Add some notes about rocket interface to design.md

docs/design.md

@@ -266,3 +266,40 @@ Each table contains:
   from the end of the file will not shorten it; it will only be
   deleted when it has been fully filled and all of the data has been
   subsequently removed.
+
+
+Rocket
+------
+
+Original design had the nilmdb.nilmdb thread (through bulkdata)
+convert from on-disk layout to a Python list, and then the
+nilmdb.server thread (from cherrypy) converts to ASCII.  For at least
+the extraction side of things, it's easy to pass the bulkdata a layout
+name instead, and have it convert directly from on-disk to ASCII
+format, because this conversion can then be shoved into a C module.
+This module, which provides a means for converting directly from
+on-disk format to ASCII or Python lists, is the "rocket" interface.
+Python is still used to manage the files and figure out where the
+data should go; rocket just puts binary data directly in or out of
+those files at specified locations.
+
+Before rocket, testing speed with uint16_6 data, with an end-to-end
+test (extracting data with nilmtool):
+
+- insert: 65 klines/sec
+- extract: 120 klines/sec
+
+After switching to the rocket design, but using the Python version
+(pyrocket):
+
+- insert: (hasn't yet been switched completely; more complicated due
+           to error handling when parsing ASCII)
+- extract: 120 klines/sec
+
+After switching to a C extension module (rocket.c)
+
+- insert:
+- extract: 
+
+
+