As I (and Stefan?) see it there are two major areas the current fetcher could be
improved (as in speed)
1. Politeness code and how it is implemented is the biggest
problem of current fetcher(together with robots.txt handling).
With a simple code changes like replacing it with a PriorityQueue
based solution showed very promising results in increased IO.
2. Changing fetcher to use non blocking io (this requires great amount
of work as we need to implement the protocols from scratch again).
I would like to start with working towards #1 by first refactoring
the current code (plugins actually) in following way:
1. Move robots.txt handling away from (lib-http)plugin.
Even if this is related only to http, leaving it to lib-http
does not allow other kinds of scheduling strategies to be implemented
(it is hardcoded to fetch robots.txt from the same thread when requesting
a page from a site from witch it hasn't tried to load robots.txt)
2. Move code for politeness away from (lib-http)plugin
It is really usable outside http and also the current design limits
changing of the implementation (to queue based)
Where to move these, well my suggestion is the nutch core, does anybody
see problems with this?
These code refactoring activities are to be done in a way that none
of the current functionality is (at least deliberately) changed leaving
current functionality as is thus leaving room and possibility to build
the next generation fetcher(s) without destroying the old one at same time.