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it doesn't have any documentation; there is a short section on it in "Essential PHP Tools", but appears to be too out of date.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@xxxxxxxxxxx> wrote in message news:42E87651.1050102@xxxxxxxxxxxxxx > 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 > >> > >> > > -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
