============================
Synapse Server-to-Server API
============================

A description of the protocol used to communicate between Synapse home servers;
also known as Federation.


Overview
========

The server-server API is a mechanism by which two home servers can exchange
Synapse event messages, both as a real-time push of current events, and as a
historic fetching mechanism to synchronise past history for clients to view. It
uses HTTP connections between each pair of servers involved as the underlying
transport. Messages are exchanged between servers in real-time by active pushing
from each server's HTTP client into the server of the other. Queries to fetch
historic data for the purpose of back-filling scrollback buffers and the like
can also be performed.


 { Synapse entities }                            { Synapse entities }
     ^          |                                    ^          |
     |  events  |                                    |  events  |
     |          V                                    |          V
 +------------------+                            +------------------+
 |                  |---------( HTTP )---------->|                  |
 |   Home Server    |                            |   Home Server    |
 |                  |<--------( HTTP )-----------|                  |
 +------------------+                            +------------------+


Transactions and PDUs
=====================

The communication between home servers is performed by a bidirectional exchange
of messages. These messages are called Transactions, and are encoded as JSON
objects with a dict as the top-level element, passed over HTTP. A Transaction is
meaningful only to the pair of home servers that exchanged it; they are not
globally-meaningful.

Each transaction has an opaque ID and timestamp (UNIX epoch time in miliseconds)
generated by its origin server, an origin and destination server name, a list of
"previous IDs", and a list of PDUs - the actual message payload that the
Transaction carries.

 {"transaction_id":"916d630ea616342b42e98a3be0b74113",
  "ts":1404835423000,
  "origin":"red",
  "destination":"blue",
  "prev_ids":["e1da392e61898be4d2009b9fecce5325"],
  "pdus":[...]}

The "previous IDs" field will contain a list of previous transaction IDs that
the origin server has sent to this destination. Its purpose is to act as a
sequence checking mechanism - the destination server can check whether it has
successfully received that Transaction, or ask for a retransmission if not.

The "pdus" field of a transaction is a list, containing zero or more PDUs.[*]
Each PDU is itself a dict containing a number of keys, the exact details of
which will vary depending on the type of PDU.

(* Normally the PDU list will be non-empty, but the server should cope with
receiving an "empty" transaction, as this is useful for informing peers of other
transaction IDs they should be aware of. This effectively acts as a push
mechanism to encourage peers to continue to replicate content.)

All PDUs have an ID, a context, a declaration of their type, a list of other PDU
IDs that have been seen recently on that context (regardless of which origin
sent them), and a nested content field containing the actual event content.

[[TODO(paul): Update this structure so that 'pdu_id' is a two-element
[origin,ref] pair like the prev_pdus are]]

 {"pdu_id":"a4ecee13e2accdadf56c1025af232176",
  "context":"#example.green",
  "origin":"green",
  "ts":1404838188000,
  "pdu_type":"m.text",
  "prev_pdus":[["blue","99d16afbc857975916f1d73e49e52b65"]],
  "content":...
  "is_state":false}

In contrast to the transaction layer, it is important to note that the prev_pdus
field of a PDU refers to PDUs that any origin server has sent, rather than
previous IDs that this origin has sent. This list may refer to other PDUs sent
by the same origin as the current one, or other origins.

Because of the distributed nature of participants in a Synapse conversation, it
is impossible to establish a globally-consistent total ordering on the events.
However, by annotating each outbound PDU at its origin with IDs of other PDUs it
has received, a partial ordering can be constructed allowing causallity
relationships to be preserved. A client can then display these messages to the
end-user in some order consistent with their content and ensure that no message
that is semantically in reply of an earlier one is ever displayed before it.

PDUs fall into two main categories: those that deliver Events, and those that
synchronise State. For PDUs that relate to State synchronisation, additional
keys exist to support this:

 {...,
  "is_state":true,
  "state_key":TODO
  "power_level":TODO
  "prev_state_id":TODO
  "prev_state_origin":TODO}

[[TODO(paul): At this point we should probably have a long description of how
State management works, with descriptions of clobbering rules, power levels, etc
etc... But some of that detail is rather up-in-the-air, on the whiteboard, and
so on. This part needs refining. And writing in its own document as the details
relate to the server/system as a whole, not specifically to server-server
federation.]]


Protocol URLs
=============

For active pushing of messages representing live activity "as it happens":

  PUT /send/:transaction_id/
    Body: JSON encoding of a single Transaction

    Response: [[TODO(paul): I don't actually understand what
    ReplicationLayer.on_transaction() is doing here, so I'm not sure what the
    response ought to be]]

  The transaction_id path argument will override any ID given in the JSON body.
  The destination name will be set to that of the receiving server itself. Each
  embedded PDU in the transaction body will be processed.


To fetch a particular PDU:

  GET /pdu/:origin/:pdu_id/

    Response: JSON encoding of a single Transaction containing one PDU

  Retrieves a given PDU from the server. The response will contain a single new
  Transaction, inside which will be the requested PDU.
  

To fetch all the state of a given context:

  GET /state/:context/

    Response: JSON encoding of a single Transaction containing multiple PDUs

  Retrieves a snapshot of the entire current state of the given context. The
  response will contain a single Transaction, inside which will be a list of
  PDUs that encode the state.


To paginate events on a given context:

  GET /paginate/:context/
    Query args: v, limit

    Response: JSON encoding of a single Transaction containing multiple PDUs

  Retrieves a sliding-window history of previous PDUs that occurred on the
  given context. Starting from the PDU ID(s) given in the "v" argument, the
  PDUs that preceeded it are retrieved, up to a total number given by the
  "limit" argument. These are then returned in a new Transaction containing all
  off the PDUs.


To stream events all the events:

  GET /pull/
    Query args: origin, v

  Response: JSON encoding of a single Transaction consisting of multiple PDUs

  Retrieves all of the transactions later than any version given by the "v"
  arguments. [[TODO(paul): I'm not sure what the "origin" argument does because
  I think at some point in the code it's got swapped around.]]