Declarative configration default values

[pellared]

TL;DR; It’s unclear whether omitted fields in declarative configuration should use SDK defaults or configuration-defined defaults. This ambiguity leads to inconsistent behavior across languages (e.g., OTLP exporter protocol defaults differ).

While reading the declarative configuration SDK specification, I’ve run into difficulty understanding what the default values should be when a field is omitted. Specifically:

• Should the omitted field take on the SDK’s built-in default, or
• Should it use a configuration-defined default?

The specification section here does not make this distinction clear:

opentelemetry-specification/specification/configuration/sdk.md

Lines 263 to 271 in 7295dc4

	If a property has a default value defined (i.e. is _not_ required) and is
	missing or present but null, Create MUST ensure the SDK component is configured
	with the default value. If a property is required and is missing or present but
	null, Create SHOULD return an error. For example, if configuring
	the [span batching processor](../trace/sdk.md#batching-processor) and
	the `scheduleDelayMillis` property is missing or present but null, the component
	is configured with the default value of `5000`. However, if the `exporter`
	property is missing or present but null, Create fails fast since there is no
	default value for `exporter`.

My assumption is that the defaults illustrated in this example are intended to apply:
https://github.com/open-telemetry/opentelemetry-configuration/blob/main/examples/sdk-migration-config.yaml

However, the specification doesn’t explicitly say so.

It’s also possible that the spec intends for SDK defaults to be used instead. If that’s the case, it could lead to inconsistencies, since SDK defaults may differ between languages. For example, the OTLP exporter’s default protocol is grpc in some SDKs and http/protobuf in others. Such discrepancies could be confusing for end users.

Additionally, default values could be explicitly defined in the JSON Schema using the default annotation:
https://json-schema.org/understanding-json-schema/reference/annotations.

[pellared]

CC @Kielek @ysolomchenko

[ysolomchenko]

While implementing the default, I used comments from this file, in most cases, it describes the behavior for If omitted or null