Spec: SpanProcessor.onStart MUST receive a mutable "read/write" span

[arminru]

See https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/sdk.md#onstart

Currently only an immutable ReadableSpan is passed:

opentelemetry-js/packages/opentelemetry-tracing/src/SpanProcessor.ts

Lines 29 to 34 in dc8082a

	/**
	* Called when a {@link ReadableSpan} is started, if the `span.isRecording()`
	* returns true.
	* @param span the Span that just started.
	*/
	onStart(span: ReadableSpan): void;

[dyladan]

Thanks for reporting this. The concrete span sent to onStart is already a Span so this should be a very simple change.

[mayurkale22]

Just curious to know the use-case where we need writeable span in SpanProcessor.onStart?

[arminru]

One use case for SpanProcessors is to add certain attributes on a span. The thread ID (of the starting span), for example, is a property that calls for being added automatically rather than bugging users with that. The code.* attributes could also be added automatically by a SpanProcessor which inspects the call stack.

[dyladan]

@mayurkale22 user @GasimGasimzada has already asked for this in gitter too

[GasimGasimzada]

@mayurkale22

Just curious to know the use-case where we need writeable span in SpanProcessor.onStart?

For example, we have a tracer in the browser and for all spans we add http user agent, the current URL, and other custom attributes that are applicable to our app for all spans. In my case, I extends SimpleSpanProcessor and overrode onStart, which adds all the tags. Since the underlying JS object already had all the necessary attributes, I casted ReadableSpan to Span (it is hacky but it got the job done for now):

const span = readableSpan as unknown as Span;