launchpad-dev team mailing list archive

[Guilherme Salgado <salgado@xxxxxxxxxxxxx>]

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


-- 
Guilherme Salgado <salgado@xxxxxxxxxxxxx>

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