MSC4235: via query param for hierarchy endpoint

proposals/4235-via-query-param-for-hierarchy.md

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+# MSC4235: `via` query param for /hierarchy endpoint
+
+To request the hierarchy of a space, clients need to call the
+[/hierarchy](https://spec.matrix.org/v1.9/client-server-api/#get_matrixclientv1roomsroomidhierarchy) endpoint. This
+endpoint does not require user to join the room before responding with hierarchy details. It instead check whether the
+room is potentially joinable (per [MSC3173](https://github.com/matrix-org/matrix-spec-proposals/pull/3173)) before
+responding.
+
+Because it doesn't not require space room to be joined before calling this endpoint, It is possible that the requested
+room is not locally available on the server and server need to request over federation to respond. In that case this
+endpoint does not provide client to express the server name's which server need to request over federation. 
+
+## Proposal
+
+The [/hierarchy](https://spec.matrix.org/v1.9/client-server-api/#get_matrixclientv1roomsroomidhierarchy) endpoint should
+include `via` as query parameter which server can use in case requested room in not locally available on the server.
+
+Following change will allow clients to use this endpoint in more efficient manner, like: 
+1. Requesting the only part of sub-spaces hierarchy which is unknown the the client.
+2. Rendering the preview of space and it's children before joining the space.
+3. Since server does not respect [order](https://spec.matrix.org/v1.9/client-server-api/#mspacechild) of the children in
+   response, client can divide the work of loading child spaces and figuring out order and re-ordering instead of
+   relying on the long paginated response for those action.
+
+## Potential issues
+Unknown
+
+## Alternative
+Unknown
+
+## Security considerations
+Unknown
+
+## Unstable prefix
+None