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+===========
+Rooms Model
+===========
+
+A description of the general data model used to implement Rooms, and the
+user-level visible effects and implications.
+
+
+Overview
+========
+
+"Rooms" in Synapse are shared messaging channels over which all the participant
+users can exchange messages. Rooms have an opaque persistent identify, a
+globally-replicated set of state (consisting principly of a membership set of
+users, and other management and miscellaneous metadata), and a message history.
+
+
+Room Identity and Naming
+========================
+
+Rooms can be arbitrarily created by any user on any home server; at which point
+the home server will sign the message that creates the channel, and the
+fingerprint of this signature becomes the strong persistent identify of the
+room. This now identifies the room to any home server in the network regardless
+of its original origin. This allows the identify of the room to outlive any
+particular server. Subject to appropriate permissions [to be discussed later],
+any current member of a room can invite others to join it, can post messages
+that become part of its history, and can change the persistent state of the room
+(including its current set of permissions).
+
+Home servers can provide a directory service, allowing a lookup from a
+convenient human-readable form of room label to a room ID. This mapping is
+scoped to the particular home server domain and so simply represents that server
+administrator's opinion of what room should take that label; it does not have to
+be globally replicated and does not form part of the stored state of that room.
+
+This room name takes the form
+
+  #localname:some.domain.name
+
+for similarity and consistency with user names on directories.
+
+To join a room (and therefore to be allowed to inspect past history, post new
+messages to it, and read its state), a user must become aware of the room's
+fingerprint ID. There are two mechanisms to allow this:
+
+ * An invite message from someone else in the room
+
+ * A referral from a room directory service
+
+As room IDs are opaque and ephemeral, they can serve as a mechanism to create
+"ad-hoc" rooms deliberately unnamed, for small group-chats or even private
+one-to-one message exchange.
+
+
+Stored State and Permissions
+============================
+
+Every room has a globally-replicated set of stored state. This state is a set of
+key/value or key/subkey/value pairs. The value of every (sub)key is a
+JSON-representable object. The main key of a piece of stored state establishes
+its meaning; some keys store sub-keys to allow a sub-structure within them [more
+detail below]. Some keys have special meaning to Synapse, as they relate to
+management details of the room itself, storing such details as user membership,
+and permissions of users to alter the state of the room itself. Other keys may
+store information to present to users, which the system does not directly rely
+on. The key space itself is namespaced, allowing 3rd party extensions, subject
+to suitable permission.
+
+Permission management is based on the concept of "power-levels". Every user
+within a room has an integer assigned, being their "power-level" within that
+room. Along with its actual data value, each key (or subkey) also stores the
+minimum power-level a user must have in order to write to that key, the
+power-level of the last user who actually did write to it, and the PDU ID of
+that state change.
+
+To be accepted as valid, a change must NOT:
+
+ * Be made by a user having a power-level lower than required to write to the
+   state key
+
+ * Alter the required power-level for that state key to a value higher than the
+   user has
+
+ * Increase that user's own power-level
+
+ * Grant any other user a power-level higher than the level of the user making
+   the change
+
+[[TODO(paul): consider if relaxations should be allowed; e.g. is the current
+outright-winner allowed to raise their own level, to allow for "inflation"?]]
+
+
+Room State Keys
+===============
+
+[[TODO(paul): if this list gets too big it might become necessary to move it
+into its own doc]]
+
+The following keys have special semantics or meaning to Synapse itself:
+
+m.member (has subkeys)
+  Stores a sub-key for every Synapse User ID which is currently a member of
+  this room. Its value gives the membership type ("knocked", "invited",
+  "joined").
+
+m.power_levels
+  Stores a mapping from Synapse User IDs to their power-level in the room. If
+  they are not present in this mapping, the default applies.
+
+  The reason to store this as a single value rather than a value with subkeys
+  is that updates to it are atomic; allowing a number of colliding-edit
+  problems to be avoided.
+
+m.default_level
+  Gives the default power-level for members of the room that do not have one
+  specified in their membership key.
+
+m.invite_level
+  If set, gives the minimum power-level required for members to invite others
+  to join, or to accept knock requests from non-members requesting access. If
+  absent, then invites are not allowed. An invitation involves setting their
+  membership type to "invited", in addition to sending the invite message.
+
+m.join_rules
+  Encodes the rules on how non-members can join the room. Has the following
+  possibilities:
+    "public" - a non-member can join the room directly
+    "knock" - a non-member cannot join the room, but can post a single "knock"
+        message requesting access, which existing members may approve or deny
+    "invite" - non-members cannot join the room without an invite from an
+        existing member
+    "private" - nobody who is not in the 'may_join' list or already a member
+        may join by any mechanism
+
+  In any of the first three modes, existing members with sufficient permission
+  can send invites to non-members if allowed by the "m.invite_level" key. A
+  "private" room is not allowed to have the "m.invite_level" set.
+
+  A client may use the value of this key to hint at the user interface
+  expectations to provide; in particular, a private chat with one other use
+  might warrant specific handling in the client.
+
+m.may_join
+  A list of User IDs that are always allowed to join the room, regardless of any
+  of the prevailing join rules and invite levels. These apply even to private
+  rooms. These are stored in a single list with normal update-powerlevel
+  permissions applied; users cannot arbitrarily remove themselves from the list.
+
+m.add_state_level
+  The power-level required for a user to be able to add new state keys.
+
+m.public_history
+  If set and true, anyone can request the history of the room, without needing
+  to be a member of the room.
+
+m.archive_servers
+  For "public" rooms with public history, gives a list of home servers that
+  should be included in message distribution to the room, even if no users on
+  that server are present. These ensure that a public room can still persist
+  even if no users are currently members of it. This list should be consulted by
+  the dirctory servers as the candidate list they respond with.
+
+The following keys are provided by Synapse for user benefit, but their value is
+not otherwise used by Synapse.
+
+m.name
+  Stores a short human-readable name for the room, such that clients can display
+  to a user to assist in identifying which room is which.
+  
+  This name specifically is not the strong ID used by the message transport
+  system to refer to the room, because it may be changed from time to time.
+
+m.topic
+  Stores the current human-readable topic
+
+
+Room Creation Templates
+=======================
+
+A client (or maybe home server?) could offer a few templates for the creation of
+new rooms. For example, for a simple private one-to-one chat the channel could
+assign the creator a power-level of 1, requiring a level of 1 to invite, and
+needing an invite before members can join. An invite is then sent to the other
+party, and if accepted and the other user joins, the creator's power-level can
+now be reduced to 0. This now leaves a room with two participants in it being
+unable to add more.
+
+
+Rooms that Continue History
+===========================
+
+An option that could be considered for room creation, is that when a new room is
+created the creator could specify a PDU ID into an existing room, as the history
+continuation point. This would be stored as an extra piece of meta-data on the
+initial PDU of the room's creation. (It does not appear in the normal previous
+PDU linkage).
+
+This would allow users in rooms to "fork" a room, if it is considered that the
+conversations in the room no longer fit its original purpose, and wish to
+diverge. Existing permissions on the original room would continue to apply of
+course, for viewing that history. If both rooms are considered "public" we might
+also want to define a message to post into the original room to represent this
+fork point, and give a reference to the new room.
+
+
+User Direct Message Rooms
+=========================
+
+There is no need to build a mechanism for directly sending messages between
+users, because a room can handle this ability. To allow direct user-to-user chat
+messaging we simply need to be able to create rooms with specific set of
+permissions to allow this direct messaging.
+
+Between any given pair of user IDs that wish to exchange private messages, there
+will exist a single shared Room, created lazily by either side. These rooms will
+need a certain amount of special handling in both home servers and display on
+clients, but as much as possible should be treated by the lower layers of code
+the same as other rooms.
+
+Specially, a client would likely offer a special menu choice associated with
+another user (in room member lists, presence list, etc..) as "direct chat". That
+would perform all the necessary steps to create the private chat room. Receiving
+clients should display these in a special way too as the room name is not
+important; instead it should distinguish them on the Display Name of the other
+party.
+
+Home Servers will need a client-API option to request setting up a new user-user
+chat room, which will then need special handling within the server. It will
+create a new room with the following 
+
+  m.member: the proposing user
+  m.join_rules: "private"
+  m.may_join: both users
+  m.power_levels: empty
+  m.default_level: 0
+  m.add_state_level: 0
+  m.public_history: False
+
+Having created the room, it can send an invite message to the other user in the
+normal way - the room permissions state that no users can be set to the invited
+state, but because they're in the may_join list then they'd be allowed to join
+anyway.
+
+In this arrangement there is now a room with both users may join but neither has
+the power to invite any others. Both users now have the confidence that (at
+least within the messaging system itself) their messages remain private and
+cannot later be provably leaked to a third party. They can freely set the topic
+or name if they choose and add or edit any other state of the room. The update
+powerlevel of each of these fixed properties should be 1, to lock out the users
+from being able to alter them.
+
+
+Anti-Glare
+==========
+
+There exists the possibility of a race condition if two users who have no chat
+history with each other simultaneously create a room and invite the other to it.
+This is called a "glare" situation. There are two possible ideas for how to
+resolve this:
+
+ * Each Home Server should persist the mapping of (user ID pair) to room ID, so
+   that duplicate requests can be suppressed. On receipt of a room creation
+   request that the HS thinks there already exists a room for, the invitation to
+   join can be rejected if:
+      a) the HS believes the sending user is already a member of the room (and
+         maybe their HS has forgotten this fact), or
+      b) the proposed room has a lexicographically-higher ID than the existing
+         room (to resolve true race condition conflicts)
+      
+ * The room ID for a private 1:1 chat has a special form, determined by
+   concatenting the User IDs of both members in a deterministic order, such that
+   it doesn't matter which side creates it first; the HSes can just ignore
+   (or merge?) received PDUs that create the room twice.