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Matthew Hodgson
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===========================
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Matrix Server-to-Server API
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===========================
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A description of the protocol used to communicate between Matrix home servers;
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also known as Federation.
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Overview
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========
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The server-server API is a mechanism by which two home servers can exchange
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Matrix event messages, both as a real-time push of current events, and as a
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historic fetching mechanism to synchronise past history for clients to view. It
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uses HTTP connections between each pair of servers involved as the underlying
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transport. Messages are exchanged between servers in real-time by active pushing
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from each server's HTTP client into the server of the other. Queries to fetch
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historic data for the purpose of back-filling scrollback buffers and the like
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can also be performed.
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{ Matrix clients } { Matrix clients }
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^ | ^ |
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| events | | events |
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| V | V
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+------------------+ +------------------+
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| |---------( HTTP )---------->| |
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| Home Server | | Home Server |
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| |<--------( HTTP )-----------| |
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+------------------+ +------------------+
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There are three main kinds of communication that occur between home servers:
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* Queries
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These are single request/response interactions between a given pair of
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servers, initiated by one side sending an HTTP request to obtain some
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information, and responded by the other. They are not persisted and contain
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no long-term significant history. They simply request a snapshot state at the
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instant the query is made.
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* EDUs - Ephemeral Data Units
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These are notifications of events that are pushed from one home server to
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another. They are not persisted and contain no long-term significant history,
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nor does the receiving home server have to reply to them.
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* PDUs - Persisted Data Units
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These are notifications of events that are broadcast from one home server to
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any others that are interested in the same "context" (namely, a Room ID).
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They are persisted to long-term storage and form the record of history for
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that context.
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Where Queries are presented directly across the HTTP connection as GET requests
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to specific URLs, EDUs and PDUs are further wrapped in an envelope called a
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Transaction, which is transferred from the origin to the destination home server
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using a PUT request.
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Transactions and EDUs/PDUs
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==========================
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The transfer of EDUs and PDUs between home servers is performed by an exchange
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of Transaction messages, which are encoded as JSON objects with a dict as the
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top-level element, passed over an HTTP PUT request. A Transaction is meaningful
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only to the pair of home servers that exchanged it; they are not globally-
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meaningful.
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Each transaction has an opaque ID and timestamp (UNIX epoch time in
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milliseconds) generated by its origin server, an origin and destination server
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name, a list of "previous IDs", and a list of PDUs - the actual message payload
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that the Transaction carries.
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{"transaction_id":"916d630ea616342b42e98a3be0b74113",
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"ts":1404835423000,
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"origin":"red",
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"destination":"blue",
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"prev_ids":["e1da392e61898be4d2009b9fecce5325"],
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"pdus":[...],
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"edus":[...]}
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The "previous IDs" field will contain a list of previous transaction IDs that
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the origin server has sent to this destination. Its purpose is to act as a
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sequence checking mechanism - the destination server can check whether it has
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successfully received that Transaction, or ask for a retransmission if not.
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The "pdus" field of a transaction is a list, containing zero or more PDUs.[*]
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Each PDU is itself a dict containing a number of keys, the exact details of
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which will vary depending on the type of PDU. Similarly, the "edus" field is
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another list containing the EDUs. This key may be entirely absent if there are
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no EDUs to transfer.
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(* Normally the PDU list will be non-empty, but the server should cope with
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receiving an "empty" transaction, as this is useful for informing peers of other
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transaction IDs they should be aware of. This effectively acts as a push
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mechanism to encourage peers to continue to replicate content.)
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All PDUs have an ID, a context, a declaration of their type, a list of other PDU
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IDs that have been seen recently on that context (regardless of which origin
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sent them), and a nested content field containing the actual event content.
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[[TODO(paul): Update this structure so that 'pdu_id' is a two-element
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[origin,ref] pair like the prev_pdus are]]
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{"pdu_id":"a4ecee13e2accdadf56c1025af232176",
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"context":"#example.green",
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"origin":"green",
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"ts":1404838188000,
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"pdu_type":"m.text",
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"prev_pdus":[["blue","99d16afbc857975916f1d73e49e52b65"]],
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"content":...
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"is_state":false}
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In contrast to the transaction layer, it is important to note that the prev_pdus
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field of a PDU refers to PDUs that any origin server has sent, rather than
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previous IDs that this origin has sent. This list may refer to other PDUs sent
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by the same origin as the current one, or other origins.
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Because of the distributed nature of participants in a Matrix conversation, it
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is impossible to establish a globally-consistent total ordering on the events.
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However, by annotating each outbound PDU at its origin with IDs of other PDUs it
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has received, a partial ordering can be constructed allowing causallity
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relationships to be preserved. A client can then display these messages to the
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end-user in some order consistent with their content and ensure that no message
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that is semantically in reply of an earlier one is ever displayed before it.
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PDUs fall into two main categories: those that deliver Events, and those that
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synchronise State. For PDUs that relate to State synchronisation, additional
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keys exist to support this:
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{...,
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"is_state":true,
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"state_key":TODO
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"power_level":TODO
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"prev_state_id":TODO
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"prev_state_origin":TODO}
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[[TODO(paul): At this point we should probably have a long description of how
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State management works, with descriptions of clobbering rules, power levels, etc
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etc... But some of that detail is rather up-in-the-air, on the whiteboard, and
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so on. This part needs refining. And writing in its own document as the details
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relate to the server/system as a whole, not specifically to server-server
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federation.]]
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EDUs, by comparison to PDUs, do not have an ID, a context, or a list of
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"previous" IDs. The only mandatory fields for these are the type, origin and
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destination home server names, and the actual nested content.
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{"edu_type":"m.presence",
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"origin":"blue",
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"destination":"orange",
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"content":...}
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Protocol URLs
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=============
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All these URLs are namespaced within a prefix of
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/_matrix/federation/v1/...
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For active pushing of messages representing live activity "as it happens":
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PUT .../send/:transaction_id/
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Body: JSON encoding of a single Transaction
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Response: [[TODO(paul): I don't actually understand what
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ReplicationLayer.on_transaction() is doing here, so I'm not sure what the
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response ought to be]]
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The transaction_id path argument will override any ID given in the JSON body.
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The destination name will be set to that of the receiving server itself. Each
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embedded PDU in the transaction body will be processed.
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To fetch a particular PDU:
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GET .../pdu/:origin/:pdu_id/
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Response: JSON encoding of a single Transaction containing one PDU
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Retrieves a given PDU from the server. The response will contain a single new
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Transaction, inside which will be the requested PDU.
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To fetch all the state of a given context:
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GET .../state/:context/
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Response: JSON encoding of a single Transaction containing multiple PDUs
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Retrieves a snapshot of the entire current state of the given context. The
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response will contain a single Transaction, inside which will be a list of
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PDUs that encode the state.
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To backfill events on a given context:
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GET .../backfill/:context/
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Query args: v, limit
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Response: JSON encoding of a single Transaction containing multiple PDUs
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Retrieves a sliding-window history of previous PDUs that occurred on the
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given context. Starting from the PDU ID(s) given in the "v" argument, the
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PDUs that preceeded it are retrieved, up to a total number given by the
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"limit" argument. These are then returned in a new Transaction containing all
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off the PDUs.
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To stream events all the events:
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GET .../pull/
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Query args: origin, v
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Response: JSON encoding of a single Transaction consisting of multiple PDUs
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Retrieves all of the transactions later than any version given by the "v"
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arguments. [[TODO(paul): I'm not sure what the "origin" argument does because
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I think at some point in the code it's got swapped around.]]
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To make a query:
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GET .../query/:query_type
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Query args: as specified by the individual query types
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Response: JSON encoding of a response object
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Performs a single query request on the receiving home server. The Query Type
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part of the path specifies the kind of query being made, and its query
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arguments have a meaning specific to that kind of query. The response is a
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JSON-encoded object whose meaning also depends on the kind of query.
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