Added design sketch

design.md

@@ -0,0 +1,109 @@
+# Weave DNS (service discovery) design
+
+The model is that each host has a service that is notified of
+hostnames and weave addresses for containers on the host. It binds to
+the host bridge to answer DNS queries from local containers; anything
+it can't answer, it asks with mDNS on the weave network.
+
+The service is divided into two components: the first is the DNS
+server, which serves the records it is told about, and otherwise asks
+via mDNS on the weave network. The other component is the DNS updater,
+specific to Docker, which monitors the running containers so to add
+and remove records from the DNS server.
+
+## DNS server API
+
+The DNS server accepts HTTP requests on the following URL (patterns)
+and methods:
+
+PUT /name/<identifier>/<ip-address>
+
+Put a record for an IP, bound to a host-scoped identifier (e.g., a
+container ID), in the DNS database.
+
+DELETE /name/<identifier>/<ip-address>
+
+Remove a specific record for an IP and host-scoped identifier.
+
+DELETE /name/<identifier>
+
+Remove all records for the host-scoped identifier.
+
+GET /name/
+
+List all the records in the database.
+
+GET /name/<identifier>
+
+List all the records for a host-scoped identifier.
+
+GET /status
+
+Give the server status.
+
+### Record structure
+
+The record structure is *either* form fields (useful for scripting
+PUTs), or a JSON object with the same fields. The fields are:
+
+```js
+    {
+        "domain": string,
+        "routing_prefix": number
+    }
+```
+
+> Also TTL?
+
+## DNS server behaviour
+
+The DNS server listens for the HTTP requests as above, and maintains a
+database of host-local names accordingly.
+
+It also listens on the host-local network for DNS queries (port
+53). Anything that is in its database it responds to immediately. For
+other requests, it asks using multicast DNS on the weave network.
+
+Meanwhile, the DNS server is listening for mDNS queries, and responds
+if it the queried name is in its database.
+
+### Caching mDNS answers
+
+The DNS server will hear the answers to its own and other servers'
+mDNS queries sent on the weave network. It can cache these, to respond
+to local DNS requests; the cache is consulted before using mDNS.
+
+## DNS updater
+
+The updater component uses the Docker remote API to monitor containers
+coming and going, and tells the DNS server to update its records via
+its HTTP interface. It does not need to be attached to the weave
+network.
+
+The updater starts by subscribing to the events, and getting a list of
+the current containers. Any containers given a domain ending with
+".weave" are considered for inclusion in the name database.
+
+When it sees a container start or stop, the updater checks the weave
+network attachment of the container, and updates the DNS server.
+
+> How does it check the network attachment from within a container?
+
+> Will it need to delay slightly so that `attach` has a chance to run?
+> Perhaps it could put containers on a watch list when it's noticed
+> them.
+
+## Container startup
+
+> TBD
+
+## Grouping and load balancing
+
+> TBD
+
+## Extensions
+
+ * An updater that checks health more specifically, e.g., by testing
+   that a particular port is being listened to, or that it can get a
+   200 OK. These could be encoded in the container environment
+   variables, as per registrator.