faas.id incorrectly used as span attribute

[dyladan]

faas.id is used as a span attribute here, but should be a resource attribute. I left it here to make this PR as small as possible while passing and changing no behavior. I will create a follow up bug to fix this.

Originally posted by @dyladan in #440 (comment)

[dyladan]

/cc @anuraaga @willarmiros

[dyladan]

@willarmiros mind if I assign this to you?

[willarmiros]

@dyladan Sure!

[willarmiros]

It turns out this is actually a known issue. The faas.id cannot be a resource attribute because resource attributes must be set at initialization time and are thereafter immutable. However a Lambda function's ARN (which is its faas.id) is only readable from the context of a function invocation. So it is impossible to assign it at initialization time and is made a span attribute as a workaround instead.

Here's the original spec issue pointing this out: open-telemetry/opentelemetry-specification#1261

This would either be solved by:

1. Getting the Lambda team to expose the ARN at startup (we're trying!), or
2. Adding resource attributes post-creation (e.g. via auto-discovery) opentelemetry-specification#1298, which would allow some resource attribs (like faas.id) to be mutable

[github-actions]

This issue is stale because it has been open 60 days with no activity. Remove stale label or comment or this will be closed in 14 days.

[dyladan]

@willarmiros looks like there has been no spec movement on this since trace GA. Think now might be a good time to try to tackle this?

[willarmiros]

We're talking about related items in the sampling SIG with respect to exposing resource attributes to samplers to inform sampling decisions & whether the Resources should be provided for every sampling decision or just at Sampler creation - I'll bring up this issue at the next sampling SIG

[dyladan]

@willarmiros any update here?

[willarmiros]

Unfortunately due to internal limitations Lambda is not able to expose its ARN at startup. So it seems we are blocked on open-telemetry/opentelemetry-specification#1298 to get this fixed.

[Flarna]

Meanwhile spec allows to set faas.id as span attribute instead as resource attribute: On some cloud providers, it may not be possible to determine the full ID at startup, so consider setting faas.id as a span attribute instead.

For AWS it would be possible to set it on resource by delaying resource detection until first invocation but on e.g. azure a single OS process (and therefore single global TracerProvider) may host several functions which have different ids therefore resource is not applicable at all.

see: open-telemetry/opentelemetry-specification#2502

[dyladan]

interesting I missed that wording in the spec. thanks for clarifying. i'll close this then