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Clarify declarative config default requirements #4780

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32 changes: 23 additions & 9 deletions specification/configuration/sdk.md
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Original file line number Diff line number Diff line change
Expand Up @@ -260,15 +260,29 @@ Interpret configuration model and return SDK components.
The multiple responses MAY be returned using a tuple, or some other data
structure encapsulating the components.

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`.
Create requirements around default and null behavior are described below. Note that
[`defaultBehavior` and `nullBehavior`](https://github.com/open-telemetry/opentelemetry-configuration/blob/main/CONTRIBUTING.md#json-schema-source-and-output)
are defined in the configuration data model.

* If a property is present and the value is non-null, Create MUST use the value.
* If a property is not present, Create MUST use `defaultBehavior`.
* If a property is present and the value is null, Create MUST use the
`nullBehavior`, or `defaultBehavior` if `nullBehavior` is not set.
* If a property is required, and not present, Create MUST return an error.
* If a property is not nullable, and present and the value is null, Create MUST
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@carlosalberto carlosalberto Dec 5, 2025

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Minor nit: I had to read this twice to understand this bullet. Maybe there's clearer wording but I couldn't come with an improvement myself though 😓

return an error.

A few examples to illustrate:

* If configuring [`BatchSpanProcessor`](https://github.com/open-telemetry/opentelemetry-configuration/blob/main/schema-docs.md#batchspanprocessor-)
and `schedule_delay` is not present or present but null, the component is
configured according to the `defaultBehavior` of `5000`.
* If configuring `BatchSpanProcessor` and `exporter` is not present or present
but null, Create returns an error since `exporter` is required and not
nullable.
* If configuring [`SpanExporter`](https://github.com/open-telemetry/opentelemetry-configuration/blob/main/schema-docs.md#spanexporter)
and `console` is present and null, the component is configured with a
`console` exporter with default configuration since `console` is nullable.

When encountering a reference to
a [SDK extension component](#sdk-extension-components) which is not built in to
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