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# MSC2324: Facilitating early releases of software dependent on spec
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*Note*: This is a process change MSC, not a change to the spec itself.
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There's currently an unanswered question by the spec process: when is it
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safe to start using stable endpoints or to present a feature as "stable"?
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Historically this question would receive very different answers depending
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on who you asked, so in an effort to come up with a concise answer the
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following process change is proposed.
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## Proposal
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The new process, from start to finish, is proposed as:
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1. Have an idea for a feature.
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2. Optionally: implement the feature using unstable endpoints, vendor prefixes,
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and unstable feature flags as appropriate.
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* When using unstable endpoints, they MUST include a vendor prefix. For
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example: `/_matrix/client/unstable/com.example/login`. Vendor prefixes
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throughout this proposal always use the Java package naming convention.
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* Unstable endpoints **do not** inherit from stable (`/r0`) APIs. Previously,
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one could access the entirety of the Matrix API through `/unstable` however
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this is generally considered a bad practice. Therefore an implementation
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can no longer assume that because its feature-specific endpoint exists that
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any other endpoint will exist in the same unstable namespace.
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* If the client needs to be sure the server supports the feature, an unstable
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feature flag that MUST be vendor prefixed is to be used. This kind of flag
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shows up in the `unstable_features` field of `/versions` as, for example,
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`com.example.new_login`.
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* You can ship the feature at *any* time, so long as you are able to accept
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the technical debt that results from needing to provide adequate backwards
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and forwards compatibility for the vendor prefixed implementation. The
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implementation MUST support the flag disappearing and be generally safe for
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users. Note that implementations early in the MSC review process may also be
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required to provide backwards compatibility with earlier editions of the
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proposal.
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* If you don't want to support the technical debt (or if it's impossible to
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provide adequate backwards/forwards compatibility - e.g. a user authentication
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change which can't be safely rolled back), do not implement the feature and
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wait for Step 7.
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* If at any point the idea changes, the feature flag should also change so
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that implementations can adapt as needed.
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3. In parallel, or ahead of implementation, open an MSC and solicit review.
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4. Before a FCP (Final Comment Period) can be called, the Spec Core Team will
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require that evidence to prove the MSC works be presented. A typical example
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of this is an implementation of the MSC (which does not necessarily need to have been shipped anywhere).
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5. FCP is gone through, and assuming nothing is flagged the MSC lands.
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6. A spec PR is written to incorporate the changes into Matrix.
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7. A spec release happens.
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8. Implementations switch to using stable prefixes (e.g.: `/r0`) if the server
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supports the specification version released. If the server doesn't advertise
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the specification version, but does have the feature flag, unstable prefixes
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should still be used.
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9. A transition period of about 2 months starts immediately after the spec release, before
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implementations start to loudly encourage other implementations to switch to stable
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endpoints. For example, the Synapse team should start asking the Riot team to
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support the stable endpoints (as per Step 8) 2 months after the spec release if they
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haven't already.
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It's worth repeating that this process generally only applies if the implementation
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wants to ship the feature ahead of the spec being available. By doing so, it takes
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on the risk that the spec/MSC may change and it must adapt. If the implementation
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is unable to take on that risk, or simply doesn't mind waiting, it should go through
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the spec process without shipping an unstable implementation.
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To help MSCs get incorporated by implementations as stable features, the spec core
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team plans to release the specification more often. How often is undefined and is
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largely a case-by-case basis.
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To reiterate:
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* Implementations MUST NOT use stable endpoints before the MSC is in the spec. This
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includes NOT using stable endpoints post-FCP.
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* Implementations CAN ship features that are exposed by default to users before an
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MSC has been merged to the spec, provided they follow the process above.
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* Implementations SHOULD be wary of the technical debt they are incurring by moving
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faster than the spec.
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To clarify:
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* The vendor prefix is chosen by the developer of the feature, using the Java package
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naming convention. For example, `org.matrix` is the foundation's vendor prefix.
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* The vendor prefixes, unstable feature flags, and unstable endpoints should be included
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in the MSC so other developers can benefit. The MSC MUST still say what the stable
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endpoints are to look like.
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### Specific examples outside of the client-server API
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There are some instances where a change might be made outside of the client-server API,
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which is where much of this proposal is targeted. The general spirit of the process
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should be followed where possible, if implementations decide to work ahead of spec releases.
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#### Room versions
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When a new room version is needed, implementations MUST use vendor-prefixed versions
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before using the namespace reserved for Matrix (see https://spec.matrix.org/unstable/rooms/).
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A room version is considered released once it is listed as an "available room version" in
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the spec. Often a new room version is accompanied with a server-server API release, but
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doesn't have to be.
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#### Server-server / Identity / Push / Appservice API
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These APIs don't yet have a `/versions` endpoint or similar. Typically behaviour changes in
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these APIs are introduced with backwards compatibility in mind (try X and if that fails fall
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back to Y) and therefore don't always need a flag to indicate support. If a flag were to
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be required, an `unstable_features` or similar array would need to be exposed somewhere.
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#### Changes to request/response parameters
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Parameters being added to request/response bodies and query strings MUST be vendor-prefixed
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per the proposed process. For example, a new JSON field might be `{"org.matrix.example": true}`
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with the proposal being for `example` being added. A query string parameter would be prefixed
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the same way: `?org.matrix.example=true`.
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If the MSC is simply adding fields to already-versioned endpoints, it does not need to put
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the whole endpoint into the `/unstable` namespace provided the new parameters are prefixed
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appropriately.
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#### .well-known and other APIs that can't be versioned
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Best effort is appreciated. Typically these endpoints will be receiving minor behavioural
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changes or new fields. New fields should be appropriately prefixed, and behaviour changes
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should be rolled out cautiously by implementations (waiting until after FCP has concluded
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is probably best to ensure there's no major problems with the new behaviour).
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