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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.