Settings Probes

A probe is a persistent system metric that's updated at a consistent rate.

Probes can be either functions, or a module that implements the Instruments.Probe behaviour. If it's a module, its state is controlled by another process, all implementers need to worry about is the state transitions.

defining-probes

Defining probes

First of all, probes must have unique names so their stats won't conflict with one another. This is enforced at runtime, since it's possible to define probes progammatically.

functions

Functions

The simplest way to define a probe is to specify a function:

Probe.define("erlang.process_count", :gauge,
  function: fn -> :erlang.system_info(:process_count) end,
  report_interval: 60_000)

Since the above definiton doesn't pass in the sample_interval options, the sample interval is the same as the report interval. The metric will be sampled and reported every 60 seconds.

mfa

MFA

A simplification of the above example uses the :mfa option to specify a module, function and arguments to be called. For example,

Probe.define("erlang.process_count", :gauge, mfa: {:erlang, :system_info, [:process_count]})

You can also have a function that returns a keyword list of stats and select which keys you want to report. The keys are added to the stat name

Probe.define("erlang.memory", :gauge,
  function: fn -> :erlang.memory() end,
  keys: [:total, :atom, :processes],
  report_interval: 60_000)

While the :erlang.memory() returns a keyword list with 9 entries, the above call will only produce three metrics, erlang.memory.total, erlang.memory.atom and erlang.memory.processes.

module-based-probes

Module based Probes

If more control is desired, you can implement a probe module yourself.

defmodule MyProbe do
  @behaviour Instruments.Probe
  # callback implementations...
end

# and then define the probe:
Probe.define("system.my_probe", :counter, module: MyProbe)

Your module is now a registered probe, and will receive all of the Probe callbacks.