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Table of Contents
Warning
The Matrix specification is still evolving: the APIs are not yet frozen and this document is in places a work in progress or stale. We have made every effort to clearly flag areas which are still being finalised. We're publishing it at this point because it's complete enough to be more than useful and provide a canonical reference to how Matrix is evolving. Our end goal is to mirror WHATWG's Living Standard.
Matrix is a set of open APIs for open-federated Instant Messaging (IM), Voice over IP (VoIP) and Internet of Things (IoT) communication, designed to create and support a new global real-time communication ecosystem. The intention is to provide an open decentralised pubsub layer for the internet for securely persisting and publishing/subscribing JSON objects. This specification is the ongoing result of standardising the APIs used by the various components of the Matrix ecosystem to communicate with one another.
The principles that Matrix attempts to follow are:
The functionality that Matrix provides includes:
The end goal of Matrix is to be a ubiquitous messaging layer for synchronising arbitrary data between sets of people, devices and services - be that for instant messages, VoIP call setups, or any other objects that need to be reliably and persistently pushed from A to B in an inter-operable and federated manner.
Matrix defines APIs for synchronising extensible JSON objects known as "events" between compatible clients, servers and services. Clients are typically messaging/VoIP applications or IoT devices/hubs and communicate by synchronising communication history with their "homeserver" using the "Client-Server API". Each homeserver stores the communication history and account information for all of its clients, and shares data with the wider Matrix ecosystem by synchronising communication history with other homeservers and their clients.
Clients typically communicate with each other by emitting events in the context of a virtual "room". Room data is replicated across all of the homeservers whose users are participating in a given room. As such, no single homeserver has control or ownership over a given room. Homeservers model communication history as a partially ordered graph of events known as the room's "event graph", which is synchronised with eventual consistency between the participating servers using the "Server-Server API". This process of synchronising shared conversation history between homeservers run by different parties is called "Federation". Matrix optimises for the the Availability and Partitioned properties of CAP theorem at the expense of Consistency.
For example, for client A to send a message to client B, client A performs an HTTP PUT of the required JSON event on its homeserver (HS) using the client-server API. A's HS appends this event to its copy of the room's event graph, signing the message in the context of the graph for integrity. A's HS then replicates the message to B's HS by performing an HTTP PUT using the server-server API. B's HS authenticates the request, validates the event's signature, authorises the event's contents and then adds it to its copy of the room's event graph. Client B then receives the message from his homeserver via a long-lived GET request.
How data flows between clients
==============================
{ Matrix client A } { Matrix client B }
^ | ^ |
| events | Client-Server API | events |
| V | V
+------------------+ +------------------+
| |---------( HTTPS )--------->| |
| homeserver | | homeserver |
| |<--------( HTTPS )----------| |
+------------------+ Server-Server API +------------------+
History Synchronisation
(Federation)
Each client is associated with a user account, which is identified in Matrix using a unique "User ID". This ID is namespaced to the homeserver which allocated the account and has the form:
@localpart:domain
The localpart of a user ID may be a user name, or an opaque ID identifying this user. The domain of a user ID is the domain of the homeserver.
All data exchanged over Matrix is expressed as an "event". Typically each client action (e.g. sending a message) correlates with exactly one event. Each event has a type which is used to differentiate different kinds of data. type values MUST be uniquely globally namespaced following Java's package naming conventions, e.g. com.example.myapp.event. The special top-level namespace m. is reserved for events defined in the Matrix specification - for instance m.room.message is the event type for instant messages. Events are usually sent in the context of a "Room".
Events exchanged in the context of a room are stored in a directed acyclic graph (DAG) called an "event graph". The partial ordering of this graph gives the chronological ordering of events within the room. Each event in the graph has a list of zero or more "parent" events, which refer to any preceding events which have no chronological successor from the perspective of the homeserver which created the event.
Typically an event has a single parent: the most recent message in the room at the point it was sent. However, homeservers may legitimately race with each other when sending messages, resulting in a single event having multiple successors. The next event added to the graph thus will have multiple parents. Every event graph has a single root event with no parent.
To order and ease chronological comparison between the events within the graph, homeservers maintain a depth metadata field on each event. An event's depth is a positive integer that is strictly greater than the depths of any of its parents. The root event should have a depth of 1. Thus if one event is before another, then it must have a strictly smaller depth.
A room is a conceptual place where users can send and receive events. Events are sent to a room, and all participants in that room with sufficient access will receive the event. Rooms are uniquely identified internally via "Room IDs", which have the form:
!opaque_id:domain
There is exactly one room ID for each room. Whilst the room ID does contain a domain, it is simply for globally namespacing room IDs. The room does NOT reside on the domain specified. Room IDs are not meant to be human readable. They are case-sensitive. The following conceptual diagram shows an m.room.message event being sent to the room !qporfwt:matrix.org:
{ @alice:matrix.org } { @bob:domain.com }
| ^
| |
[HTTP POST] [HTTP GET]
Room ID: !qporfwt:matrix.org Room ID: !qporfwt:matrix.org
Event type: m.room.message Event type: m.room.message
Content: { JSON object } Content: { JSON object }
| |
V |
+------------------+ +------------------+
| homeserver | | homeserver |
| matrix.org | | domain.com |
+------------------+ +------------------+
| ^
| [HTTP PUT] |
| Room ID: !qporfwt:matrix.org |
| Event type: m.room.message |
| Content: { JSON object } |
`-------> Pointer to the preceding message ------`
PKI signature from matrix.org
Transaction-layer metadata
PKI Authorization header
...................................
| Shared Data |
| State: |
| Room ID: !qporfwt:matrix.org |
| Servers: matrix.org, domain.com |
| Members: |
| - @alice:matrix.org |
| - @bob:domain.com |
| Messages: |
| - @alice:matrix.org |
| Content: { JSON object } |
|...................................|
Federation maintains shared data structures per-room between multiple home servers. The data is split into message events and state events.
The state of the room at a given point is calculated by considering all events preceding and including a given event in the graph. Where events describe the same state, a merge conflict algorithm is applied. The state resolution algorithm is transitive and does not depend on server state, as it must consistently select the same event irrespective of the server or the order the events were received in. Events are signed by the originating server (the signature includes the parent relations, type, depth and payload hash) and are pushed over federation to the participating servers in a room, currently using full mesh topology. Servers may also request backfill of events over federation from the other servers participating in a room.
Each room can also have multiple "Room Aliases", which look like:
#room_alias:domain
A room alias "points" to a room ID and is the human-readable label by which rooms are publicised and discovered. The room ID the alias is pointing to can be obtained by visiting the domain specified. Note that the mapping from a room alias to a room ID is not fixed, and may change over time to point to a different room ID. For this reason, Clients SHOULD resolve the room alias to a room ID once and then use that ID on subsequent requests. Room aliases MUST NOT exceed 255 bytes (including the domain).
When resolving a room alias the server will also respond with a list of servers that are in the room that can be used to join via.
HTTP GET
#matrix:domain.com !aaabaa:matrix.org
| ^
| |
_______V____________________|____
| domain.com |
| Mappings: |
| #matrix >> !aaabaa:matrix.org |
| #golf >> !wfeiofh:sport.com |
| #bike >> !4rguxf:matrix.org |
|________________________________|
Users in Matrix are identified via their matrix user ID (MXID). However, existing 3rd party ID namespaces can also be used in order to identify Matrix users. A Matrix "Identity" describes both the user ID and any other existing IDs from third party namespaces linked to their account. Matrix users can link third-party IDs (3PIDs) such as email addresses, social network accounts and phone numbers to their user ID. Linking 3PIDs creates a mapping from a 3PID to a user ID. This mapping can then be used by Matrix users in order to discover the MXIDs of their contacts. In order to ensure that the mapping from 3PID to user ID is genuine, a globally federated cluster of trusted "Identity Servers" (IS) are used to verify the 3PID and persist and replicate the mappings.
Usage of an IS is not required in order for a client application to be part of the Matrix ecosystem. However, without one clients will not be able to look up user IDs using 3PIDs.
Users may publish arbitrary key/value data associated with their account - such as a human readable display name, a profile photo URL, contact information (email address, phone numbers, website URLs etc).
Users may also store arbitrary private key/value data in their account - such as client preferences, or server configuration settings which lack any other dedicated API. The API is symmetrical to managing Profile data.