Client-side replay does what it says on the tin: you provide a previously saved
HTTP conversation, and mitmproxy replays the client requests one by one. Note
that mitmproxy serializes the requests, waiting for a response from the server
before starting the next request. This might differ from the recorded
conversation, where requests may have been made concurrently.

You may want to use client-side replay in conjunction with the __anticache__
option. This will modify requests to remove headers (e.g.  if-modified-since)
that might cause a server to reply with a 304-not-modified.