Serialization of GenServer/:rpc requests to remote Nodes - clarification sought

Questions / Help
1

Could someone clarify what is meant here by serialize your requests on a single server?

I’m assuming the receiving GenServer would also be running asynchronously on the remote node, so what is the difference with Tasks?

Taken from the docs

Both :rpc and using a GenServer would serialize your requests on a single server, while tasks are effectively running asynchronously on the remote node, with the only serialization point being the spawning done by the supervisor.

2

All the approaches run code on the remote machine, but which process they do it in is different:

  • a named GenServer will handle the message in its single process
  • :rpc will perform the specified apply in the RPC server process. There’s one of these per node
  • a remote Task.Supervisor will handle the “start a task” request in its single process, which starts a separate process where the Task does its work

The difference is easier to think about if you imagine the “actual work” part of each of these takes a while - imagine Process.sleep(2000) as a stand-in. If three requests in close succession are sent to the three approaches, here’s what happens:

  • the GenServer and :rpc approach both take slightly more than 3x the time of one “actual work” step, because they handle messages one-at-a-time
  • the Task.Supervisor approach will take slightly more than 1x the time of an “actual work” step, as it handles messages by starting a process for each
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3

Thanks a lot for the thorough reply

So my initial assumption that the GenServer runs synchronously was wrong. Is this specific to call? Would cast take the same time as the Task.Supervisor approach?

4

Both call and cast send messages to the GenServer, but they differ on what they do next.

  • cast returns immediately; it cannot fail, even if the recipient PID has exited or never existed in the first place.

  • call waits for a reply, blocking the process until the GenServer sends a reply

In the three-requests example, the three casts will all return immediately to their callers but the GenServer will process their messages one at a time

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5

Makes perfect sense. Thanks a ton