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- ============================
- 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.]]
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