Add documentation about Federation Queries and EDUs

docs/server-server/specification.rst

@@ -29,15 +29,40 @@ can also be performed.
  |                  |<--------( HTTP )-----------|                  |
  +------------------+                            +------------------+
 
+There are three main kinds of communication that occur between home servers:
 
-Transactions and PDUs
+ * Queries
-=====================
+   These are single request/response interactions between a given pair of
+   servers, initiated by one side sending an HTTP request to obtain some
+   information, and responded by the other. They are not persisted and contain
+   no long-term significant history. They simply request a snapshot state at the
+   instant the query is made.
 
-The communication between home servers is performed by a bidirectional exchange
+ * EDUs - Ephemeral Data Units
-of messages. These messages are called Transactions, and are encoded as JSON
+   These are notifications of events that are pushed from one home server to
-objects with a dict as the top-level element, passed over HTTP. A Transaction is
+   another. They are not persisted and contain no long-term significant history,
-meaningful only to the pair of home servers that exchanged it; they are not
+   nor does the receiving home server have to reply to them.
-globally-meaningful.
+
+ * PDUs - Persisted Data Units
+   These are notifications of events that are broadcast from one home server to
+   any others that are interested in the same "context" (namely, a Room ID).
+   They are persisted to long-term storage and form the record of history for
+   that context.
+
+Where Queries are presented directly across the HTTP connection as GET requests
+to specific URLs, EDUs and PDUs are further wrapped in an envelope called a
+Transaction, which is transferred from the origin to the destination home server
+using a PUT request.
+
+
+Transactions and EDUs/PDUs
+==========================
+
+The transfer of EDUs and PDUs between home servers is performed by an exchange
+of Transaction messages, which are encoded as JSON objects with a dict as the
+top-level element, passed over an HTTP PUT request. 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
@@ -49,7 +74,8 @@ Transaction carries.
   "origin":"red",
   "destination":"blue",
   "prev_ids":["e1da392e61898be4d2009b9fecce5325"],
-  "pdus":[...]}
+  "pdus":[...],
+  "edus":[...]}
 
 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
@@ -58,7 +84,9 @@ 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.
+which will vary depending on the type of PDU. Similarly, the "edus" field is
+another list containing the EDUs. This key may be entirely absent if there are
+no EDUs to transfer.
 
 (* 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
@@ -112,6 +140,15 @@ 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.]]
 
+EDUs, by comparison to PDUs, do not have an ID, a context, or a list of
+"previous" IDs. The only mandatory fields for these are the type, origin and
+destination home server names, and the actual nested content.
+
+ {"edu_type":"m.presence",
+  "origin":"blue",
+  "destination":"orange",
+  "content":...}
+
 
 Protocol URLs
 =============
@@ -179,3 +216,16 @@ To stream events all the events:
   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.]]
+
+
+To make a query:
+
+  GET .../query/:query_type
+    Query args: as specified by the individual query types
+
+  Response: JSON encoding of a response object
+
+  Performs a single query request on the receiving home server. The Query Type
+  part of the path specifies the kind of query being made, and its query
+  arguments have a meaning specific to that kind of query. The response is a
+  JSON-encoded object whose meaning also depends on the kind of query.