MSC2844: Global version number for the whole spec

proposals/2844-global-versioning.md

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+# MSC2844: Using a global version number for the entire specification
+
+Currently we have 4 kinds of versions, all of which have slightly different use cases and semantics
+which apply:
+
+1. The individual API spec document versions, tracked as revisions (`r0.6.1`, for example).
+2. Individual endpoint versioning underneath an API spec document version (`/v1/`, `/v2/`, etc). Note
+   that the client-server API currently ties the major version of its spec document version to the
+   endpoint, thus making most endpoints under it `/r0/` (currently).
+3. Room versions which define a set of behaviour and algorithms on a per-room basis. These are well
+   defined in the spec and are not covered here: https://matrix.org/docs/spec/#room-versions
+4. An overarching "Matrix" version, largely for marketing purposes. So far we've only cut Matrix 1.0
+   back when we finalized the initial versions of the spec documents, but have not cut another one
+   since.
+
+This current system is slightly confusing, and has some drawbacks for being able to compile builds of
+the spec documents (published on matrix.org) and generally try and communicate what supported versions
+an implementation might have. For example, Synapse currently supports 4 different APIs, all of which
+have their own versions, and all of which would need to be considered and compared when validating
+another implementation of Matrix such as a client or push gateway. Instead, Synapse could say it
+supports "Matrix 1.1", making compatibility much easier to determine - this is what this proposal aims
+to define.
+
+## Proposal
+
+Instead of having per-API versions (`r0.6.1`, etc), we have a version that spans the entire specification.
+This version represents versioning for the index (which has quite a bit of unversioned specification on
+it currently), the APIs, room versions, and the appendices (which are also currently unversioned but
+contain specification). Room versions are a bit more nuanced though, and are covered later in this MSC.
+
+The version which covers the entire specification and all its parts is called the "Matrix version", and
+is a promotion of the previously marketing-only version number assigned to the spec. The first version
+after this proposal is expected to be Matrix 1.1, though the spec core team will make that distinction.
+v1.0 would be left in the marketing era and recorded for posterity (though still retains no significant
+meaning).
+
+Doing this has the benefits previously alluded to:
+
+* Implementations of Matrix can now easily compare their supported versions using a single identifier
+  without having to (potentially) indicate which API they built support for.
+* Publishing the specification is less likely to contain broken or outdated links due to API versions
+  not matching up properly. This is currently an issue where if we want to release a new version of
+  the server-server specification then we must also either rebuild or manually fix the blob of HTML
+  known as the client-server API to account for the new version - we often forget this step, sometimes
+  because it's just too difficult.
+* Explaining to people what version Matrix or any of the documents is at becomes incredibly simplified.
+  No longer will we have to explain most of what the introduction to this proposal covers to every new
+  person who asks.
+
+### Full Matrix version grammar
+
+The Matrix versioning scheme takes heavy inspiration from semantic versioning, though intentionally does
+not follow it for reasons described throughout this proposal. Primarily, the argument against semantic
+versioning is held in the alternatives section below.
+
+Given a version number `MAJOR.MINOR`, incremement the:
+
+* `MAJOR` version when a substantial change is made to the core of the protocol. This is reserved for
+  interpretation by the Spec Core Team, though is intended to be for extremely invasive changes such
+  as switching away from JSON, introducing a number of features where a `MINOR` version increase just
+  doesn't feel good enough, or changes to the signing algorithms.
+* `MINOR` version when a feature is introduced, or a backwards incompatible change has been managed
+  through the specification. Later on, this proposal explains what it means to manage a breaking change.
+
+When present in the protocol itself, the Matrix version will always be prefixed with `v`. For example,
+`v1.1`.
+
+Additional information can be supplied in the version number by appending a dash (`-`) to the end of the
+version and including any relevant information. This is typically used to denote alpha, beta, unstable,
+or other similar off-cycle release builds. This MSC does not propose a scheme for RCs or pre-releases,
+though the Spec Core Team may wish to do so. This can also be used to represent patch builds for the
+documentation itself, such as correcting spelling mistakes. An example would be `v1.1-patch.20210109`.
+
+See the section on brewing Matrix versions for information on how the unstable version is decided.
+
+`MINOR` versions have a backwards compatibility scheme described later in this proposal. `MAJOR`
+versions are expected to have zero backwards compatibility guarantees to them.
+
+For clarity, `v1.2.0` will probably work with `v1.1.0`, though implementations should be wary if they
+depend on a version. As mentioned, the backwards compatibility scheme section goes into more detail on
+this.
+
+Most notably, this MSC does not propose including a patch version at all. The specifics of what would
+be included in a patch version (spelling changes, release process bug fixes, etc) do not impact any
+implementations of Matrix and thus are not needing of a patch version.
+
+### Structure changes and changelogs
+
+The API documents remain mostly unchanged. We'll still have a client-server API, server-server API, etc,
+but won't have versions associated with those particular documents. This also means they would lose their
+individual changelogs in favour of a more general changelog. An exception to this rule is room versions,
+which are covered later in this proposal.
+
+### Endpoint versioning
+
+Under this MSC, all HTTP endpoints in the specification are to be per-endpoint versioned. This is already
+the case for all APIs except the Client-Server API, and so this section deals specifically with that API.
+The deprecation of endpoints is handled later in this proposal.
+
+Under this proposal, all endpoints in the client-server API get assigned `v3` as their per-endpoint
+version as a starting point. This is primarily done to avoid confusion with the ancient client-server API
+versions which had `v1` and called the `rN` system "v2". Though many of the endpoints available today
+are not present in those older API editions, it is still proposed that they start at `v3` to avoid
+confusion with long-standing implementations.
+
+Servers would advertise support for the new Matrix version by appending it to the array in `/versions`.
+If the sever also supports an older `rN` version, it would include those too.
+For example: `["v1.1", "r0.6.1"]`.
+
+### Room versions
+
+*Author's note*: Having many things with the root word "version" can be confusing, so for this section
+"room versions" are called "room editions" and the Matrix version refers to what this proposal is
+introducing. This MSC does not propose renaming "room versions" - that is another MSC's problem.
+
+Room editions are a bit special in that they have their own versioning scheme as servers and, potentially,
+clients need to be aware of how to process the room. As such, a room edition's versioning scheme is not
+altered by this proposal, however the publishing of the (in)stability of a given edition is now covered by the
+newly proposed Matrix version.
+
+Whenever a room edition transitions from stable to unstable, or unstable to stable, or is introduced
+then it would get counted as a feature for a `MINOR` release of Matrix. We don't currently have a plan
+to remove any room editions, so they are not covered as a potential process for this MSC.
+
+### Deprecation approach
+
+This MSC doesn't change much about how things get deprecated, though it would be codified by this MSC.
+
+An MSC is required to transition something from stable (the default) to deprecated. Once something has
+been deprecated for suitably long enough, it is eligible for removal from the specification with another
+MSC. Today's process is the same, though not defined explicitly.
+
+Also not mentioned in today's system is that implementations are not required to implement deprecated
+endpoints. This MSC doesn't change that, but does put some rules around how deprecation works for a
+given endpoint. Specifically, if a server wants to support a Matrix version where an endpoint is *not*
+deprecated, then it must serve it.
+
+As an example, if `/test` were introduced in v1.1, deprecated in v1.2, and removed in v1.3 then an
+implementation can support v1.1, v1.2, and v1.3 by implementing `/test` as it was defined in v1.2 (minus
+the deprecation flag). If the implementation wanted to support just v1.2 and v1.3, then it could
+optionally implement `/test`, though is encouraged to. Further, if the implementation only wanted to
+support v1.3, then it *should not* implement `/test` at all because it was removed.
+
+Generally deprecation is paired with replacement or breaking changes. For example, if `/v3/sync` were
+to be modified such that it needed to be bumped to `v4`, the MSC which does so would deprecate `/v3/sync`
+in favour of its proposed `/v4/sync`. Because endpoints are versioned on a per-endpoint basis, `/v4/sync`
+will still work with a server that supports `/v3/profile` (for example) - the version number doesn't mean
+an implementation can only use v4 endpoints.
+
+This sort of approach could be potentially confusing and non-standard as it would mean for an amount of
+time in the specification there would be two versions of an endpoint: one deprecated and one not. Most
+specifications and protocols do not use this sort of approach and instead opt to replace the whole API
+or outright remove the endpoint, however the Matrix specification tends to have a longer-lived cycle
+associated with it and thus means we should support a larger than average backwards compatibility
+period.
+
+## Potential issues
+
+None appear to be relevant to be discussed on their own - they are discussed in their respective
+sections above when raised.
+
+## Alternatives
+
+There are some strong opinions that we should use proper semantic versioning for the specification
+instead of the inspired system proposed here. So, why shouldn't we use semantic versioning?
+
+1. It's meant for software and library compatibility, not specifications. Though it could theoretically
+   be used as a specification version, the benefits of doing so are not immediately clear. The scheme
+   proposed here is simple enough where rudimentary comparisons are still possible between versions,
+   and existing semantic versioning libraries can still be made to work. Further, the specification's
+   version number should not be relied upon by a library for its versioning scheme - libraries,
+   applications, etc should have their own versioning scheme so they may work independently of the
+   spec's release schedule.
+
+2. It has potential for causing very high major version numbers. Though largely an aesthetic concern,
+   it can be hard to market Matrix v45 (or even Matrix v4) to potential ecosystem adopters due to
+   the apparant unstable-ness of the specification. Similarly, the major version is used for advertising
+   purposes which could be confusing or overly noisy to say there's a major version every few
+   releases. By instead staying in the 1.x series for a long period of time, the specification appears
+   stable and easy to work with, attracting potential adopters and making that 2.0 release feel all
+   that more special.
+
+3. The semantic versioning spec is not followed in practice. Most uses of semantic versioning are
+   actually off-spec adaptations which are largely compatible with the ideals of the system. This, however,
+   puts Matrix in a difficult spot as it would want to say we follow semantic versioning, but can't
+   because there's no relevant specification document to link to. Even if there was, it would appear
+   as though we were encouraging the idea of forking a specification as a specification ourselves,
+   which may be confusing if not sending the wrong message entirely. Though the system proposed here
+   is a reinvention of semantic versioning to a degree, this proposed system is different from how
+   semantic versioning works in so many ways it is not entirely comparable.
+
+4. The benefit of saying we use a well-popularized versioning system is not a strong enough argument
+   to be considered here.
+
+This MSC is also inherently incompatible with semantic versioning due to its approach to deprecation.
+Instead of encouraging breaking changes (removal of endpoints) be major version changes, this MSC
+says that happens at the minor version change level. As mentioned in the relevant section, this is
+not foreseen to be an issue for Matrix given its a system already used by the protocol and is common
+enough to at least be moderately familiar - the arguments for using semantic versioning in this respect
+do not hold up, per above.
+
+## Security considerations
+
+None relevant - if we need to make a security release for Matrix then we simply make a release and
+advertise accordingly.
+
+## Unstable prefix
+
+The author does not recommend that this MSC be implemented prior to it landing due to the complexity
+involved as well as the behavioural changes not being possible to implement. However, if an implementation
+wishes to try anyways, it should use `org.matrix.msc2844` in the `unstable_features` of `/versions`
+and use `/_matrix/client/unstable/org.matrix.msc2844` in place of `/_matrix/client/r0`.
+
+This MSC is largely proven as possible through an in-development build of the specification which uses
+an alternative toolchain for rendering the specification: https://adoring-einstein-5ea514.netlify.app/
+(see the 'releases' dropdown in the top right; link may not be available or even the same as described
+here due to development changes - sorry).