launchpad-dev team mailing list archive

["Francis J. Lacoste" <francis.lacoste@xxxxxxxxxxxxx>]

On January 6, 2010, Guilherme Salgado wrote:
> On Wed, 2010-01-06 at 20:26 +0700, Stuart Bishop wrote:
> > On Wed, Jan 6, 2010 at 8:20 PM, Guilherme Salgado <salgado@xxxxxxxxxxxxx> 
wrote:
> > >> > After realizing that I came up with another approach, which relies
> > >> > only on the presence/absence of the read-only.txt file to figure out
> > >> > the mode we're on.  On this approach,
> > >> > config.database.main_master/slave are gone and we use
> > >> > dbconfig.main_master/slave instead, which are properties in
> > >> > DatabaseConfig that return the appropriate value according to the
> > >> > mode we're on.
> > >>
> > >> Does this mean we're checking for the presence of this text file
> > >> before every database operation? That sounds quite IO intensive.
> > >
> > > ISTM that the presence of the file would be checked only a couple times
> > > (once for each of the properties in DatabaseConfig that look for that
> > > file) for each handler thread, as a consequence of storm creating the
> > > DB connections when they're first used.
> > >
> > > If that's correct, then we'll have to find a way to reset the stores in
> > > all threads when we switch modes -- something I didn't realize before.
> >
> > You can look for this file at the start of the request when installing
> > the database policy and change the database settings for just the
> > current thread. That is just one stat() per request rather than per
> > database operation. We certainly wouldn't want the appserver to be
> > changing from normal to read-only mode half way through handling a
> > request.
> 
> I guess that'd work, but since we have other places[1] which check
> whether or not we're in read only mode on every request, I think we
> should cache that information somewhere so that we don't issue more than
> one stat() call (on read-only.txt) for each request.  Since we'll be
> checking that file at the beginning of every request, we could cache it
> in request.annotations, maybe?
> 
> [1] LaunchpadDatabase.connection_factory
>     LaunchpadSecurityPolicy.checkPermission()
>     LaunchpadDatabasePolicyFactory and WebServiceDatabasePolicyFactory
> 

Yes, I think request.annotations is the right place to save this information.

-- 
Francis J. Lacoste
francis.lacoste@xxxxxxxxxxxxx

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