Propagate context from front-end to back-end with custom instrumentation

[ppatel890]

Hi all,

We have setup some custom instrumentation for websockets to mimic a request/response lifecyle, and are trying to connect spans created on the front-end with spans created on the backend.

We have passed the span to the backend, and have access to the span context/traceId, etc. When we attempt to set the parent of the custom span we create on the backend, the two spans don't seem to get linked into a single trace:

const parentSpan = getSpanFromFrontend();
const span = tracer.startSpan(spanName, { parent: parentSpan })
// do some work
span.end()

I imagine one of the problems is the parentSpan has been serialized when sent through the websocket, and is a plain JS object, so setting the parent in startSpan doesn't work as expected. Any pointers on how we can connect these two manually instrumented spans would be super helpful.

Thanks!

[dyladan]

If you are using the latest version, the parent option to startSpan has been deprecated. You can either pass a context to startSpan or make the context active using the context API.

Pass context directly:

const span = tracer.startSpan(spanName, {}, api.setSpan(api.context.active(), parent))

set context active:

api.context.with(api.setSpan(api.context.active(), parent), () => {
  const span = tracer.startSpan(spanName); // this span gets the parent from the active context automatically
});

[vmarchaud]

I would just add to look how the B3 Propagator works, specially getting the current span context and injecting it back into the context.

[mannyluvstacos]

Off-topic comment.

These discussions have been really helpful.

I am new to the Opentelemetry-js ecosystem and I am trying to wrap my head around setting the active context.
I will post my own question specifically around opentelemetry-node and opentelemetry/instrumentation-graphql