Query an Erlang process for its state?

Question

A common pattern in Erlang is the recursive loop that maintains state:

loop(State) ->
  receive
    Msg ->
      NewState = whatever(Msg),
      loop(NewState)
  end.

Is there any way to query the state of a running process with a bif or tracing or something? Since crash messages say "...when state was..." and show the crashed process's state, I thought this would be easy, but I was disappointed that I haven't been able to find a bif to do this.

So, then, I figured using the dbg module's tracing would do it. Unfortunately, I believe because these loops are tail call optimized, dbg will only capture the first call to the function.

Any solution?

Answer 1

If your process is using OTP, it is enough to do sys:get_status(Pid).

The error message you mentions is displayed by SASL. SASL is an error reporting daemon in OTP.

The state you are referring in your example code is just an argument of tail recursive function. There is no way to extract it using anything except for tracing BIFs. I guess this would be not a proper solution in production code, since tracing is intended to be used only for debug purposes.

Proper, and industry tested, solution would be make extensive use of OTP in your project. Then you can take full advantage of SASL error reporting, rb module to collect these reports, sys - to inspect the state of the running OTP-compatible process, proc_lib - to make short-lived processes OTP-compliant, etc.

Answer 2

It turns out there's a better answer than all of these, if you're using OTP:

sys:get_state/1

Probably it didn't exist at the time.