Re: DRB and threads

On Tue, 11 Mar 2003, Brian Candler wrote:

> > the problem is that this drops the current request, if you are using a
> > transactional database that might be fine - otherwise...
>
> .. the client gets half a page, which IMO is no worse than getting a 500
> Server Failed error :-)

the problem would be when using a non-transactional db, say mysql, and blowing
up midway through the application - bad thing.

> > > I mean using the drb protocol over HTTP as an API: e.g. front-end server
> > > talks to the world, and talks DRB-over-HTTP to the back end system, which
> > > has a pool of database processes run under fastcgi. It requires Ruby at both
> > > ends of course, but it should be a darned sight faster than SOAP or
> > > YAML/OKAY, and is *so* easy to use because you just make object calls on the
> > > front-end (which magically perform actions on the backend)
> >
> > i'm fuzzy on why you need fastcgi for the database backend?
>
> Well, it's like this. If I run a raw DRb server, I get into threading
> issues. Firstly, the way that a DRb server works, the object being served
> may get multiple method calls simultaneously from concurrent threads as
> multiple requests come in. So you can't just write:
>
>    db = DBI.connect('dbi:foo:','bar','baz')
>    DRb.start_service('druby://localhost:9000', db)
> because that would be highly dangerous. That can be fixed by wrapping it in
> an object which serialises the requests. In fact the that object can hold an
> array of separate DBI handles, which is even better: in principle, one
> client doing a long query then doesn't starve out other clients.

i did exactly this for postgresql -

http://raa.ruby-lang.org/list.rhtml?name=pgconngroup

> In practice though, calls to DBI libraries are likely to block Ruby's
> threading. Maybe some don't, but at best it will depend on what type of DB
> backend you are using. That is the killer for me. And finally, on a
> multi-CPU box, I don't want it all taking place on one CPU.
>
> The solution then is to fork into multiple processes, each handling one
> request at a time. I didn't fancy writing the code to do that; furthermore,
> if doing it in Ruby I'd also have to modify DRb to work over a stdin/stdout
> pipe for the parent to pass messages to the child.

or create a DRb object to handle the request, fork start this object on a port
in the child, contact the parent using drb:// protocol to register with the
parent, and then child and parent can communticate using the drb protocol
exclusively.  the parent could also return a handle on the forked child DRb
object to the client - and then communication would be between them from there
on out.  i guess this would be like a normal forking server, only using DRb
instead of sockets...

> Now, mod_fastcgi handles process creation automatically, even dynamically
> sizing the pool, and passing requests to individual children (using the
> fastcgi protocol). This seemed to be ideal. I made a small mod to DRb so
> that it does not accept() another incoming request until the current one has
> completed, and hey presto.

sounds really cool.  how do you prompt mod_fastcgi to spwan a process?  are
you hitting it via http and the web server or using it directly?

> My other solution would have been mod_ruby, which I've not tried yet - then
> I get one database handle for each Apache worker process, which also gives
> me the concurrency I need.

i haven't tried it yet but the issues i've seen on this list with building are
not motivating...

> > > Interestingly, the Ruby MOD_FCGI/CGI module itself isn't particularly
> > > speedly when compared with raw fcgi:
> >
> > there are **alot** more function calls in mod_fcgi...
>
> I think the overhead is in the Ruby standard CGI library, not in mod_fcgi.
> The attached program on my little laptop takes about 0.10 seconds per
> iteration.
>
> It turns out that the vast majority of this is the 'html3' bit, since it
> does a whole load of dynamic method additions:
>
>     case type
>     when "html3"
>       extend Html3
>       element_init()
>       extend HtmlExtension
>
> Changing CGI.new('html3') to CGI.new(nil) makes it run approximately 180
> times faster, at 0.00055 seconds per iteration :-)

ouch.

> The moral is: if you can avoid using CGI's tag-generating feature (which I
> don't think is friendly anyway) then you get a big improvement gain...

funny, i NEVER use this except for in my example program.  i use amrita for
all my stuff.

i love amrita.  anyone used to new C version?

-a


--
  ====================================
  | Ara Howard
  | NOAA Forecast Systems Laboratory
  | Information and Technology Services
  | Data Systems Group
  | R/FST 325 Broadway
  | Boulder, CO 80305-3328
  | Email: [e-mail]
  | Phone:  [phone]
  | Fax:    [phone]
  ====================================