Update the ARCHITECTURE document

Also has a pretty line diagram

docs/ARCHITECTURE.rst

@@ -1,85 +1,85 @@
 Components
 ==========
 
-The components of the server are roughly like this::
-
-   -------
-   - Env -
-   -------
-
-     -------
-     - IRC -
-     -------
-            <
-             -------------     ------------
-             - ElectrumX -<<<<<- LocalRPC -
-             -------------     ------------
-              <      >
-    ----------        -------------------
-    - Daemon -<<<<<<<<- Block processor -
-    ----------        -------------------
-        <              <               >
-         --------------                 -----------
-         - Prefetcher -                 - FS + DB -
-         --------------                 -----------
-
+.. image:: https://docs.google.com/drawings/d/1Su_DR2c8__-4phm12hAzV65fL2tNm_1IhKr4XivkW6Q/pub?w=720&h=540
+    :target: https://docs.google.com/drawings/d/1Su_DR2c8__-4phm12hAzV65fL2tNm_1IhKr4XivkW6Q/pub?w=960&h=720
 
 Env
 ---
 
-Holds configuration taken from the environment.  Handles defaults
-appropriately.  Generally passed to the constructor of other
-components which take their settings from it.
+Holds configuration taken from the environment, with apprioriate
+defaulting appropriately.  Generally passed to the constructor of
+other components which take their settings from it.
+
+Controller
+----------
+
+The central part of the server process initialising and coordinating
+all the others.  Manages resource usage.
 
 
 LocalRPC
 --------
 
 Handles local JSON RPC connections querying ElectrumX server state.
-Not started until the block processor has caught up with the daemon.
+Started when the ElectrumX process starts.
 
 ElectrumX
 ---------
 
 Handles JSON Electrum client connections over TCP or SSL.  One
 instance per client session.  Should be the only component concerned
-with the details of the Electrum wire protocol.  Responsible for
-caching of client responses.  Not started until the block processor
-has caught up with the daemon.  Logically, if not yet in practice, a
-coin-specific class.
+with the details of the Electrum wire protocol.
+
+Not started until the block processor has caught up with the daemon.
 
 Daemon
 ------
 
-Used by the block processor, ElectrumX servers and prefetcher.
-Encapsulates daemon RPC wire protcol.  Logically, if not yet in
-practice, a coin-specific class.
+Encapsulates the RPC wire protcol with bitcoind for the whole server.
+Transparently handles temporary daemon errors, and fails over if
+necessary.
+
 
 Block Processor
 ---------------
 
 Responsible for managing block chain state (UTXO set, history,
-transaction and undo information) and processing towards the chain
-tip.  Uses the caches for in-memory state updates since the last
-flush.  Flushes state to the storage layer.  Reponsible for handling
-block chain reorganisations.  Once caught up maintains a
-representation of daemon mempool state.
+transaction and undo information) and for handling block chain
+reorganisations.
 
-Database
---------
+When caught up, processes new blocks as they are found, and flushes
+the updates to the database immediately.
 
-The database.  Along with the host filesystem stores flushed chain state.
+When syncing uses caches for in-memory state updates since the prior
+flush.  Occasionally flushes state to the storage layer when caches
+get large.
 
 Prefetcher
 ----------
 
-Used by the block processor to asynchronously prefetch blocks from the
-daemon.  Holds fetched block height.  Once it has caught up
-additionally obtains daemon mempool tx hashes.  Serves blocks and
-mempool hashes to the block processor via a queue.
+Cooperates with the block processor to asynchronously prefetch blocks
+from bitcoind.  Once it has caught up it additionally requests mempool
+transaction hashes from bitcoind.  Serves blocks to the block
+processor via a queue, and the mempool hashes to the Mempool object.
+
+Mempool
+-------
+
+Handles all the details of maintaining a representation of bitcoind's
+mempool state.  Obtains the list of current mempool transaction hashes
+from the Daemon when notified by the Prefetcher.
+
+Notifies the controller that addresses have been touched when the
+mempool refreshes (or implicitly when a new block is found).
+
+Database
+--------
+
+The database.  Flushed chain state is stored in the DB backend, such
+as leveldb, along with metadata on the host filesystem.
 
 IRC
 ---
 
-Not currently imlpemented; will handle IRC communication for the
-ElectrumX servers.
+Handles advertising of ElectrumX services via IRC.