Getting the total message queue size in Erlang/Elixir

Question

I'm building some monitoring for an Elixir app and want to be tracking the total length of process' message queues - if they go over any reasonable value it would mean the system isn't keeping up.

How would one query in a real-life system? :erlang.system_info/1 doesn't seem to be providing it, and you can only get per-process information from :erlang.process_info/1. I don't like the idea of using the latter and manually aggregating the message queue lengths, since there's potentially thousands of relevant GenServer processes and instrumenting them feels like unnecessary overhead.

I seem to remember Wombat OAM having this functionality which makes me think it's possible, but I might be mistaken.

Answer 1

In Erlang:

All info: :erlang.process_info(self())

Just the message queue length: :erlang.process_info(self(), :message_queue_len) # => {:message_queue_len, 2}

Just the messages: :erlang.process_info(self(), :messages) #=> {:messages, [:hello, :world]}

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In Elixir:

All the info: Process.info(self())

Just the message queue length: Process.info(self(), :message_queue_len)

Just the messages: Process.info(self(), :messages)

Both give the same output which includes