Separate semantic conventions from the specification.

text/0227-separate-semantic-conventions.md

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+# Separate Semantic Conventions
+
+Move Semantic Conventions outside of the main Specifications and version them
+separately.
+
+## Motivation
+
+We need to allow semantic conventions to evolve mostly independent of the
+overall OpenTelemetry specification. Today, any breaking change in a semantic
+convention would require bumping the version number of the entirety of the
+OpenTelemetry specification.
+
+## Explanation
+
+A new GitHub repository called `semantic-conventions` would be created in the
+OpenTelemetry organization.
+
+This would *initially* have the following structure:
+
+- Boilerplate files, e.g. `README.md`, `LICENSE`, `CODEOWNERS`, `CONTRIBUTING.md`
+- `Makefile` that allows automatic generation of documentation from model.
+- `semantic_conventions/` The set of YAML files that exist in
+  `{specification}/semantic_conventions` today.
+- `docs/` A new directory that contains human readable documentation for how to
+  create instrumentation compliant with semantic conventions.
+  - `resource/` - Contents of `{specification}/resource/semantic_conventions`
+  - `trace/` - Contents of `{specification}/trace/semantic_conventions`
+  - `metrics/` - Contents of `{specification}/metrics/semantic_conventions`
+  - `logs/`- Contents of `{specification}/logs/semantic_conventions`
+  - `schemas/` - A new location for [Telemetry Schemas](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/schemas/README.md)
+    to live. This directory will be hosted at
+    `https://opentelemetry.io/schemas/`
+
+Existing semantic conventions in the specification would be marked as
+moved, with documentation denoting the move, but preserving previous contents.
+
+Additionally, if the semantic conventions eventually move to domain-specific
+directory structure (e.g. `docs/{domain}/README.md`, with trace, metrics, events
+in the same file), then this can be refactored in the new repository, preserving
+git history.
+
+There will also be the following exceptions in the specification:
+
+- Semantic conventions used to implement API/SDK details will be fully specified in the `opentelemetry-specification` repo
+  and will not be allowed to change in the Semantic Convention directory.
+  - Error/Exception handling will remain in the specification.
+  - SDK configuration interaction w/ semantic convention will remain in the
+    specification. Specifically `service.name`.
+- The specification may elevate some semantic conventions as necessary for
+  compatibility requirements, e.g. `service.instance.id` and
+  [Prometheus Compatibility](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/compatibility/prometheus_and_openmetrics.md).
+
+These exceptions exist because:
+
+- Stable portions of the specification already rely on these conventions.
+- These conventions are required to implement an SDK today.
+
+As such, the Specification should define the absolute minimum of reserved or
+required attribute names and their interaction with the SDK.
+
+## Internal details
+
+The following process would be used to ensure semantic conventions are
+seamlessly moved to their new location. This process lists steps in order:
+
+- A moratorium will be placed on Semantic Convention PRs to the specififcation
+  repository. (Caveat that PRs related to this proposal would be allowed).
+- Interactions between Semantic Conventions and the Specification will be
+  extracted such that the Specification can place requirements on Semantic
+  Conventions and *normative* specification language will remain in the
+  core specification directories.
+- A new repository `open-telemetry/semantic-conventions` will be constructed with
+  the proposed format and necessary `Makefile` / tooling.
+  - The new repository would be created by using `git filter-branch` to preserve
+    all existing semantic convention history. *This means all existing
+    semantic conventions will be in the new repository*.
+  - GitHub Actions, `Makefile` tooling and contributing / readmes would be
+    updated for the separate repository.
+  - **Note: At this point, the new location for semantic conventions should
+      be adoptable/usable.**
+- Semantic conventions in the Specification will be marked as moved with
+  links to the new location.
+  - The semconv YAML files in the specification repository *will be deleted*.
+  - All semconv markdown files will be updated such that:
+    - They no longer generate from YAML files.
+    - They include a header denoting deprecation and move to the new repository.
+- Instrumentation authors will update their code generation to pull from the new
+  semconv repository instead of the specification repository.
+
+## Trade-offs and mitigations
+
+This proposal has a few drawbacks:
+
+- The semantic conventions will no longer be easily referenceable form the specification.
+  - This is actually a benefit. We can ensure isolation of convention from
+    specification and require the Specification to use firm language for
+    attributes it requires, like `service.name`.
+  - We will provide links from existing location to the new location.
+- Semantic Convention version will no longer match the specification version.
+  - Instrumentation authors will need to consume a separate semantic-convention
+    bundle from Specification bundle. What used to be ONE upgrade effort will
+    now be split into two (hopefully smaller) efforts.
+  - We expect changes from Semantic Conventions and Specification to be
+    orthogonal, so this should not add significant wall-clock time.
+- Existing PRs against semantic conventions will need to be regenerated.
+
+Initially this repository would have the following ownership:
+
+- Approvers
+  - [Christian Neumüller](https://github.com/Oberon00), Dynatrace
+  - [James Moessis](https://github.com/jamesmoessis), Atlassian
+  - [Joao Grassi](https://github.com/joaopgrassi), Dynatrace
+  - [Johannes Tax](https://github.com/pyohannes), Microsoft
+  - [Liudmila Molkova](https://github.com/lmolkova), Microsoft
+  - [Sean Marcinak](https://github.com/MovieStoreGuy), Atlassian
+  - [Ted Young](https://github.com/tedsuo), Lightstep
+- Approvers (HTTP Only)
+  - [Trask Stalnaker](github.com/trask)
+- Approvers (SchemaUrl Files)
+  - [Tigran Najaryan](github.com/tigrannajaryan)
+- Maintainers
+  - [Josh Suereth](github.com/jsuereth)
+  - [Armin Ruech](github.com/arminru)
+  - [Reiley Yang](github.com/reyang)
+
+That is, Maintenance would initially continue to fall on (a subset of) the
+Technical Committee. Approvers would start with existing semconv approvers in
+addition to targeted at HTTP semantic convention stability approvers and
+expand rapidly as we build momentum on semantic conventions.
+
+## Prior art and alternatives
+
+When we evaluate equivalent communities and efforts we see the following:
+
+- `OpenTracing` - had specification and [semantics](https://github.com/opentracing/specification/blob/master/semantic_conventions.md)
+  merged.
+- `OpenCensus` - had specification and semantics merged. However, OpenCensus
+  merged with OpenTelemetry prior to mass adoption or stable release of its
+  specification.
+- `Elastic Common Schema` - the schema is its own project / document.
+- `Prometheus` - Prometheus does not define rigid guidelines for telemetry, like
+  semantic conventions, instead relying on naming conventions and
+  standardization through mass adoption.
+
+## Open questions
+
+This OTEP doesn't address the full needs of tooling and codegen that will be
+needed for the community to shift to a separate semantic convention directory.
+This will require each SIG that uses autogenerated semantic conventions to
+adapt to the new location.
+
+The first version of the new location for semantic conventions may not follow
+the latest of the specification. There is reasoning to desire a `2.0` but the
+details will be discussed in the new repository location upon execution of this
+OTEP.
+
+## Future possibilities
+
+This OTEP paves way for the following desirable features:
+
+- Semantic Conventions can decide to bump major version numbers to accommodate
+  new signals or hard-to-resolve new domains without breaking the Specification
+  version number.
+- Semantic Conventions can have dedicated maintainers and approvers.
+- Semantic Conventions can restructure to better enable subject matter experts
+  (SMEs) to have approver/CODEOWNER status on relevant directories.
+- Semantic Conventions can adopt semantic versioning for itself, helping clearly
+  denote breaking changes to users.
+
+There is a desire to move semantic conventions to domain-specific directories
+instead of signal-specific. This can occur after the separation of the repository
+and will be proposed and discussed separately from this OTEP.
+
+For example:
+
+- `docs/`
+  - `signals/` - Conventions for metrics, traces + logs
+    - `http/`
+    - `db/`
+    - `messaging/`
+    - `client/`
+  - `resource/` - We still need resource-specific semantic conventions