We run a process that iterates through a set of documents across two collections. For each document, the PHP script will do some processing and, once it's done, the changes are saved back into the current document. Please note that this operation is spread across several threads which are connected to a replica set.

As you can tell, this operation does tons of find and update requests, these operations are currently failing at random. Knowing that issue, we've set up a log file on which we're currently outputting every error as it occurs.

This is the result:

2021-09-14 10:35:21|WARN| Failed to send "find" command with database "<hidden>": Malformed message from server
2021-09-14 10:35:22|WARN| Failed to send "find" command with database "<hidden>": Malformed message from server
2021-09-14 10:35:22|WARN| Failed to send "find" command with database "<hidden>": Message size 300723383 is not within expected range 16-48000000 bytes
2021-09-14 10:35:22|WARN| Failed to send "find" command with database "<hidden>": Message size 300724015 is not within expected range 16-48000000 bytes
2021-09-14 10:35:22|WARN| Failed to send "find" command with database "<hidden>": Malformed message from server
2021-09-14 10:35:22|WARN| Failed to send "find" command with database "<hidden>": Message size 1711538176 is not within expected range 16-48000000 bytes

The log above shows just several lines of this long set of errors, the messages are quite random and resemble a network error, although it's important to note that this isn't the case, as we're still running the exact same codebase on PHP 5.6, this is just an in-progress migration to PHP 7.4.