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api: add API_VERSIONING.md. (envoyproxy#8399)
This captures the API versioning guidelines implied by https://docs.google.com/document/d/1xeVvJ6KjFBkNjVspPbY_PwEDHC7XPi0J5p1SqUXcCl8/edit#heading=h.xgk8xel154p and splits apart the Envoy API and internal implementation breaking change policies. Some of the policy decisions (e.g. not allowing vNalpha to be hand edited, how we do manual breaking changes, etc.) have been added to these guidelines based on recent experience with protoxform and mechanical major version upgrade work, they are not part of the original stable API versioning work. Risk level: Low Testing: Formatting and docs build. Fixes envoyproxy#8371 Signed-off-by: Harvey Tuch <[email protected]>
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| # API versioning guidelines | ||
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| The Envoy project (and in the future [UDPA](https://github.com/cncf/udpa)) takes API stability and | ||
| versioning seriously. Providing stable APIs is a necessary step in ensuring API adoption and success | ||
| of the ecosystem. Below we articulate the API versioning guidelines that aim to deliver this | ||
| stability. | ||
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| # API semantic versioning | ||
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| The Envoy APIs consist of a family of packages, e.g. `envoy.admin.v2alpha`, | ||
| `envoy.service.trace.v2`. Each package is independently versioned with a protobuf semantic | ||
| versioning scheme based on https://cloud.google.com/apis/design/versioning. | ||
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| The major version for a package is captured in its name (and directory structure). E.g. version 2 | ||
| of the tracing API package is named `envoy.service.trace.v2` and its constituent protos are located | ||
| in `api/envoy/service/trace/v2`. Every protobuf must live directly in a versioned package namespace, | ||
| we do not allow subpackages such as `envoy.service.trace.v2.somethingelse`. | ||
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| Minor and patch versions will be implemented in the future, this effort is tracked in | ||
| https://github.com/envoyproxy/envoy/issues/8416. | ||
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| In everyday discussion and GitHub labels, we refer to the `v2`, `v3`, `vN`, `...` APIs. This has a | ||
| specific technical meaning. Any given message in the Envoy API, e.g. the `Bootstrap` at | ||
| `envoy.config.bootstrap.v3.Boostrap`, will transitively reference a number of packages in the Envoy | ||
| API. These may be at `vN`, `v(N-1)`, etc. The Envoy API is technically a DAG of versioned package | ||
| namespaces. When we talk about the `vN xDS API`, we really refer to the `N` of the root | ||
| configuration resources (e.g. bootstrap, xDS resources such as `Cluster`). The | ||
| v3 API bootstrap configuration is `envoy.config.bootstrap.v3.Boostrap`, even | ||
| though it might might transitively reference `envoy.service.trace.v2`. | ||
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| # Backwards compatibility | ||
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| In general, within a package's major API version, we do not allow any breaking changes. The guiding | ||
| principle is that neither the wire format nor protobuf compiler generated language bindings should | ||
| experience a backward compatible break on a change. Specifically: | ||
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| * Fields should not be renumbered or have their types changed. This is standard proto development | ||
| procedure. | ||
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| * Renaming of fields or package namespaces for a proto must not occur. This is inherently dangerous, | ||
| since: | ||
| * Field renames break wire compatibility. This is stricter than standard proto development | ||
| procedure in the sense that it does not break binary wire format. However, it **does** break | ||
| loading of YAML/JSON into protos as well as text protos. Since we consider YAML/JSON to be first | ||
| class inputs, we must not change field names. | ||
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| * For service definitions, the gRPC endpoint URL is inferred from package namespace, so this will | ||
| break client/server communication. | ||
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| * For a message embedded in an `Any` object, the type URL, which the package namespace is a part | ||
| of, may be used by Envoy or other API consuming code. Currently, this applies to the top-level | ||
| resources embedded in `DiscoveryResponse` objects, e.g. `Cluster`, `Listener`, etc. | ||
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| * Consuming code will break and require source code changes to match the API changes. | ||
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| * Some other changes are considered breaking for Envoy APIs that are usually considered safe in | ||
| terms of protobuf wire compatibility: | ||
| * Upgrading a singleton field to a repeated, e.g. `uint32 foo = 1;` to `repeated uint32 foo = 1`. | ||
| This changes the JSON wire representation and hence is considered a breaking change. | ||
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| * Wrapping an existing field with `oneof`. This has no protobuf or JSON/YAML wire implications, | ||
| but is disruptive to various consuming stubs in languages such as Go, creating unnecessary | ||
| churn. | ||
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| * Increasing the strictness of | ||
| [protoc-gen-validate](https://github.com/envoyproxy/protoc-gen-validate) annotations. Exceptions | ||
| may be granted for scenarios in which these stricter conditions model behavior already implied | ||
| structurally or by documentation. | ||
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| The exception to the above policy is for API versions tagged `vNalpha`. Within an alpha major | ||
| version, arbitrary breaking changes are allowed. | ||
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| Note that changes to default values for wrapped types, e.g. `google.protobuf.UInt32Value` are not | ||
| governed by the above policy. Any management server requiring stability across Envoy API or | ||
| implementations within a major version should set explicit values for these fields. | ||
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| # API lifecycle | ||
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| The API lifecycle follows a calendar clock. At the end of Q3 each year, a major API version | ||
| increment may occur for any Envoy API package, in concert with the quarterly Envoy release. | ||
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| Envoy will support at most three major versions of any API package at all times: | ||
| * The current stable major version, e.g. v3. | ||
| * The previous stable major version, e.g. v2. This is needed to ensure that we provide at least 1 | ||
| year for a supported major version to sunset. By supporting two stable major versions | ||
| simultaneously, this makes it easier to coordinate control plane and Envoy | ||
| rollouts as well. This previous stable major version will be supported for 1 | ||
| year after the introduction of the new current stable major version. | ||
| * Optionally, the next experimental alpha major version, e.g. v4alpha. This is a release candidate | ||
| for the next stable major version. This is only generated when the current stable major version | ||
| requires a breaking change at the next cycle, e.g. a deprecation or field rename. This release | ||
| candidate is mechanically generated via the | ||
| [protoxform](https://github.com/envoyproxy/envoy/tree/master/tools/protoxform) tool from the | ||
| current stable major version, making use of annotations such as `deprecated = true`. This is not a | ||
| human editable artifact. | ||
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| An example of how this might play out is that at the end of September in 2020, we will freeze | ||
| `envoy.config.bootstrap.v4alpha` and this package will become the current stable major version | ||
| `envoy.config.bootstrap.v4`. The `envoy.config.bootstrap.v3` package will become the previous stable | ||
| major version and support for `envoy.config.bootstrap.v2` will be dropped from the Envoy | ||
| implementation. Note that some transitively referenced package, e.g. | ||
| `envoy.config.filter.network.foo.v2` may remain at version 2 during this release, if no changes were | ||
| made to the referenced package. | ||
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| The implication of this API lifecycle and clock is that any deprecated feature in the Envoy API will retain | ||
| implementation support for 1-2 years (1.5 years on average). | ||
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| # New API features | ||
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| The Envoy APIs can be [safely extended](https://cloud.google.com/apis/design/compatibility) with new | ||
| packages, messages, enums, fields and enum values, while maintaining [backwards | ||
| compatibility](#backwards-compatibility). Additions to the API for a given package should normally | ||
| only be made to the *current stable major version*. The rationale for this policy is that: | ||
| * The feature is immediately available to Envoy users who consume the current stable major version. | ||
| This would not be the case if the feature was placed in `vNalpha`. | ||
| * `vNalpha` can be mechanically generated from `vN` without requiring developers to maintain the new | ||
| feature in both locations. | ||
| * We encourage Envoy users to move to the current stable major version from the previous one to | ||
| consume new functionality. | ||
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| # When can an API change be made to a package's previous stable major version? | ||
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| As a pragmatic concession, we allow API feature additions to the previous stable major version for a | ||
| single quarter following a major API version increment. Any changes to the previous stable major | ||
| version must be manually reflected in a consistent manner in the current stable major version as | ||
| well. | ||
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| # How to make a breaking change across major versions | ||
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| We maintain [backwards compatibility](#backwards-compatibility) within a major version but allow | ||
| breaking changes across major versions. This enables API deprecations, cleanups, refactoring and | ||
| reorganization. The Envoy APIs have a stylized workflow for achieving this. There are two prescribed | ||
| methods, depending on whether the change is mechanical or manual. | ||
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| ## Mechanical breaking changes | ||
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| Field deprecations, renames, etc. are mechanical changes that will be supported by the | ||
| [protoxform](https://github.com/envoyproxy/envoy/tree/master/tools/protoxform) tool. These are | ||
| guided by annotations in protobuf. | ||
| * Deprecations are specified with the built-in protobuf deprecated option set on a message, enum, | ||
| field or enum value. No field may be marked as deprecated unless a replacement for this | ||
| functionality exists and the corresponding Envoy implementation is production ready. | ||
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| * Renames are specified with a `[#rename-at-next-major-version: <new name>]` protobuf comment | ||
| annotation. | ||
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| * We anticipate that `protoxform` will also support `oneof` promotion, package movement, etc. via | ||
| similar annotations. | ||
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| ## Manual breaking changes | ||
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| A manual breaking change is distinct from the mechanical changes such as field deprecation, since in | ||
| general it requires new code and tests to be implemented in Envoy by hand. For example, if a developer | ||
| wants to unify `HeaderMatcher` with `StringMatcher` in the route configuration, this is a likely | ||
| candidate for this class of change. The following steps are required: | ||
| 1. The new version of the feature, e.g. the `NewHeaderMatcher` message should be added, together | ||
| with referencing fields, in the current stable major version for the route configuration proto. | ||
| 2. The Envoy implementation should be changed to consume configuration from the fields added in (1). | ||
| Translation code (and tests) should be written to map from the existing field and messages to | ||
| (1). | ||
| 3. The old message/enum/field/enum value should be annotated as deprecated. | ||
| 4. At the next major version, `protoxform` will remove the deprecated version automatically. | ||
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| This approach ensures that API major version releases are predictable and mechanical, and has the | ||
| bulk of the Envoy code and test changes owned by feature developers, rather than the API owners. | ||
| There will be no major `vN` initiative to address technical debt beyond that enabled by the above | ||
| process. | ||
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| # One Definition Rule (ODR) | ||
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| To avoid maintaining more than two stable major versions of a package, and to cope with diamond | ||
| dependency, we add a restriction on how packages may be referenced transitively; a package may have | ||
| at most one version of another package in its transitive dependency set. This implies that some | ||
| packages will have a major version bump during a release cycle simply to allow them to catch up to | ||
| the current stable version of their dependencies. | ||
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| Some of this complexity and churn can be avoided by having strict rules on how packages may | ||
| reference each other. Package organization and `BUILD` visibility constraints should be used | ||
| restrictions to maintain a shallow depth in the dependency tree for any given package. | ||
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| # Minimizing the impact of churn | ||
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| In addition to stability, the API versioning policy has an explicit goal of minimizing the developer | ||
| overhead for the Envoy community, other clients of the APIs (e.g. gRPC), management server vendors | ||
| and the wider API tooling ecosystem. A certain amount of API churn between major versions is | ||
| desirable to reduce technical debt and to support API evolution, but too much creates costs and | ||
| barriers to upgrade. | ||
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| We consider deprecations to be *mandatory changes*. Any deprecation will be removed at the next | ||
| stable API version. | ||
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| Other mechanical breaking changes are considered *discretionary*. These include changes such as | ||
| field renames and are largely reflected in protobuf comments. The `protoxform` tool may decide to | ||
| minimize API churn by deferring application of discretionary changes until a major version cycle | ||
| where the respective message is undergoing a mandatory change. | ||
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| The Envoy API structure helps with minimizing churn between versions. Developers should architect | ||
| and split packages such that high churn protos, e.g. HTTP connection manager, are isolated in | ||
| packages and have a shallow reference hierarchy. |
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