Error Reporting in Rails Applications

This guide introduces ways to manage errors in a Rails application.

After reading this guide, you will know:

How to use Rails' error reporter to capture and report errors.
How to create custom subscribers for your error-reporting service.

1 Error Reporting

The Rails error reporter provides a standard way to collect errors that occur in your application and report them to your preferred service or location (e.g. you could report the errors to a monitoring service such as Sentry).

It aims to replace boilerplate error-handling code like this:

begin
  do_something
rescue SomethingIsBroken => error
  MyErrorReportingService.notify(error)
end

with a consistent interface:

Rails.error.handle(SomethingIsBroken) do
  do_something
end

Rails wraps all executions (such as HTTP requests, jobs, and rails runner invocations) in the error reporter, so any unhandled errors raised in your app will automatically be reported to your error-reporting service via their subscribers.

This means that third-party error-reporting libraries no longer need to insert a Rack middleware or do any monkey-patching to capture unhandled errors. Libraries that use Active Support can also use this to non-intrusively report warnings that would previously have been lost in logs.

Using the Rails error reporter is optional, as other means of capturing errors still work.

1.1 Subscribing to the Reporter

To use the error reporter with an external service, you need a subscriber. A subscriber can be any Ruby object with a report method. When an error occurs in your application or is manually reported, the Rails error reporter will call this method with the error object and some options.

Some error-reporting libraries, such as Sentry's and Honeybadger's, automatically register a subscriber for you.

You may also create a custom subscriber. For example:

# config/initializers/error_subscriber.rb
class ErrorSubscriber
  def report(error, handled:, severity:, context:, source: nil)
    MyErrorReportingService.report_error(error, context: context, handled: handled, level: severity)
  end
end

After defining the subscriber class, you can register it by calling the Rails.error.subscribe method:

Rails.error.subscribe(ErrorSubscriber.new)

You can register as many subscribers as you wish. Rails will call them in the order in which they were registered.

It is also possible to unregister a subscriber by calling Rails.error.unsubscribe. This may be useful if you'd like to replace or remove a subscriber added by one of your dependencies. Both subscribe and unsubscribe can take either a subscriber or a class as follows:

subscriber = ErrorSubscriber.new
Rails.error.unsubscribe(subscriber)
# or
Rails.error.unsubscribe(ErrorSubscriber)

The Rails error reporter will always call registered subscribers, regardless of your environment. However, many error-reporting services only report errors in production by default. You should configure and test your setup across environments as needed.

1.2 Using the Error Reporter

Rails error reporter has four methods that allow you to report methods in different ways:

Rails.error.handle
Rails.error.record
Rails.error.report
Rails.error.unexpected

1.2.1 Reporting and Swallowing Errors

The Rails.error.handle method will report any error raised within the block. It will then swallow the error, and the rest of your code outside the block will continue as normal.

result = Rails.error.handle do
  1 + "1" # raises TypeError
end
result # => nil
1 + 1 # This will be executed

If no error is raised in the block, Rails.error.handle will return the result of the block, otherwise it will return nil. You can override this by providing a fallback:

user = Rails.error.handle(fallback: -> { User.anonymous }) do
  User.find(params[:id])
end

1.2.2 Reporting and Re-raising Errors

The Rails.error.record method will report errors to all registered subscribers and then re-raise the error, meaning that the rest of your code won't execute.

Rails.error.record do
  1 + "1" # raises TypeError
end
1 + 1 # This won't be executed

If no error is raised in the block, Rails.error.record will return the result of the block.

1.2.3 Manually Reporting Errors

You can also manually report errors by calling Rails.error.report:

begin
  # code
rescue StandardError => e
  Rails.error.report(e)
end

Any options you pass will be passed on to the error subscribers.

1.2.4 Reporting Unexpected Errors

You can report any unexpected error by calling Rails.error.unexpected.

When called in production, this method will return nil after the error is reported and the execution of your code will continue.

When called in development, the error will be wrapped in a new error class (to ensure it's not being rescued higher in the stack) and surfaced to the developer for debugging.

For example:

def edit
  if published?
    Rails.error.unexpected("[BUG] Attempting to edit a published article, that shouldn't be possible")
    false
  end
  # ...
end

This method is intended to gracefully handle any errors that may occur in production, but that aren't anticipated to be the result of typical use.

1.3 Error-reporting Options

The reporting APIs #handle, #record, and #report support the following options, which are then passed along to all registered subscribers:

handled: a Boolean to indicate if the error was handled. This is set to true by default. #record sets this to false.
severity: a Symbol describing the severity of the error. Expected values are: :error, :warning, and :info. #handle sets this to :warning, while #record sets it to :error.
context: a Hash to provide more context about the error, like request or user details
source: a String about the source of the error. The default source is "application". Errors reported by internal libraries may set other sources; the Redis cache library may use "redis_cache_store.active_support", for instance. Your subscriber can use the source to ignore errors you aren't interested in.

Rails.error.handle(context: { user_id: user.id }, severity: :info) do
  # ...
end

1.4 Setting Context Globally

In addition to setting context through the context option, you can use Rails.error.set_context. For example:

Rails.error.set_context(section: "checkout", user_id: @user.id)

Any context set this way will be merged with the context option

Rails.error.set_context(a: 1)
Rails.error.handle(context: { b: 2 }) { raise }
# The reported context will be: {:a=>1, :b=>2}
Rails.error.handle(context: { b: 3 }) { raise }
# The reported context will be: {:a=>1, :b=>3}

1.5 Filtering by Error Classes

With Rails.error.handle and Rails.error.record, you can also choose to only report errors of certain classes. For example:

Rails.error.handle(IOError) do
  1 + "1" # raises TypeError
end
1 + 1 # TypeErrors are not IOErrors, so this will *not* be executed

Here, the TypeError will not be captured by the Rails error reporter. Only instances of IOError and its descendants will be reported. Any other errors will be raised as normal.

1.6 Disabling Notifications

You can prevent a subscriber from being notified of errors for the duration of a block by calling Rails.error.disable. Similarly to subscribe and unsubscribe, you can pass in either the subscriber itself, or its class.

Rails.error.disable(ErrorSubscriber) do
  1 + "1" # TypeError will not be reported via the ErrorSubscriber
end

This can also be helpful for third-party error reporting services who may want to manage error handling a different way, or higher in the stack.

2 Error-reporting Libraries

Error-reporting libraries can register their subscribers in a Railtie:

module MySdk
  class Railtie < ::Rails::Railtie
    initializer "my_sdk.error_subscribe" do
      Rails.error.subscribe(MyErrorSubscriber.new)
    end
  end
end

If you register an error subscriber, but still have other error mechanisms like a Rack middleware, you may end up with errors reported multiple times. You should either remove your other mechanisms or adjust your report functionality so it skips reporting an error it has seen before.