Logging in Rails with a per-request ID

Backupify produced a great article about this: http://blog.backupify.com/2012/06/27/contextual-logging-with-log4r-and-graylog/

We wanted the request_id (that is generated by rails and available at request.uuid to be present on all messages throughout the request. In order to get it into the rack logging (the list of parameters and the timing among others), we added it to the MDC in a rack middleware.

application.rb:

config.middleware.insert_after "ActionDispatch::RequestId", "RequestIdContext"

app/controllers/request_id_context.rb: (had trouble finding it in lib for some reason)

class RequestIdContext
  def initialize(app)
    @app = app
  end

  def call(env)
    Log4r::MDC.get_context.keys.each {|k| Log4r::MDC.remove(k) }
    Log4r::MDC.put("pid", Process.pid)
    Log4r::MDC.put("request_id", env["action_dispatch.request_id"])
    @app.call(env)
  end
end

If you push jobs onto delay job/resque, put the request_id into the queue. and in your worker pull it off and set into the MDC. Then you can trace the requests the whole way through