How to disable OpenTelemetry after it is initialized?

Question

I'm pretty new to OpenTelemetry world.

We have a PHP backend and Node.js Express server for SSR. For starters we want to collect tracing data for some subset of the requests. So, when a request comes to PHP server it calculates some chances, and if it is true it starts collecting tracing data from PHP side. Then comes to a point that it makes a HTTP request to Express server to render some JS and HTML. And the option of if the tracing is enabled or not is passed to Express server endpoint via PHP.

The problem: Tracing is setup since the beginning of the Express server, to track everything correctly. But the option to stop tracing or not, comes from backend to Express server in a route. So, when the route is invoked I need to check the body and decide to stop tracing or not.

I can't do it with samplers. Because like I said the decision to not send any tracing comes to the endpoint. Not since the beginning.

How can I stop the tracing and don't send anything? Even if some data were already collected.

Answer 1

In OpenTelemetry, the signal between services on whether to trace or not is called Context Propagation and should be propagated between services (in your case, from PHP to Express) via the traceparent header. This header contains the parent trace id, span id, flags which indicate whether the parent is sampled or not. This is how traces across your two different services can be linked together (by having the same trace id, and having the root Express span linked to the PHP span which initiated the http request).

What you are describing, where PHP makes the decision to trace (via sampling), and Express should only trace based on PHP's decision, can be handled by a "Parent-based, always-off" sampler in Express, and probably a "Parent-based, trace id ratio-based" sampler in PHP. You won't need to discard any data from Express, since it will only trace if the parent was traced.

Lastly, you need to ensure that your outbound HTTP requests from PHP inject the appropriate traceparent header:

$request = new Request('GET', 'express-app/some/route');
$carrier = [];
TraceContextPropagator::getInstance()->inject($carrier);
foreach ($carrier as $name => $value) {
    $request = $request->withAddedHeader($name, $value);
}
$request->send();