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ReStructuredText
114 lines
4.7 KiB
ReStructuredText
==================
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Room Join Workflow
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==================
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An outline of the workflows required when a user joins a room.
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Discovery
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=========
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To join a room, a user has to discover the room by some mechanism in order to
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obtain the (opaque) Room ID and a candidate list of likely homeservers that
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contain it.
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Sending an Invitation
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---------------------
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The most direct way a user discovers the existence of a room is from a
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invitation from some other user who is a member of that room.
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The inviter's HS sets the membership status of the invitee to "invited" in the
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"m.members" state key by sending a state update PDU. The HS then broadcasts this
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PDU among the existing members in the usual way. An invitation message is also
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sent to the invited user, containing the Room ID and the PDU ID of this
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invitation state change and potentially a list of some other homeservers to use
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to accept the invite. The user's client can then choose to display it in some
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way to alert the user.
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[[TODO(paul): At present, no API has been designed or described to actually send
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that invite to the invited user. Likely it will be some facet of the larger
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user-user API required for presence, profile management, etc...]]
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Directory Service
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-----------------
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Alternatively, the user may discover the channel via a directory service; either
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by performing a name lookup, or some kind of browse or search acitivty. However
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this is performed, the end result is that the user's homeserver requests the
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Room ID and candidate list from the directory service.
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[[TODO(paul): At present, no API has been designed or described for this
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directory service]]
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Joining
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=======
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Once the ID and homeservers are obtained, the user can then actually join the
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room.
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Accepting an Invite
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-------------------
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If a user has received and accepted an invitation to join a room, the invitee's
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homeserver can now send an invite acceptance message to a chosen candidate
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server from the list given in the invitation, citing also the PDU ID of the
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invitation as "proof" of their invite. (This is required as due to late message
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propagation it could be the case that the acceptance is received before the
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invite by some servers). If this message is allowed by the candidate server, it
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generates a new PDU that updates the invitee's membership status to "joined",
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referring back to the acceptance PDU, and broadcasts that as a state change in
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the usual way. The newly-invited user is now a full member of the room, and
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state propagation proceeds as usual.
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Joining a Public Room
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---------------------
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If a user has discovered the existence of a room they wish to join but does not
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have an active invitation, they can request to join it directly by sending a
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join message to a candidate server on the list provided by the directory
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service. As this list may be out of date, the HS should be prepared to retry
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other candidates if the chosen one is no longer aware of the room, because it
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has no users as members in it.
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Once a candidate server that is aware of the room has been found, it can
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broadcast an update PDU to add the member into the "m.members" key setting their
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state directly to "joined" (i.e. bypassing the two-phase invite semantics),
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remembering to include the new user's HS in that list.
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Knocking on a Semi-Public Room
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------------------------------
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If a user requests to join a room but the join mode of the room is "knock", the
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join is not immediately allowed. Instead, if the user wishes to proceed, they
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can instead post a "knock" message, which informs other members of the room that
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the would-be joiner wishes to become a member and sets their membership value to
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"knocked". If any of them wish to accept this, they can then send an invitation
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in the usual way described above. Knowing that the user has already knocked and
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expressed an interest in joining, the invited user's homeserver should
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immediately accept that invitation on the user's behalf, and go on to join the
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room in the usual way.
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[[NOTE(Erik): Though this may confuse users who expect 'X has joined' to
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actually be a user initiated action, i.e. they may expect that 'X' is actually
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looking at synapse right now?]]
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[[NOTE(paul): Yes, a fair point maybe we should suggest HSes don't do that, and
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just offer an invite to the user as normal]]
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Private and Non-Existent Rooms
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------------------------------
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If a user requests to join a room but the room is either unknown by the home
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server receiving the request, or is known by the join mode is "invite" and the
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user has not been invited, the server must respond that the room does not exist.
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This is to prevent leaking information about the existence and identity of
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private rooms.
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Outstanding Questions
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=====================
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* Do invitations or knocks time out and expire at some point? If so when? Time
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is hard in distributed systems.
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