Mathias Meyer
Mathias Meyer


Wherein I write about Rails’ current implementation of logging and ActiveSupport’s greatest feature that was added in 3.0.

I’ve been thinking a lot about logging lately. I’m a big fan of logging. I also spent some quality time with a Rails app again recently. I’m not a big fan of Rails’ logging. Something needs to change, either me or Rails’ logging. I opted for the latter.

After some digging in I found that Rails 3 has improved quite significantly on the logging front. The output is still the same unparsable mess it used to be, but the way logging is implemented has changed quite drastically, and much for the better, I’d argue.

With Rails 3, one important thing was added, that drives parts of the both logging and benchmarking (e.g. for ActiveRecord’s query measurements). Everything is now built around notifications and instrumentation. When ActiveRecord fires a query, it measure the time required and then triggers an event with the recorded time. Whether someone picks up the event is not of its concern. Which is exactly what the new notifications are about: separation of concern.

I was surprised to find that not a lot of people seemed to know about them. Let’s have a short look at what it allows you to do.


The idea is far from new. The Pragmatic Programmer talked about blackboards, a similar mechanism, Cocoa has had something similar for ages, and now Rails has something like it too.

The basic idea is that you have a centralized repository where you can subscribe to events, for example to the event ActionController triggers when it process an action. Here’s the code to extract the path that was requested by a user.

ActionSupport::Notifications.subscribe('process_action.action_controller') do |event|
  puts event.payload[:path]

Any number of subscribers can be attached to a message, they’ll get notified. The default implementation is synchronous, but nothing could and should keep you from adding an implementation that uses a message queue instead.

This is pretty cool. I started using it to track metrics and not clutter the code with the specifics. When something of interest happens, an event is triggered. If someone listens, cool, if they don’t we’ll keep going. I could have one subscriber that collects metrics and another one for tracer logging.

Rails uses these notifications all over the place, in particular for logging. To avoid having lots of manual subscriptions to specific events, Rails also added a mechanism to subscribe to events for the sole purpose of logging them and without adding subscriptions manually for all of them. The LogSubscriber was born.


LogSubscribers are exactly that: easy ways to subscribe to events whose purpose is logging. Of course what you do with the events is up to you, but that’s their main purpose. Every Rails component uses them, and every component has its own implementation of a LogSubscriber. Here’s an excerpt of the one used by ActionController.

Every public method defined, except for logger, will be attached to an event of the same name. So the three relevant events for this subscriber are start_processing and process_action.

To make the improvement of notifications and log subscribers more visible, here’s the code that did the same in Rails 2.3. You decide which you like better. I certainly enjoy the decoupled-ness of the log subscriber a lot more.

When you defined your LogSubscriber, you can attach it to a namespace:

ActionSupport::LogSubscriber.attach_to :action_view

This creates a new instance of the LogSubscriber and attaches all methods to their corresponding events. The event names are along the same lines as the example with process_action.action_controller. First the event’s name, then the component’s name.

Rails’ Logging

The example above nicely brings me to Rails’ logging. First up: I like Rails’ idea of logging everything, for development purposes it’s awesome and pretty helpful. When things are moved into production the fun stops for me though. The logging out put is hard to parse, and it’s hard to make sense of because it’s usually multiple lines per request.

Rails 3.2 recently added tagged logging and a request identifier to work around that. But that still doesn’t solve the problem of the output in general being too noisy and hard to parse for a centralized logging service, or any logging service. If you don’t care about your logs then I’m sure you’re fine, but I care a great deal about my logs. When things break, they’re the sole source of truth, and I like to make sure they’re valuable enough to fulfill that premise. Rails’ request logging gets in the way of that for the reasons outlined above.

To remind you of what we’re talking about, here’s an example log output for a single request:

Started GET "/" for at 2012-03-10 14:28:14 +0100
Processing by HomeController#index as HTML
  Rendered text template within layouts/application (0.0ms)
  Rendered layouts/_assets.html.erb (2.0ms)
  Rendered layouts/_top.html.erb (2.6ms)
  Rendered layouts/_about.html.erb (0.3ms)
  Rendered layouts/_google_analytics.html.erb (0.4ms)
Completed 200 OK in 79ms (Views: 78.8ms | ActiveRecord: 0.0ms)

It reads pretty nicely for sure, but as soon as you add more processes that dump their output in the same log, things get mingled and some of the information is, in my opinion just not necessary in production.

So we have a pretty centralized approach to logging, and me wanting to do something to improve the logging. Clearly the two could be made to work together.

Towards a Better Logging (in Production)

My ideal request logging is a single line per request, nothing more. That’s clearly at odds with the output above, but thanks to the fact that (almost) everything is wrapped in log subscribers. Here’s a line of something that would fit my purpose pretty nicely:

GET / format=html action=home#index status=200 duration=58.33 view=40.43 db=15.26

It’s one line, it contains all the relevant information, it’s pretty easy to parse for a machine, and it’s easy to read for the human eye. Some told me the latter shouldn’t be necessary, and I’m certainly not hung up on it, but I like to be able to skim logs.

The message is pretty clear: the HTTP method, the URL and an slew of optional parameters. If there’s an exception that too should end up as single line. You can log the stacktrace of course, but it’s much harder to make sense of for the same reason multiple lines of log output are hard to make sense of. They drown in a river of log output from multiple sources. I thought about only storing relations to an exception in the logs instead, e.g. an identifier or an error code. Throw in a request identifier as metadata stored with the exception, and you’ve got yourself a nice way to correlate exceptions and log lines.

How can we get to the output above? It turns out, it’s actually pretty simple. We need to unhook the log subscribers for ActionView and ActionController events and hook in our own. The result is Lograge, a logging experiment I extracted into a library from the Travis CI source code, where I first started playing with the ideas I had around logging.

It adds its own log subscriber, discarding all irrelevant events, only accepting two events instead of the whole bunch included by default. The result is a single log line. Easy on the eyes, easy on the machine.

An Experiment in Logging

My main goal is to eventually have a saner way of logging requests in Rails (or any web framework, for that matter). Lograge is the beginning of that. I already got great feedback on the general idea and on specifics like the log output.

I have also yet to solve how to properly log request parameters, so this is only the beginning. I care a great deal about logging and I’d like to see that eventually improve in Rails so that other people start caring for their logs too, if only in production mode or as an optional feature. If not, Lograge will be here for you. I also have Rails 2.3 on my radar if you’re still using that. It’s a lot messier to implement, but not impossible.

If you want to know a bit more about the internals, the README is a good place to start. If you have any input on the ideas implemented in Lograge, the log output or anything else, feel free to open an issue. Let’s talk about logging, and let’s make it better.