Cross application tracing in Ruby

Important

As of version 8.0.0 of the Ruby agent, distributed tracing is on by default. Distributed tracing improves on cross application tracing and is recommended for large, distributed systems.

Cross application tracing is deprecated in favor of distributed tracing and will be removed in a future agent version. If you need to continue using cross application tracing, such as for a non-standard middleware framework, see the configuration information in this document.

Requirements

Follow these requirements to use cross application tracing with the Ruby agent:

Make sure the requests being instrumented use a supported HTTP client library.
Install or update to the latest Ruby agent (version 3.5.5.38 or higher).
Follow the requirements for middleware installation.

Middleware installation

Cross application tracing works with Rack, and therefore requires Rails 2.3 or greater, or another compatible framework.

If you use Rails, the Ruby agent will install the middleware automatically.
If you use a different Rack-based framework, manually add the NewRelic::Rack::AgentHooks middleware to your stack.

Configuration

Cross application tracing can be controlled by a configuration flag. As of version 8.0.0 of the Ruby agent, the default for cross_application_tracer.enabled is false, even when unspecified. To enable cross application tracing, you must set this flag to true amd set distributed_tracing.enabled to false.

cross_application_tracer:
  enabled: true
distributed_tracing:
  enabled: false

For versions below 8.0.0, cross application can be configured using the following setting.

cross_application_tracer:
  enabled: true

Cross application trace measurements

The external measurement (from the calling application) will always be larger than the internal measurement (from the called application). The external measurement is collected by New Relic's instrumentation of the HTTP client library (such as Net::HTTP). The internal measurement is taken by New Relic's instrumentation of the web framework (such as Rails) in the called application.

Here are some of the major components included in the external measurement that are not included in the internal measurement:

Time for the frontend web server on the receiving host to process the request and send it to the backend web server on the receiving host
Time for the request to be parsed in the backend web server on the receiving host
Time for the request to "percolate" through Rack middlewares on the receiving host
Time for the web framework to route the request to the appropriate controller action
Once the web framework has routed it to the appropriate controller action, this is where the internal measurement happens. Then:
Time for the request to "percolate" back up through the Rack middlewares
Network latency to write the response back to the requesting server
Time on the requesting host for the HTTP response to be parsed by the HTTP client library

Some of these components are easier to control than others. For example, to capture timings for the Receiving host items above, make sure you have request queue monitoring set up on the receiving application.

Get distributed tracing

As of version 8.0.0 of the Ruby agent, distributed tracing is on by default. Distributed tracing is an improvement on cross application tracing and is recommended for large, distributed systems.