AppProfiler

Profiling is critical to providing an understanding of an application's performance.

AppProfiler aims to provide a common framework for performance profiling for Rails applications.

Installation

To install app_profiler you need to include it in your Gemfile.

Profiling middleware

Configuration

This gem automatically injects the AppProfiler::Middleware middleware into your Rails application.

This middleware can be disabled by using:

AppProfiler.middleware.disabled = true
# OR
Rails.application.config.app_profiler.middleware_disabled = true

Trigger profiling

Profiling can be triggered in one of two ways:

1. Using the profile key in the query string of the URL.
   • Query string format: /?[key=value]&...
2. Using the X-Profile key in the request headers.
   • X-Profile header format: [<key>=<value>];...

You can configure the profile header using:

AppProfiler.profile_header = "X-Profile"
# OR
Rails.application.config.app_profiler.profile_header = "X-Profile"

Here are some examples:

1. /?profile=cpu&interval=2000&autoredirect=1&ignore_gc=1
2. Set X-Profile to mode=wall;interval=1000;context=test-directory;autoredirect=1

Possible keys:

Note that the autoredirect feature can be turned on for all requests by doing the following:

AppProfiler.autoredirect = true
# OR
Rails.application.config.app_profiler.autoredirect = true

To customize the redirect location you can provide a proc:

AppProfiler.profile_url_formatter = ->(upload) { "https://host.com/custom/#{upload.name}" }
# OR
Rails.application.config.app_profiler.profile_url_formatter = ->(upload) { "https://host.com/custom/#{upload.name}" }

When profiling is triggered, the middleware will generate the profile through StackProf and upload the profiles to your specified storage. For example, the default configuration would upload profiles to file storage.

When using a cloud storage provider, you can configure the target bucket name using:

AppProfiler.storage.bucket_name = "new-bucket-name"
# OR
Rails.application.config.app_profiler.storage_bucket_name = "new-bucket-name"

Access control

You may restrict the storing of profiling results by defining your own Middleware based on AppProfiler::Middleware and changing the after_profile hook method to return false for such cases.

For example, the following middleware only stores the profiling results if a disallow_profiling key was not added to the request.env while processing the request.

class AppProfilerAuthorizedMiddleware < AppProfiler::Middleware
  def after_profile(env, params)
    !env.key?("disallow_profiling")
  end
end

You can also restrict running profiling at all by using before_profile. For example you may wish to prevent anonymous users triggering the profiler:

class AppProfilerAuthorizedMiddleware < AppProfiler::Middleware
  def before_profile(env, params)
    current_user.present?
  end
end

The custom middleware can then be configured like the following:

Rails.application.config.app_profiler.middleware = AppProfilerAuthorizedMiddleware

Profile Server

This option allows for profiles to be passively collected via an HTTP endpoint, inspired by golang's built-in pprof server.

A minimal Rack app runs a minimal (non-compliant) HTTP server, which exposes an endpoint that allows for profiling. For security purposes, the server is bound to localhost only. The HTTP server is built using standard library modules only, in order to keep dependencies minimal. Because it is an entirely separate server, listening on an entirely separate socket, this should not interfere with any existing application routes, and should even be usable in non-web apps.

This allows for two main use cases:

• Passive profile collection in production
  • Periodically profiling production apps to analyze them responding to real workloads
  • Providing a statistical, long-term view of the "hot paths" of the workload
• Local development profiling
  • Can be used to get a profile "on demand" against a development server

Configuration

If using as a railtie, only a single option needs to be set:

config.app_profiler.server_enabled = true

Alternatively, the server can be directly started with:

AppProfiler::Server.start

The default duration (in seconds), for requests without a duration parameter, can also be set via the railtie config.

AppProfiler.server.duration = 30
# OR
Rails.application.config.app_profiler.server_duration = 30

The server supports both TCP and Unix sockets for its transport. It is recommended to use TCP for local development, and Unix sockets for production:

AppProfiler.server.transport = AppProfiler::Server::TRANSPORT_UNIX
# OR
Rails.application.config.app_profiler.server_transport = AppProfiler::Server::TRANSPORT_UNIX

It is possible, but not recommended, to hardcode the listen port to be used in TCP server mode with:

AppProfiler.server.port = 8080
# OR
Rails.application.config.app_profiler.server_port = 8080

If this is done in production and it can cause port conflicts with multiple instances of the app, which is another reason why the Unix transport is preferred for production.

Discovering the port or socket path

In general, the server should be run without setting the port, in which case any free TCP port may be used. To determine what the port is, check the application logs, or resolve it from the special "Magic file" which contains a mapping of pid to port:

$ PID=49825
$ port_file=$(ls -1 /tmp/app_profiler/profileserver-$PID-port-*)
$ PORT=$(echo $port_file | sed 's/.*port-\([[:digit:]]*\)-.*/\1/g')
$ echo $PORT
60160

This approach is intended to be "machine friendly" so that an external profiling agent can easily detect what port to profile on.

For the Unix mode, this is even easier as the file simply includes the PID in it, and this will be the file handle to use for the Unix socket:

$ PID=41016
$ SOCK=$(ls -1d /tmp/app_profiler/* | grep $PID.sock)
$ echo $SOCK
/tmp/app_profiler/app-profiler-41016.sock

Collecting a profile

The API is very simple, and passes supported parameters directly to stackprof.

See the possible keys for additional documentation on the supported parameters.

For example, to collect a heap profile for 60 seconds, counting every 10th allocation:

curl "http://127.0.0.1:$PORT/profile?duration=60&mode=object&interval=10"

Usage with speedscope directly

By default the server will allow CORS. This can be disabled if it presents a problem, but it should be generally safe given that the server listens for requests on localhost only, which is already a private network address.

This can be used with a local instance of speedscope to directly initiate profiling from the browser. Assuming speedscope is running locally on port 9292, and the profile server is running on port 57510, the server address can be URL encoded, and passed to speedscope via #profileURL:

http://127.0.0.1:9292/#profileURL=http%3A%2F%2F127.0.0.1%3A57510%2Fprofile%3Fduration%3D1

Profiling manually

AppProfiler can be used more simply to profile blocks of code. Here's how:

report = AppProfiler.run(mode: :cpu) do
  # ...
end

report.view # opens the profile locally in speedscope.

Profile files can be found locally in your rails app at tmp/app_profiler/*.json.

Storage backends

Profiles are stored based on the defined storage class. At the moment, the gem only supports file-based and remote storage via Google Cloud Storage. the default backend is file storage.

You can use a different backend with the following configuration:

AppProfiler.storage = AppProfiler::Storage::GoogleCloudStorage
# OR
Rails.application.config.app_profiler.storage = AppProfiler::Storage::GoogleCloudStorage

Credentials for the selected storage can be set using the following configuration (Google Cloud Storage expects the path to a JSON file, or the JSON contents):

AppProfiler.storage.credentials = { "key" => "value" }
# OR
Rails.application.config.app_profiler.storage_credentials = { "key" => "value" }

Note that in development mode the file isn't uploaded. Instead, it is viewed via the Middleware::ViewAction. If you want to change that, use the middleware_action configuration:

Rails.application.config.app_profiler.middleware_action = AppProfiler::Middleware::UploadAction

Running tests

bin/setup && bundle exec rake