Initial TCP protocol implementation

This defines the low level TCP replication protocol

synapse/replication/tcp/__init__.py

@@ -12,3 +12,35 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
+"""This module implements the TCP replication protocol used by synapse to
+communicate between the master process and its workers (when they're enabled).
+
+The protocol is based on fire and forget, line based commands. An example flow
+would be (where '>' indicates master->worker and '<' worker->master flows)::
+
+    > SERVER example.com
+    < REPLICATE events 53
+    > RDATA events 54 ["$foo1:bar.com", ...]
+    > RDATA events 55 ["$foo4:bar.com", ...]
+
+The example shows the server accepting a new connection and sending its identity
+with the `SERVER` command, followed by the client asking to subscribe to the
+`events` stream from the token `53`. The server then periodically sends `RDATA`
+commands which have the format `RDATA <stream_name> <token> <row>`, where the
+format of `<row>` is defined by the individual streams.
+
+Error reporting happens by either the client or server sending an `ERROR`
+command, and usually the connection will be closed.
+
+
+Structure of the module:
+ * client.py   - the client classes used for workers to connect to master
+ * command.py  - the definitions of all the valid commands
+ * protocol.py - contains bot the client and server protocol implementations,
+                 these should not be used directly
+ * resource.py - the server classes that accepts and handle client connections
+ * streams.py  - the definitons of all the valid streams
+
+Further details can be found in docs/tcp_replication.rst
+"""