How does processing work with multiple span processors?

[DenisAngeletta]

Hi everyone!

Having multiple span processors added to the TracerProvider is possible. I wanted to ask how processing looks like in that case. I'd imagine something like:
onStart(1) -> onEnd(1) -> onStart(2) -> onEnd(2)

There are multiple ways to add SpanProcessors as far as I know:

1:

@Override
public void customize(AutoConfigurationCustomizer autoConfiguration) {
    autoConfiguration.addTracerProviderCustomizer(
        (sdkTracerProviderBuilder, configProperties) -> sdkTracerProviderBuilder.addSpanProcessor(new SpanProcessorOne())
                .addSpanProcessor(new SpanProcessorTwo()));
}

2:

@Override
public void customize(AutoConfigurationCustomizer autoConfiguration) {
    autoConfiguration.addTracerProviderCustomizer(
        (sdkTracerProviderBuilder, configProperties) -> sdkTracerProviderBuilder.addSpanProcessor(
    SpanProcessor.composite(new SpanProcessorOne(), new SpanProcessorTwo())));
}

I'm not sure how (1) would work but (2) essentially creates a MultiSpanProcessor in which to my understanding onStart is called twice before any call of onEnd. Am I understanding this correctly?

The reason I am asking this because I need a workaround where I setAttributes when a Span is ending. If the processing of spans happens like onStart(1) -> onEnd(1) -> onStart(2) -> onEnd(2) then I would also be happy.

[laurit]

When span is started onStart is called on all span processors when span is ended onEnd is called. These methods are called synchronously.
I believe that whichever way you add the span processors they'll be wrapped in a MultiSpanProcessor.