Apply rate sampling on ending spans instead when starting

[xolf]

I've introduced spans into my Laravel project. To test the integration I've set the sample rate to 1. I've also added some custom spans into the code for long running/repeating tasks, like

$span = Flare::tracer()->startSpan('sleeping');
sleep(5);
Flare::tracer()->endSpan($span); // Throws "No span to end" Exception when the request is sampled

For production use I have now reduced the sample rate and start received some No span to end exceptions. After some quick digging I've found that startSpan() does return null if the trace is not recorded due to sampling.

A more clean code/less noisy behaviour for seems to be that startSpan() returns a span and endSpan() decided due to sampling whether or not it's going to be recorded or are there any ideas/preferences behind the current implementation?

[rubenvanassche]

Hi @xolf,

While I get that this behaviour is a bit weird it actually allows us to keep track if a span did start a trace or not, that's the reason why we return null. Also it makes creating spans more lightweight since they will only be created when actually needed (although that behaviour is more usefull in the recorders).

Have you seen the span method which takes a closure? This might solve the issue since it only requires one call the tracer. Albeit that sometimes adding extra closures to your code isn't possible.