Matrix Specification

Matrix defines a set of open APIs for decentralised communication, suitable for securely publishing, persisting and subscribing to data over a global open federation of servers with no single point of control. Uses include Instant Messaging (IM), Voice over IP (VoIP) signalling, Internet of Things (IoT) communication, and bridging together existing communication silos - providing the basis of a new open real-time communication ecosystem.

To propose a change to the Matrix Spec, see the explanations at Proposals for Spec Changes to Matrix.

Matrix APIs

The specification consists of the following parts:

• Client-Server API
• Server-Server API
• Application Service API
• Identity Service API
• Push Gateway API
• Room Versions
• Appendices

Additionally, this introduction page contains the key baseline information required to understand the specific APIs, including the section the overall architecture.

The Matrix API Viewer is useful for browsing the Client-Server API.

Introduction to the Matrix APIs

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:

• Pragmatic Web-friendly APIs (i.e. JSON over REST)
• Keep It Simple & Stupid
  • provide a simple architecture with minimal third-party dependencies.
• Fully open:
  • Fully open federation - anyone should be able to participate in the global Matrix network
  • Fully open standard - publicly documented standard with no IP or patent licensing encumbrances
• Empowering the end-user
  • The user should be able to choose the server and clients they use
  • The user should be able to control how private their communication is
  • The user should know precisely where their data is stored
• Fully decentralised - no single points of control over conversations or the network as a whole
• Learning from history to avoid repeating it
  • Trying to take the best aspects of XMPP, SIP, IRC, SMTP, IMAP and NNTP whilst trying to avoid their failings

The functionality that Matrix provides includes:

• Creation and management of fully distributed chat rooms with no single points of control or failure
• Eventually-consistent cryptographically secure synchronisation of room state across a global open network of federated servers and services
• Sending and receiving extensible messages in a room with (optional) end-to-end encryption
• Extensible user management (inviting, joining, leaving, kicking, banning) mediated by a power-level based user privilege system.
• Extensible room state management (room naming, aliasing, topics, bans)
• Extensible user profile management (avatars, display names, etc)
• Managing user accounts (registration, login, logout)
• Use of 3rd Party IDs (3PIDs) such as email addresses, phone numbers, Facebook accounts to authenticate, identify and discover users on Matrix.
• Trusted federation of identity servers for:
  • Publishing user public keys for PKI
  • Mapping of 3PIDs to Matrix IDs

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 interoperable and federated manner.

Requirement levels

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” across all parts of the specification are to be interpreted as described in RFC 2119.

Some entire sections of the specification might be optional depending on the circumstances. For example, the modules of the client-server API apply depending on the use case. The requirement level expressed by the above key words appearing in such a section is only relevant if the section itself is applicable.

Spec Change Proposals

To propose a change to the Matrix Spec, see the explanations at Proposals for Spec Changes to Matrix.

Architecture

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 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)

Users

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

See ‘Identifier Grammar’ in the appendices for full details of the structure of user IDs.

Devices

The Matrix specification has a particular meaning for the term “device”. As a user, I might have several devices: a desktop client, some web browsers, an Android device, an iPhone, etc. They broadly relate to a real device in the physical world, but you might have several browsers on a physical device, or several Matrix client applications on a mobile device, each of which would be its own device.

Devices are used primarily to manage the keys used for end-to-end encryption (each device gets its own copy of the decryption keys), but they also help users manage their access - for instance, by revoking access to particular devices.

When a user first uses a client, it registers itself as a new device. The longevity of devices might depend on the type of client. A web client will probably drop all of its state on logout, and create a new device every time you log in, to ensure that cryptography keys are not leaked to a new user. In a mobile client, it might be acceptable to reuse the device if a login session expires, provided the user is the same.

Devices are identified by a device_id, which is unique within the scope of a given user.

A user may assign a human-readable display name to a device, to help them manage their devices.

Events

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”.

Event Graphs

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.

Room structure

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.

See ‘Identifier Grammar’ in the appendices for full details of the structure of a room ID.

The following conceptual diagram shows an m.room.message event being sent to the room !qporfwt:matrix.org:

{ @alice:matrix.org }                             { @bob:example.org }
        |                                                 ^
        |                                                 |
[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     |                          |   example.org    |
+------------------+                          +------------------+
        |                                                 ^
        |         [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, example.org |
             |   Members:                         |
             |    - @alice:matrix.org             |
             |    - @bob:example.org              |
             | Messages:                          |
             |   - @alice:matrix.org              |
             |     Content: { JSON object }       |
             |....................................|

Federation maintains shared data structures per-room between multiple homeservers. The data is split into message events and state events.

Message events: These describe transient ‘one-off’ activity in a room such as instant messages, VoIP call setups, file transfers, etc. They generally describe communication activity.

State events: These describe updates to a given piece of persistent information (‘state’) related to a room, such as the room’s name, topic, membership, participating servers, etc. State is modelled as a lookup table of key/value pairs per room, with each key being a tuple of state_key and event type. Each state event updates the value of a given key.

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.

Room Aliases

Each room can also have multiple “Room Aliases”, which look like:

#room_alias:domain

See ‘Identifier Grammar’ in the appendices for full details of the structure of a room alias.

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.

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:example.org      !aaabaa:matrix.org
       |                    ^
       |                    |
_______V____________________|____
|          example.org           |
| Mappings:                      |
| #matrix >> !aaabaa:matrix.org  |
| #golf   >> !wfeiofh:sport.com  |
| #bike   >> !4rguxf:matrix.org  |
|________________________________|

Identity

Users in Matrix are identified via their Matrix user ID. 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 user IDs 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.

Profiles

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).

Private User Data

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.

Common concepts

Various things are common throughout all of the Matrix APIs. They are documented here.

Namespacing

Namespacing helps prevent conflicts between multiple applications and the specification itself. Where namespacing is used, m. prefixes are used by the specification to indicate that the field is controlled by the specification. Custom or non-specified namespaces used in the wild MUST use the Java package naming convention to prevent conflicts.

As an example, event types defined in the specification are namespaced under the special m. prefix, however any client can send a custom event type, such as com.example.game.score (assuming the client has rights to the com.example namespace) without needing to put the event into the m. namespace.

Timestamps

Unless otherwise stated, timestamps are the number of milliseconds elapsed since the unix epoch (1970-01-01 00:00:00 UTC), but not counting leap seconds, so that each day is precisely 86,400,000 milliseconds.

This means that timestamps can repeat during leap seconds. Most programming languages provide timestamps in that format natively, e.g. ECMAScript. Throughout the specification this may be referred to as POSIX, Unix, or just “time in milliseconds”.

Specification Versions

Matrix as a whole is released under a single specification number in the form vX.Y.

• A change to X reflects a breaking or substantially invasive change. When exactly to increment this number is left to the Spec Core Team, however it is intended for changes such as moving away from JSON, altering the signing algorithm, or when a large number of Y changes feel deserving of a major version increase.
• A change to Y represents a backwards compatible or “managed” backwards compatible change to the specification, usually in the form of features.

Additionally, the spec version may have arbitrary metadata applied to it when followed by a -. For example, v1.1-alpha. Usage of this is not strictly specified but is intended for usage of pre-release builds of the specification.

Note that while v1.2 is meant to be backwards compatible with v1.1, it is not guaranteed that future versions will be fully backwards compatible with v1.1. For example, if /test were to be introduced in v1.1 and deprecated in v1.2, then it can be removed in v1.3. More information about this is described in the deprecation policy below.

Endpoint versioning

All API endpoints within the specification are versioned individually. This means that /v3/sync (for example) can get deprecated in favour of /v4/sync without affecting /v3/profile at all. A server supporting /v4/sync would keep serving /v3/profile as it always has.

When an MSC proposes a breaking change to an endpoint it should also deprecate the existing endpoint. For some endpoints this might be implicit, such as /v4/sync being introduced (deprecating /v3/sync), however for more nuanced examples the MSC should deprecate the endpoint explicitly.

Deprecation policy

An MSC is required to transition something from stable (the default) to deprecated. Once something has been deprecated for suitably long enough (usually 1 version), it is eligible for removal from the specification with another MSC.

Implementations of Matrix are required to implement deprecated functionality of the specification, though when the functionality is later removed then the implementation is welcome to drop support (if they don’t advertise support for a version which includes deprecated functionality). For example, if /test were deprecated in v1.2 and removed in v1.3, then an implementation which wants to advertise support for v1.2 would have to implement /test, even if the implementation also advertises support for v1.3. If that implementation only advertises support for v1.3 then it would not be required to implement /test.

Legacy versioning

Prior to this system, the different APIs of Matrix were versioned individually. This is no longer possible with the new specification versioning approach.

For historical reference, the APIs were versioned as rX.Y.Z where X roughly represents a breaking change, Y a backwards-compatible change, and Z a patch or insignificant alteration to the API.

v1.0 of Matrix was released on June 10th, 2019 with the following API versions:

API/Specification	Version
Client-Server API	r0.5.0
Server-Server API	r0.1.2
Application Service API	r0.1.1
Identity Service API	r0.1.1
Push Gateway API	r0.1.0
Room Version	v5

License

The Matrix specification is licensed under the Apache License, Version 2.0.