How to end a span in a different service? Job queue use case.

[KyleAMathews]

I have a service where a webhook comes in and then a job is queued. I'd like to start a span there and then end it in the worker that processes the job.

How do I do this? My thought is we start a span when the webhook comes in and then set a traceparent on the job and then rehydrate the span somehow in the job worker?

[Flarna]

I think you should create two spans here. One which is started when the job is queued and ended when the job is done. And a second span in the worker which is started when the job itself starts and ended when it is done in worker. This has the extra benefit that you see also the time a job lies around in the queue.

As main and worker thread can't share JS objects the SDK the OTel setup would be somewhat similar as if you would have two processes. One SDK instance in the main thread and one in each worker. To link the span you have to inject the span in main thread using the propagation API and extract it in worker. So some side channel in your workitem to transfer traceparent (or similar) is needed.

[KyleAMathews]

hmm yeah that works — the bummer is we'd prefer a single parent span to easily track how long the whole operation takes. Thanks!

[Flarna]

Well, then just skip starting and stopping the second span - inject/extract is also no longer needed. But this avoids that you may add linked spans inside worker to get more details.
I assume here that your worker tasks send something back to main thread once they finished. If this is not the case it will be hard to end the span at correct time.

[KyleAMathews]

Another way to do this I thought of is passing the start time to the worker and then setting that there as the startTime to the worker span. We can't see the enqueue work spans this way but the worker spans are much more important.

[Flarna]

If you do this in worker you have to instantiate one SDK per worker thread so depending on the number of workers there might be quite some overhead.
In case the spans in worker should be linked/a child of to some parent span (either from calling process or main thread) you have to ensure to forward also the corresponding traceparent.