Instant Messaging ================= .. _module:im: This module adds support for sending human-readable messages to a room. It also adds support for associating human-readable information with the room itself such as a room name and topic. Events ------ {{m_room_message_event}} {{m_room_message_feedback_event}} Usage of this event is discouraged for several reasons: - The number of feedback events will grow very quickly with the number of users in the room. This event provides no way to "batch" feedback, unlike the `receipts module`_. - Pairing feedback to messages gets complicated when paginating as feedback arrives before the message it is acknowledging. - There are no guarantees that the client has seen the event ID being acknowledged. .. _`receipts module`: `module:receipts`_ {{m_room_name_event}} {{m_room_topic_event}} {{m_room_avatar_event}} m.room.message msgtypes ~~~~~~~~~~~~~~~~~~~~~~~ Each `m.room.message`_ MUST have a ``msgtype`` key which identifies the type of message being sent. Each type has their own required and optional keys, as outlined below. If a client cannot display the given ``msgtype`` then it SHOULD display the fallback plain text ``body`` key instead. {{msgtype_events}} Client behaviour ---------------- Clients SHOULD verify the structure of incoming events to ensure that the expected keys exist and that they are of the right type. Clients can discard malformed events or display a placeholder message to the user. Redacted ``m.room.message`` events MUST be removed from the client. This can either be replaced with placeholder text (e.g. "[REDACTED]") or the redacted message can be removed entirely from the messages view. Events which have attachments (e.g. ``m.image``, ``m.file``) SHOULD be uploaded using the `content repository module`_ where available. The resulting ``mxc://`` URI can then be used in the ``url`` key. .. _`content repository module`: `module:content`_ Recommendations when sending messages ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In the event of send failure, clients SHOULD retry requests using an exponential-backoff algorithm for a certain amount of time T. It is recommended that T is no longer than 5 minutes. After this time, the client should stop retrying and mark the message as "unsent". Users should be able to manually resend unsent messages. Users may type several messages at once and send them all in quick succession. Clients SHOULD preserve the order in which they were sent by the user. This means that clients should wait for the response to the previous request before sending the next request. This can lead to head-of-line blocking. In order to reduce the impact of head-of-line blocking, clients should use a queue per room rather than a global queue, as ordering is only relevant within a single room rather than between rooms. Local echo ~~~~~~~~~~ Messages SHOULD appear immediately in the message view when a user presses the "send" button. This should occur even if the message is still sending. This is referred to as "local echo". Clients SHOULD implement "local echo" of messages. Clients MAY display messages in a different format to indicate that the server has not processed the message. This format should be removed when the server responds. Clients need to be able to match the message they are sending with the same message which they receive from the event stream. The echo of the same message from the event stream is referred to as "remote echo". Both echoes need to be identified as the same message in order to prevent duplicate messages being displayed. Ideally this pairing would occur transparently to the user: the UI would not flicker as it transitions from local to remote. Flickering cannot be fully avoided in the current client-server API. Two scenarios need to be considered: - The client sends a message and the remote echo arrives on the event stream *after* the request to send the message completes. - The client sends a message and the remote echo arrives on the event stream *before* the request to send the message completes. In the first scenario, the client will receive an event ID when the request to send the message completes. This ID can be used to identify the duplicate event when it arrives on the event stream. However, in the second scenario, the event arrives before the client has obtained an event ID. This makes it impossible to identify it as a duplicate event. This results in the client displaying the message twice for a fraction of a second before the the original request to send the message completes. Once it completes, the client can take remedial actions to remove the duplicate event by looking for duplicate event IDs. A future version of the client-server API will resolve this by attaching the transaction ID of the sending request to the event itself. Calculating the display name for a user ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Clients may wish to show the human-readable display name of a room member as part of a membership list, or when they send a message. However, different members may have conflicting display names. Display names MUST be disambiguated before showing them to the user, in order to prevent spoofing of other users. To ensure this is done consistently across clients, clients SHOULD use the following algorithm to calculate a disambiguated display name for a given user: 1. Inspect the ``m.room.member`` state event for the relevant user id. 2. If the ``m.room.member`` state event has no ``displayname`` field, or if that field has a ``null`` value, use the raw user id as the display name. Otherwise: 3. If the ``m.room.member`` event has a ``displayname`` which is unique among members of the room with ``membership: join`` or ``membership: invite``, use the given ``displayname`` as the user-visible display name. Otherwise: 4. The ``m.room.member`` event has a non-unique ``displayname``. This should be disambiguated using the user id, for example "display name (@id:homeserver.org)". .. TODO-spec what does it mean for a ``displayname`` to be 'unique'? Are we case-sensitive? Do we care about homograph attacks? See https://matrix.org/jira/browse/SPEC-221. Developers should take note of the following when implementing the above algorithm: * The user-visible display name of one member can be affected by changes in the state of another member. For example, if ``@user1:matrix.org`` is present in a room, with ``displayname: Alice``, then when ``@user2:example.com`` joins the room, also with ``displayname: Alice``, *both* users must be given disambiguated display names. Similarly, when one of the users then changes their display name, there is no longer a clash, and *both* users can be given their chosen display name. Clients should be alert to this possibility and ensure that all affected users are correctly renamed. * The display name of a room may also be affected by changes in the membership list. This is due to the room name sometimes being based on user display names (see `Calculating the display name for a room`_). * If the entire membership list is searched for clashing display names, this leads to an O(N^2) implementation for building the list of room members. This will be very inefficient for rooms with large numbers of members. It is recommended that client implementations maintain a hash table mapping from ``displayname`` to a list of room members using that name. Such a table can then be used for efficient calculation of whether disambiguation is needed. Displaying membership information with messages ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Clients may wish to show the display name and avatar URL of the room member who sent a message. This can be achieved by inspecting the ``m.room.member`` state event for that user ID (see `Calculating the display name for a user`_). When a user paginates the message history, clients may wish to show the **historical** display name and avatar URL for a room member. This is possible because older ``m.room.member`` events are returned when paginating. This can be implemented efficiently by keeping two sets of room state: old and current. As new events arrive and/or the user paginates back in time, these two sets of state diverge from each other. New events update the current state and paginated events update the old state. When paginated events are processed sequentially, the old state represents the state of the room *at the time the event was sent*. This can then be used to set the historical display name and avatar URL. Calculating the display name for a room ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Clients may wish to show a human-readable name for a room. There are a number of possibilities for choosing a useful name. To ensure that rooms are named consistently across clients, clients SHOULD use the following algorithm to choose a name: 1. If the room has an `m.room.name`_ state event, use the name given by that event. #. If the room has an `m.room.canonical_alias`_ state event, use the alias given by that event. #. If neither of the above events are present, a name should be composed based on the members of the room. Clients should consider `m.room.member`_ events for users other than the logged-in user, with ``membership: join`` or ``membership: invite``. .. _active_members: i. If there is only one such event, the display name for the room should be the `disambiguated display name`_ of the corresponding user. #. If there are two such events, they should be lexicographically sorted by their ``state_key`` (i.e. the corresponding user IDs), and the display name for the room should be the `disambiguated display name`_ of both users: " and ", or a localised variant thereof. #. If there are three or more such events, the display name for the room should be based on the disambiguated display name of the user corresponding to the first such event, under a lexicographical sorting according to their ``state_key``. The display name should be in the format " and others" (or a localised variant thereof), where N is the number of `m.room.member`_ events with ``membership: join`` or ``membership: invite``, excluding the logged-in user and "user1". For example, if Alice joins a room, where Bob (whose user id is ``@superuser:example.com``), Carol (user id ``@carol:example.com``) and Dan (user id ``@dan:matrix.org``) are in conversation, Alice's client should show the room name as "Carol and 2 others". .. TODO-spec Sorting by user_id certainly isn't ideal, as IDs at the start of the alphabet will end up dominating room names: they will all be called "Arathorn and 15 others". Furthermore - user_ids are not necessarily ASCII, which means we need to either specify a collation order, or specify how to choose one. Ideally we might sort by the time when the user was first invited to, or first joined the room. But we don't have this information. See https://matrix.org/jira/browse/SPEC-267 for further discussion. #. If the room has no ``m.room.name`` or ``m.room.canonical_alias`` events, and no active members other than the current user, clients should consider ``m.room.member`` events with ``membership: leave``. If such events exist, a display name such as "Empty room (was and others)" (or a localised variant thereof) should be used, following similar rules as for active members (see `above `_). #. A complete absence of ``m.room.name``, ``m.room.canonical_alias``, and ``m.room.member`` events is likely to indicate a problem with creating the room or synchronising the state table; however clients should still handle this situation. A display name such as "Empty room" (or a localised variant thereof) should be used in this situation. .. _`disambiguated display name`: `Calculating the display name for a user`_ Clients SHOULD NOT use `m.room.aliases`_ events as a source for room names, as it is difficult for clients to agree on the best alias to use, and aliases can change unexpectedly. .. TODO-spec How can we make this less painful for clients to implement, without forcing an English-language implementation on them all? Server behaviour ---------------- Homeservers SHOULD reject ``m.room.message`` events which don't have a ``msgtype`` key, or which don't have a textual ``body`` key, with an HTTP status code of 400. Security considerations ----------------------- Messages sent using this module are not encrypted. Messages can be encrypted using the `E2E module`_. Clients should sanitise **all displayed keys** for unsafe HTML to prevent Cross-Site Scripting (XSS) attacks. This includes room names and topics. .. _`E2E module`: `module:e2e`_