Trying to understand why Elixir has a Logger when Lager exists

[Jason Harrelson]

I am interested in why Elixir has a built in Logger when the Lager project already exists within the Erlang ecosystem?  I am sure there are good reasons, but I cannot find any articles that explain why.  Seems to me that Lager has some pretty killer features, such as the detailed crash reports and the source code line reporting with each statement.  Any help is appreciated as I am evaluating which logger to utilize within a Phoenix application I am working on.

Thanks!

[José Valim]

Logger also provide crash reports and source code line reporting (just ask the line metadata to be printed). The reason Logger exists is because we want terms and crash reports to be formatted in Elixir terms and syntax and not in Erlang's. Although we agree Lager is great and it was an inspiration when writing Logger, I believe the only big feature in Lager not in Logger is tracing which we hope to be added with Elixir 1.1.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

On Fri, Jun 26, 2015 at 7:29 PM, Jason Harrelson <cjhar...@gmail.com> wrote:

I am interested in why Elixir has a built in Logger when the Lager project already exists within the Erlang ecosystem?  I am sure there are good reasons, but I cannot find any articles that explain why.  Seems to me that Lager has some pretty killer features, such as the detailed crash reports and the source code line reporting with each statement.  Any help is appreciated as I am evaluating which logger to utilize within a Phoenix application I am working on.

Thanks!

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[Jason Harrelson]

Thanks Jose.  I knew you would no waste your time on Logger if there was not a good reason for it.  Can you give me a reference to what metadata is actually available and how to request it (or provide an example)?  I have done a good bit of research today and have read about the subject, but I am still unclear on it.

[José Valim]

Please check the logger docs:

http://elixir-lang.org/docs/v1.0/logger/

There is an example with :user_id as metadata. :line is added automatically when you use the Logger macros.

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[Jason Harrelson]

Jose,

I checked out the docs you recommended (I had read them prior).  After several days of trying to configure the logger in my Phoenix application's dev environment to output metadata, I cannot get the request_id to log.  I also cannot understand from the docs how to add additional custom metadata, etc.  I have followed the instructions at the docs exactly to no avail.  Any additional guidance is greatly appreciated.  Thanks!

[Pedro Medeiros]

Jason, can you share a project that exemplify the way you are configuring the Logger in the context of your app?

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[Jason Harrelson]

Unfortunately, I cannot share the project as it is on a private server.  However, it is a Phoenix application and following is the contents of my config/dev.exs file.  Notice that I updated the default logging configuration with something very similar to the Elixir Logger docs example (:request_id vs :user_id).

...

config :logger, :console,

  format: "\n$date $time [$level] $metadata$message",

  metadata: [:request_id]

...

I have not made any other changes.  I am confused on what populates the "metadata" data structure.  I am assuming it is most likely a map?  Do I need to add a plug to my pipeline to do this, or some other manual way of populating the metedata data structure?  I checked the conn in a IEx.pry session and did not see anything that was named metadata or looked like it could be the metadata.

Thanks!

[José Valim]

Which Phoenix version? Do you have a Plug.RequestId in your lib/YOUR_APP/endpoint.ex?

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

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[Jason Harrelson]

Phoenix v0.13.1.  No, there is not a Plug.RequestId in my lib/YOUR_APP/endpoint.ex.  The redacted contents are as follows:

defmodule AppName.Endpoint do

  use Phoenix.Endpoint, otp_app: :app_name

  # Serve at "/" the static files from "priv/static" directory.

  #

  # You should set gzip to true if you are running phoenix.digest

  # when deploying your static files in production.

  plug Plug.Static,

    at: "/", from: : app_name, gzip: false,

    only: ~w(css images js favicon.ico robots.txt)

  # Code reloading can be explicitly enabled under the

  # :code_reloader configuration of your endpoint.

  if code_reloading? do

    plug Phoenix.LiveReloader

    plug Phoenix.CodeReloader

  end

  plug Plug.Logger

  plug Plug.Parsers,

    parsers: [:urlencoded, :multipart, :json],

    pass: ["*/*"],

    json_decoder: Poison

  plug Plug.MethodOverride

  plug Plug.Head

  plug Plug.Session,

    store: :cookie,

    key: "_app_name_key",

    signing_salt: "[REDACTED]"

  plug :router, AppName.Router

end

Thanks

[José Valim]

Just to be sure, please add

    plug Plug.RequestId

Before Plug.Logger and let me know how it goes.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

[Jason Harrelson]

That did it.  It must not have made it into the endpoint template for v0.13.1.

I do see where the endpoint template has been updated: https://github.com/phoenixframework/phoenix/blob/c6b336be2104989c4c4eb36b6288d537c9369316/installer/templates/new/lib/application_name/endpoint.ex#L19. However, I cannot find where Plug.RequestId is defined in the Phoenix source.  I would like to examine it to see how it works if you can give me a link to it.  In addition, I took a look at the conn in the controller and still do not see anything that looks like the metadata that is used in the logger.  Where is this data stored?

As always, thanks for your help!

[Jason Harrelson]

Nevermind my last set of questions. I finally located the Plug.RequstId in the elixir-lang repo: https://github.com/elixir-lang/plug/blob/master/lib/plug/request_id.ex.  I incorrectly assumed it was defined in the Phoenix repo.  The source code there will answer my remaining questions.

[Lance Halvorsen]

Hi Jason,

Plug.RequestId is defined in Plug itself, not Phoenix. Here's a link:

http://hexdocs.pm/plug/Plug.RequestId.html

.L

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[Jason Harrelson]

OK, everything is working now.  However, If I use the Logger inside of a module that is not a controller or plug, the metadata is empty, so of course it does not get output for those specific logging statements.  Is this anticipated behavior?  Is there a workaround besides passing the request_id value to any modules that need to use the Logger?

Thanks

[Ben Wilson]

If I recall correctly the meta-data is stored on the process dictionary, which is to say it matters what process you're in not what module.