ri > ActiveSupport::LogSubscriber

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📛 NAME

ActiveSupport::LogSubscriber < Subscriber

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

And it's finally registered as:

ActiveRecord::LogSubscriber.attach_to :active_record

Since we need to know all instance methods before attaching the log subscriber, the line above should be called after your ActiveRecord::LogSubscriber definition.

A logger also needs to be set with ActiveRecord::LogSubscriber.logger=. This 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

Log subscriber also has some helpers to deal with logging and automatically flushes all logs when the request finishes (via action_dispatch.callback notification) in a Rails environment.

🚀 Quick Reference

Use Case	Command	Description
📝 Define a custom log subscriber	class MyLogSubscriber < ActiveSupport::LogSubscriber; def my_event(event) ... end; end	Create a subscriber class and define methods for each notification namespace
🔗 Attach subscriber to a namespace	MyLogSubscriber.attach_to :my_namespace	Register the subscriber to listen for notifications under the given namespace
⚙️ Set a custom logger	MyLogSubscriber.logger = Logger.new(STDOUT)	Assign a logger instance (automatically set in Rails)
❌ Handle exceptions in instrumented code	if event.payload[:exception] ... end	Check for exceptions and log error details
🎨 Colorize log output	color(text, :RED)	Use the color helper to apply ANSI colors

📋 Constants

• BLACK — Colors
• BLUE — [not documented]
• BOLD — [not documented]
• CLEAR — Embed in a String to clear all previous ANSI sequences.
• CYAN — [not documented]
• GREEN — [not documented]
• MAGENTA — [not documented]
• RED — [not documented]
• WHITE — [not documented]
• YELLOW — [not documented]

🔧 Class Methods

• flush_all!
• log_subscribers
• logger

🛠️ Instance Methods

• color
• finish
• logger
• start

📦 Attributes

• logger (attr_writer)

📛 NAME 🚀 Quick Reference 📋 Constants 🔧 Class Methods 🛠️ Instance Methods 📦 Attributes