Custom Instrumentation

Distributed tracing works out of the box for supported frameworks and when tracing is enabled. If you're using an unsupported framework or don't want to turn on tracing, you can set up custom instrumentation for distributed tracing.

This page describes how to manually propagate trace information into and out of your Ruby application. All you have to do is to make sure your application extracts incoming headers and to set those headers again when making an outgoing request within your application.

Incoming tracing information has to be extracted and stored in memory for later use. Sentry provides the continue_trace function to help you with this. Incoming tracing information can come from different places:

  • In a web environment, it's sent with HTTP headers, for example, by another Sentry SDK used in your frontend project.
  • In a job queue, like Sidekiq / DelayedJob, it can be retrieved from meta or header variables.
  • You also can pick up tracing information from environment variables.

Here's an example of how to extract and store incoming tracing information using continue_trace in a Rack application where env is a typical hash of incoming HTTP headers:

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# rack application
def call(env)
  transaction = Sentry.continue_trace(env, name: 'transaction', op: 'op')
  Sentry.start_transaction(transaction: transaction)
end

Sentry's continue_trace function will extract the given headers, try to find the sentry-trace and baggage headers, and store them in memory for later use.

For distributed tracing to work, the two headers sentry-trace and baggage, must be added to outgoing requests. If you pregenerate HTML on the server-side, you might want to take a look at Inject Tracing Information into Rendered HTML, which describes how to pass on tracing information through HTML meta tags.

The Ruby SDK intercepts all requests using net/http and adds this tracing information automatically to outgoing requests.

If the gem you're using for making requests doesn't use net/http internally, you can generate this tracing information with the Sentry SDK's get_traceparent, get_baggage and get_trace_propagation_headers functions. Here's an example:

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# using custom HTTPRequest implementation
HTTPRequest.get('https://example.com', headers: Sentry.get_trace_propagation_headers)

In this example, tracing information is propagated to the project running at https://example.com. If this project also uses a supported Sentry SDK, it will extract and save the tracing information for later use.

The two services are now connected with your custom distributed tracing implementation.

To propagate tracing information into JavaScript running in rendered HTML, you have to inject HTML meta tags in your rendered HTML. For example, in a rails layout, you can inject the meta tags as follows:

app/views/layouts/application.html.erb
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<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8" />
    <%= Sentry.get_trace_propagation_meta.html_safe %>
  </head>
  <body>
    <p>This is a website.</p>
  </body>
</html>

The rendered template will then look something like this:

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<!doctype html>
<html>
  <head>
    <meta charset="UTF-8" />
    <meta
      name="sentry-trace"
      content="98e5de1c09e444469475855d313e22c3-b995922394e8433d"
    />
    <meta
      name="baggage"
      content="sentry-trace_id=98e5de1c09e444469475855d313e22c3,sentry-environment=prod,sentry-public_key=..."
    />
  </head>
  <body>
    <p>This is a website.</p>
  </body>
</html>

If you make outgoing requests from your project to other services, check if the headers sentry-trace and baggage are present in the request. If so, distributed tracing should work.

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