class ActiveSupport::LogSubscriber

ActiveSupport::LogSubscriber is an object set to consume ActiveSupport::Notifications with the sole purpose of logging them. The log subscriber dispatches notifications to a registered object based on its given namespace.

An example would be Active Record log subscriber responsible for logging queries:

module ActiveRecord
  class LogSubscriber < ActiveSupport::LogSubscriber
    attach_to :active_record

    def sql(event)
      info "#{event.payload[:name]} (#{event.duration}) #{event.payload[:sql]}"
    end
  end
end

ActiveRecord::LogSubscriber.logger must be set as well, but it is assigned automatically in a Rails environment.

After configured, whenever a "sql.active_record" notification is published, it will properly dispatch the event (ActiveSupport::Notifications::Event) to the sql method.

Being an ActiveSupport::Notifications consumer, ActiveSupport::LogSubscriber exposes a simple interface to check if instrumented code raises an exception. It is common to log a different message in case of an error, and this can be achieved by extending the previous example:

module ActiveRecord
  class LogSubscriber < ActiveSupport::LogSubscriber
    def sql(event)
      exception = event.payload[:exception]

      if exception
        exception_object = event.payload[:exception_object]

        error "[ERROR] #{event.payload[:name]}: #{exception.join(', ')} " \
              "(#{exception_object.backtrace.first})"
      else
        # standard logger code
      end
    end
  end
end

ActiveSupport::LogSubscriber also has some helpers to deal with logging. For example, ActiveSupport::LogSubscriber.flush_all! will ensure that all logs are flushed, and it is called in Rails::Rack::Logger after a request finishes.