MSC4335: M_USER_LIMIT_EXCEEDED error code

proposals/4335-user-limit-exceeded.md

@@ -0,0 +1,188 @@
+# MSC4335: M_USER_LIMIT_EXCEEDED error code
+
+Currently, Matrix homeservers lack a standardized error code to indicate when user-related limits
+have been exceeded. This creates inconsistent client experiences when homeservers need to reject
+operations due to per-user quotas, rate limits, or resource constraints.
+
+Different implementations may return generic error codes like `M_FORBIDDEN` or `M_TOO_LARGE`, making
+it difficult for clients to provide appropriate user feedback or implement proper retry logic.
+
+A concrete use case for this is the
+[fair usage limits introduced on the matrix.org homeserver](https://matrix.org/homeserver/pricing/#usage-limits).
+
+This proposal introduces a new error code `M_USER_LIMIT_EXCEEDED` that homeservers can use to
+signal when a user has exceeded their allocated limits, distinct from general rate limiting or
+server-wide constraints. This improves the user experience by allowing clients to provide more
+specific error messages and handle user-specific limitations appropriately.
+
+## Proposal
+
+This proposal adds a new [common error code]
+`M_USER_LIMIT_EXCEEDED` to the Matrix specification. This error code should be returned when a user has exceeded
+limits that are specifically associated with their account, such as:
+
+* **Storage quotas**: When a user has exceeded their allocated storage space for media uploads,
+  message history, or other persistent data.
+* **Resource limits**: When a user has reached their maximum number of allowed rooms, devices,
+  or other account-scoped resources.
+* **Feature limits**: When a user has exceeded usage limits for specific features (e.g., number
+  of public rooms they can create, number of invites they can send).
+* **Account tier restrictions**: When a user's account type (free, premium, etc.) prevents them
+  from performing certain operations.
+
+The error response must also contain additional fields:
+
+* `info_uri` string (required) - an opaque URI that the client can link the user to in order to get more context on the
+  error.
+* `soft_limit` boolean (optional, default `false`) - `true` means that the specific limit encountered can be increased.
+  Otherwise it is a hard limit that cannot be increased.
+* `increase_uri` (required if `soft_limit` is `true`) - an opaque URI where the user can undertake actions to increase
+  the encountered limit.
+
+The `info_uri` and `increase_uri` are "opaque" in the sense that the homeserver implementation may choose to encode
+information, such as the type of limit encountered, within the URI but it may do so using an encoding of its choosing.
+
+The HTTP response code should be chosen based on the specification for the individual endpoint. For
+example, the most appropriate code for [`POST /_matrix/media/v3/upload`] would be `403 Forbidden`.
+
+An example response body for the error might look as follows:
+
+```json
+{
+  "errcode": "M_USER_LIMIT_EXCEEDED",
+  "error": "User has exceeded their storage quota of 10GB",
+  "info_uri": "https://example.com/homeserver/about?limit_type=quota",
+  "soft_limit": true,
+  "increase_uri": "https://example.com/homeserver/upgrade"
+}
+```
+
+For a hard limit:
+
+```json
+{
+  "errcode": "M_USER_LIMIT_EXCEEDED",
+  "error": "User has exceeded their storage quota of 10GB",
+  "info_uri": "https://example.com/homeserver/about?limit_type=quota"
+}
+```
+
+### Applicable Endpoints
+
+As it is a [common error code], `M_USER_LIMIT_EXCEEDED` may be returned by any Matrix Client-Server API endpoint.
+
+For the purpose of illustration, practical examples could include:
+
+* [`POST /_matrix/media/v3/upload`] - When the server is enforcing a storage quota.
+* [`POST /_matrix/client/v3/rooms/{roomId}/invite`] - When invite limits (like maximum participant count) are exceeded.
+* [`POST /_matrix/client/v3/createRoom`] - When the number or type of rooms is constrained.
+
+### Distinction from Other Error Codes
+
+This error code is distinct from:
+
+* `M_LIMIT_EXCEEDED`: Used for general rate limiting that applies to all users or based on IP/client
+* `M_FORBIDDEN`: Used for authorization failures or policy violations not related to usage limits
+* `M_RESOURCE_LIMIT_EXCEEDED`: Used for server-wide resource constraints affecting all users
+* `M_TOO_LARGE`: Used when a request is too large (file size, message length, etc.) regardless of user limits
+
+## Potential issues
+
+This error code does not specify the exact nature of the limit that was exceeded, which could
+potentially lead to ambiguity. However, this is consistent with other Matrix error codes that
+rely on the human-readable `error` field to provide specific details. Instead the `info_uri`
+provides a way for the homeserver to apply arbitrary limits without the client having to understand
+every type in advance.
+
+The homeserver can choose to provide localised and personalised content on the `info_uri` and `increase_uri` if it
+wishes.
+
+The error code does not provide machine-readable information about current usage or limits,
+which could be useful for clients to display progress bars or usage statistics. However, adding
+such fields would require a more complex specification change and could be addressed in a future
+MSC if deemed necessary.
+
+## Alternatives
+
+Several alternatives were considered for this proposal:
+
+### Use M_RESOURCE_LIMIT_EXCEEDED
+
+The existing [`M_RESOURCE_LIMIT_EXCEEDED`] looks very similar at first glance.
+
+However, this code is explicitly referring to a limit being applied to the *server* and not to the *user* themselves:
+
+> The request cannot be completed because the homeserver has reached a resource limit imposed on it.
+> For example, a homeserver held in a shared hosting environment may reach a resource limit if it starts using too much
+> memory or disk space.
+
+As such this error code is currently rendered by
+[existing](https://github.com/element-hq/element-web/blob/c96da5dbf8e20ced4a512a03a75c91f8680e8d40/src/i18n/strings/en_EN.json#L1112)
+[clients](https://github.com/element-hq/element-ios/blob/2dc7b76c44545b3d027cdf0196c6af6eba8932f4/Riot/Assets/en.lproj/Vector.strings#L615)
+as something similar to:
+
+> This homeserver has exceeded one of its resource limits
+
+As such, I think this message would be confusing to users the interim whilst clients updated their implementations and
+that a new error code would be the best way forward.
+
+### Add structured error information
+
+Instead of a simple error code, a more complex error format
+could include machine-readable fields for limit types, current usage, and maximum limits. While
+this would provide more information, it would require a more significant change to the error
+response format and could be added in a future MSC if needed.
+
+### Multiple specific error codes
+
+Separate error codes could be introduced for different types
+of limits (e.g., `M_STORAGE_LIMIT_EXCEEDED`, `M_ROOM_LIMIT_EXCEEDED`). However, this approach
+would require many new error codes and doesn't provide significant benefits over a single code
+with descriptive error messages.
+
+### Define specific endpoints
+
+Instead of making this a [common error code] instead it could be an
+["other" error code](https://spec.matrix.org/v1.16/client-server-api/#other-error-codes) and have the specific endpoints
+listed.
+
+A downside of this is that if a homeserver wished to introduce a new type of limit or quota that was not foreseen, then
+another MSC would be required to introduce it.
+
+Instead, by making it a [common error code] the homeserver operator has flexibility over what types of limit they
+choose without requiring further coordination with clients.
+
+## Security considerations
+
+None as only adding a new error code.
+
+## Unstable prefix
+
+While this proposal is being developed and refined, implementations should use the following:
+
+* `ORG.MATRIX.MSC4335_USER_LIMIT_EXCEEDED` instead of `M_USER_LIMIT_EXCEEDED`
+* `org.matrix.msc4335.info_uri` instead of `info_uri`
+* `org.matrix.msc4335.soft_limit` instead of `soft_limit`
+* `org.matrix.msc4335.increase_uri` instead of `increase_uri`
+
+For example:
+
+```json
+{
+  "errcode": "ORG.MATRIX.MSC4335_USER_LIMIT_EXCEEDED",
+  "error": "User has exceeded their fair usage limit of 2GB",
+  "org.matrix.msc4335.info_uri": "https://example.com/homeserver/about?limit_type=quota",
+  "org.matrix.msc4335.soft_limit": true,
+  "org.matrix.msc4335.increase_uri": "https://example.com/homeserver/upgrade"
+}
+```
+
+## Dependencies
+
+None.
+
+[`POST /_matrix/media/v3/upload`]: https://spec.matrix.org/v1.16/client-server-api/#post_matrixmediav3upload
+[`POST /_matrix/client/v3/rooms/{roomId}/invite`]: https://spec.matrix.org/v1.16/client-server-api/#thirdparty_post_matrixclientv3roomsroomidinvite
+[`M_RESOURCE_LIMIT_EXCEEDED`]: https://spec.matrix.org/v1.16/client-server-api/#other-error-codes
+[common error code]: https://spec.matrix.org/v1.16/client-server-api/#common-error-codes
+[`POST /_matrix/client/v3/createRoom`]: https://spec.matrix.org/v1.16/client-server-api/#post_matrixclientv3createroom