Specify whether keys in attribute lists are expected to be unique

[achille-roussel]

Hello,

I'm opening this issue as a follow up to a discussion in open-telemetry/opentelemetry-proto#346 (comment) where it appeared unclear whether processors of telemetry data should be designed to support repeating keys in attribute lists.

I do not know what the right answer should be, but I couldn't find a definitive statement in the Attributes Specification to help guide design decisions that would depend on this information.

There is one mention of "unique keys" in the Attributes Limits, but it is unclear whether "unique" refers to keys being unique or the number of keys in the attribute lists:

• set a limit of unique attribute keys such that:
  • for each unique attribute key, addition of which would result in exceeding the limit, SDK MUST discard that key/value pair.

Let me know if I can provide any more context that could help address this issue.

[bogdandrutu]

@tigrannajaryan do you remember this?

I remember for sure that we said we have a map "semantic" there https://github.com/open-telemetry/opentelemetry-proto/blob/main/opentelemetry/proto/resource/v1/resource.proto#L28, if that is not the case we have huge problem in the collector :))

[Oberon00]

I remember (actually I just saw it by accident when I opened my Thread list) we had a short Thread on Slack 4 months ago: https://cloud-native.slack.com/archives/C01N7PP1THC/p1630920628020600?thread_ts=1630790115.020300&cid=C01N7PP1THC

At that time I wrote:

There is https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#set-attributes

Setting an attribute with the same key as an existing attribute SHOULD overwrite the existing attribute's value.

Which currently only applies to Span.SetAttribute. IMHO we should move that to the overall attribute definition, otherwise it doesn't even apply to https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#span-creation

Would not be surprised if it is still unspecified for Span Creation, Resources, Metric attributes (at least I did not follow up at that time).

[tigrannajaryan]

We need to clarify the spec. It is not allowed (and never was AFAIK) to have more than one attribute with the same key.

[tigrannajaryan]

Clarification in proto open-telemetry/opentelemetry-proto#350

Probably also need a clarification in the spec.