY for monitoring. And as was discovered in CDRIVER-3404, may use OP_MSG for the handshake if the last server description had a high enough maxWireVersion.