Logger with uptime

Questions / Help
1

Hi,

I would like to configure Logger to show uptime in the dev mode. Is it possible? I found it very helpful in rust with tracing.

2

The “just make it work” way is probably:

Alternatively (for better resolution maybe?) you could probably set some kv pair in config, to the current time (i.e. boot time), then do the same as above but do a now-then.

Phoenix might have it’s own statistic too. Might even already exist in the telemetry metrics.

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3

Well, if you are using Elixir 1.10 (or later) then you can use magic of “primary filters”:

:logger.add_primary_filter(:add_uptime, {fn log, _args ->
  put_in(log, [:meta, :uptime], :erlang.statistics(:wall_clock))
end)

And then you will have :uptime field available in all logs metadata. Without need for any additional magic.

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4

Another option is to give custom formatting to the console logger used by default in the development environment: Logger — Logger v1.11.4

5

Thank you all! I will probably try first the approach with the custom formatter. As far as I understand, I need to store a kind of application start time in a GenServer and then subtract the log event timestamp from that stored time.

6

I’m also curious what you like about showing the uptime in dev. Is that the only timestamp you use in dev? Or do you also use the absolute time?

7

You wont need a separate gen server, you can piggyback your phx app.

#config/dev.exs
config :my_app,
  booted_at: System.monotonic_time(:microsecond)

config :logger, :console
  metadata: [
    :uptime
  ]

# whatever telemetry handler for phoenix endpoint stop
# or in your formatter or whatever
    boot_time = application.get_env(:my_app, :booted_at)
    now = system.monotonic_time(:microsecond)
    t = now - boot_time
    logger.metadata(uptime: "#{t}ns")

See also Precise timings with `monotonic_time` - Today I Learned

Though I would go with hauleth’s option, should be able to stick that in Application.ex I think.

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8

I am mostly interested in the time difference between two log events. Thanks that you asked :slight_smile: I came to Elixir from Rust where I used to use the uptime in the dev mode. So I wanted to have this in Elixir too. But simply adding the $time to the logger format does actually more or less the same.

config :logger, :console, 
  format: "$time [$level] $message\n"

If it were possible to specify the format like $uptime [$level].... I would probably pick this option, but $time is also fine.

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