How does MPOS communicate with stratum server?
NOMP actually implements MPOS compatibility mode, so you can reference the data format it writes to MySQL: mposCompatibility.js. For example:

Storing shares:

INSERT INTO `shares` SET time = NOW(), rem_host = ?, username = ?, our_result = ?, upstream_result = ?, difficulty = ?, reason = ?, solution = ?


Updating Miner Difficulty:

UPDATE `pool_worker` SET `difficulty` = ' + diff + ' WHERE `username` = ' + connection.escape(workerName)


New Pool Worker:

INSERT INTO `pool_worker` (`account_id`, `username`, `password`) VALUES (?, ?, ?)