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Posted by "Aiguo Fei" on 11/14/04 11:22
Thanks your insightful comments. Clearly scalability is a concern for large
scale applications. One has to be discreet with it if such a facility is
available. For the dictionary example, I feel it definitely can use such a
facility and DB approach is much slower; and scalability/load balancing is
not an issue since no synchronization of any kind is necessary.
I checked APC, but unfortunately it doesn't have any documentation; there is
a short section on it in "Essential PHP Tools", but appears to be too out of
date. I checked out the chapter on PHP extentions in "Power PHP5
programming" book, seems "persistent resource that can survive in between
requests" seems to do what I have in mind, but again there is no more than
just mentioning it.
a.f.
"Rasmus Lerdorf" <rasmus@lerdorf.com> wrote in message
news:42E87651.1050102@lerdorf.com...
> This is the sort of thing we call a scalability killer. Loading
> everything into memory on one machine and then adding locks presumably
> for writing to this memory means you are limiting any sort of horizontal
> scalability and you are slowing down the one machine you do have with
> lock contention.
>
> But yes, there are a number of ways to do this. PHP has all the
> low-level functions you need to create, access and lock shared memory
> segments. If you look at pecl/apc you can use the apc_store/apc_fetch
> calls to shove an entire array or object into shared memory just once at
> the beginning and then pull it out very quickly directly from memory on
> each request.
>
> Or you could write yourself a simple extension that creates the
> appropriate structures in memory in the MINIT hook.
>
> But, consider scalability issues. If you are at the point where you are
> looking at concepts like this, you are probably also at the point where
> you need to start thinking about horizontal scaling, load balancers,
> failover, etc. A replicated database backend with a replicated copy of
> the data on each web server gets you data distribution and robust
> locking on the write to the master while all the reads from the local
> clients are likely to come right out of the database's memory-based
> request cache. The performance hit for this extra data replication and
> scalability is probably smaller than you think and the added value is
> extensive.
>
> -Rasmus
>
> Joe Wollard wrote:
> > Aiguo,
> >
> >
> >> To achieve good performance, I want to load the
> >> whole dictionary into memory when the "application" starts (e.g.,
when
> >> the web server starts...
> >
> >
> > I'm not sure how ASP works, but in PHP you can't load something into
> > memory when the web server starts and keep it there. PHP 'compiles'
> > each script upon request and then frees up the memory it occupied. I'd
> > think that this would be counter productive if your looking to achieve
> > good performance because you'll be re-building the dictionary on every
> > page load.
> >
> >
> > It might be better to let another application load the data into memory
> > and hold it there instead of making PHP load the whole thing every time
> > a user requests the page/data. If it were me I think I'd simply load
> > the dictionary into a database and build a simple library or class to
> > perform queries and calculations against the dictionary.
> >
> > Assuming your class definition takes care of connecting to the database
> > server you could lookup a word in almost the exact way you
demonstrated.
> > e.g.
> > $dict = new dictionary;
> > $definition = $dict->lookup($word);
> >
> >
> > If you'll need this object to be available on every page the user sees
> > you could always make a 'common.php' (or whatever) which would create
> > the object and then include that at the top of each page.
> >
> >
> >> For a complete solution, it should support object locking/exclusive
> >> access as well.
> >
> >
> > Maybe I've been a spoiled PHP programmer for too long. ;-) I'm not sure
> > what object locking/ exclusive access is but from the sounds of it,
> > there isn't a way to do this in PHP that I'm aware of. This is assuming
> > that you want to be able to basically remove the object from the global
> > scope. In PHP objects are globally accessible from the moment they're
> > created to the moment they're destroyed.
> >
> > Good luck!
> > -Joe
> >
> >
> > On Jul 27, 2005, at 8:40 PM, Aiguo Fei wrote:
> >
> >> In ASP.Net there is the concept of "application" which is a virtual
> >> directory on a Web server. An application can have application-wide
> >> shared data/objects, which can be accessed by any script of that
> >> application (i.e., scripts under the virtual directory). I have gone
> >> through several PHP books, haven't seen it mentioned anywhere that PHP
> >> has similar facility. I don't know if anyone has written an extention
> >> to do something similar. Or any experience/suggestion on this matter.
> >>
> >> To make it clear, consider the following example. I want to do
> >> web-based dictionary. To achieve good performance, I want to load the
> >> whole dictionary into memory when the "application" starts (e.g., when
> >> the web server starts, or triggered when the first script under a
> >> certain directory is requested), and build a lookup table; then create
> >> a globally-accessible object, let's say, Application["my_dict"]; and
> >> it provides a function to do the dictionary lookup. In a script, one
> >> could do something like:
> >> $definition=Application["my_dict"]->lookup( $word );
> >>
> >> For a complete solution, it should support object locking/exclusive
> >> access as well.
> >>
> >> --
> >> PHP General Mailing List (http://www.php.net/)
> >> To unsubscribe, visit: http://www.php.net/unsub.php
> >>
> >>
> >
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