Source Locations

How does Skylight track source locations?

Source Locations for Skylight

Skylight can help you pinpoint the locations in your code that correspond to events in the Event Sequence. As Skylight traces your code it will report the file names and line numbers (for your application code) or gem names (for external libraries). No more scouring your code trying to find out exactly where an expensive operation originated! As you browse your endpoint data, you will find these source locations in the detail card for each event:

If your app is connected to GitHub, you can click the link to go directly to that line in your source code for the specified commit.

What is a Source Location?

Skylight considers the ‘source location’ of an event to be the line in your code (or gem) that triggered the event. Because it would have significant performance costs to compute a complete call-stack for each event (and because that volume of data is usually unnecessary to provide answers), we instead locate the most relevant point of the call stack and report that as the source location.

Application Code

For trace events that originate in your app, we collect the file path and line number as the source location (we consider in-project files to be those that live within Rails.root or the directory where your Gemfile lives). Consider the following controller action, where we load a set of monsters from an Active Record scope:

  class MonstersController < ApplicationController
    def update
      monsters = Monster.active_at_night # <- The relation is built here

      monsters.each do |monster| # <- `SELECT * FROM monsters` is executed here
        monster.update(crepuscular: true)
      end

      render plain: :ok
    end
  end

The active_at_night scope (defined elsewhere) provides the logic needed to build the query, and the query is actually built on line 3 of the controller. However, since Active Record relations are lazy, and the query is not executed until data is accessed, the event is triggered on line 5 (in monsters.each), so that is what Skylight records as the source location for SELECT FROM monsters:

Gem Code

For trace events that originate in gems, we only report the gem name. For example, the Rails router comes from Action Pack, so we report that as the source location:

Synthetic Events

Occasionally you may see an event that does not have a source location, but instead says “source locations do not exist for synthetic events.” Synthetic events are events that are added by Skylight for informational purposes, but that do not necessarily represent code that was traced during a request. Examples include garbage collection and the root event for Rack requests; because these do not correspond to specific application or library code, they do not have a source location.

Change Detection

Skylight tracks changes across deploys for each source location. If the call site has moved between deploys (indicated by a yellow circle), we show the most recent location at the top with previous locations listed underneath.

In the above example, the same SQL query was detected in three different locations for the first deploy, and only one location for the subsequent deploy. Note that the deploys (and corresponding source locations) that are displayed are dependent on the range you have selected in the time explorer.

Source Locations Configuration

Enabling Source Locations

The Source Locations feature is enabled by default in Skylight 5.0.0 and above.

Disabling Source Locations

If you need to disable source locations you can set one of the following config options to false. If you do so, please get in touch with us and let us know why you needed to disable it.

# config/skylight.yml

source_locations_enabled: false

Or set SKYLIGHT_SOURCE_LOCATIONS_ENABLED=false in your environment.

Deploy Tracking

Source Locations works best if you have enabled deploy tracking, because it allows you to find an event’s source location for a particular moment in time, and track changes across deploys

Ignored Gems

Note that an event may sometimes be attributed to gem code that you did not expect (for example, a logger that wraps existing code). To avoid this you can add gem names to source_location_ignored_gems in your skylight.yml file. By default, we ignore skylight, activesupport, and activerecord.

# config/skylight.yml

source_location_ignored_gems:
  - dalli
  - redis

Alternatively, set SKYLIGHT_SOURCE_LOCATION_IGNORED_GEMS=dalli,redis in your environment.