MSC2324: Facilitating early releases of software dependent on spec (#2324)

proposals/2324-when-to-ship.md

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