What is the point of the output attribute of a stanza/operator?

[ChristianCiach]

The filelog README says:

Operators will output to the next operator in the pipeline. The last operator in the pipeline will emit from the receiver. Optionally, the output parameter can be used to specify the id of another operator to which logs will be passed directly.

What's the point? Since the output value cannot be an evaluated expression (I think), this output attribute can only be used to skip certain operators unconditionally. At this point, I could also just remove/comment the operators I want to skip. Or is there some use-case I am missing?

As far as I know, the only way to conditionally skip operators is to use the router operator.

[ChristianCiach]

I found the documentation that contains the necessary information: https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/v0.128.0/pkg/stanza/docs/types/operators.md#non-linear-sequences

So I was half-right: To construct conditional operator-sequences, you have to use the router operator. But the output attribute of an operator is still useful for operators that are themselves only executed conditionally as a router target, in cases where you want to break out of a branch, similar to the break keyword for switch-cases in most programming languages.

[ChristianCiach]

This also means that the output attribute is not even strictly necessary. I guess this example from the docs...

receivers:
  filelog:
    include: my-log.json
    operators:
      - type: router
        routes:
          - expr: 'body matches "^{.*}$"'
            output: json_parser
          - expr: 'body startsWith "ERROR"'
            output: error_parser
      - type: json_parser
        output: remove # send from here directly to the 'remove' operator
      - type: regex_parser
        id: error_parser
        regex: ... # regex appropriate to parsing error logs
      - type: remove
        field: attributes.foo
      - type: add
        key: attributes.bar
        value: baz

could be rewritten like this:

receivers:
  filelog:
    include: my-log.json
    operators:
      - type: router
        routes:
          - expr: 'body matches "^{.*}$"'
            output: json_parser
          - expr: 'body startsWith "ERROR"'
            output: error_parser
      - type: json_parser
      - type: router
        id: poor-mans-output-hack
        routes: []
        default: remove
      - type: regex_parser
        id: error_parser
        regex: ... # regex appropriate to parsing error logs
      - type: remove
        field: attributes.foo
      - type: add
        key: attributes.bar
        value: baz

Of course, this is ugly and I am glad we don't have to do that. But it still shows that we could do without the output field in the operator struct.

[ChristianCiach]

I am happy to admit that my initial assumption was wrong: Operators can be conditional without a router!

It looks like every operator that has an on_error attribute also has an if attribute:

opentelemetry-collector-contrib/pkg/stanza/operator/helper/transformer.go

Lines 30 to 34 in 723a25e

	type TransformerConfig struct {
	WriterConfig `mapstructure:",squash"`
	OnError string `mapstructure:"on_error"`
	IfExpr string `mapstructure:"if"`
	}

I found this imformation burried in the container parser docs: https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/v0.128.0/pkg/stanza/docs/operators/container.md#configuration-fields

It doesn't seem to be mentioned anywhere in any document in https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/v0.128.0/pkg/stanza/docs/types, which seems like an oversight.