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Developing "Nth-Tier" Web Applications For Unix?
Kismet asks: "I've recently been involved in a largish Web application written using Microsoft tools. It consists of a UI layer written in VBScript for Microsoft ASP, a business tier of COM objects, and a backend on SQL Server 7. How can I implement something like this with Linux? It's really easy to use server-side COM objects with ASP, but is there anything that easy for PHP or Mason or something else on Linux? PHP's database stuff is neat, but I'd like to move that out of the site script."
I think you might have got a little obsessed with the concept of a "teir" for a start.
Essentially, PHP by itself, or any language that supports abstraction, which is pretty much all of them, can be teired, this is simply dividing your program logically into n parts, and having each part communicate with the others only through a tightly defined interface.
The benefits of the 3 teir webscript/operations/database model are laid precisely in this tightly defined interface, benefits such as better security (fine-grained permissions checks can be made within the second teir, avoiding the possibility of last teir compormise, and logical simplicity within the webscripts themselves. However it is not necessary to have a three teir architecture actually run on three different applications as such.
It is entirely practical for example to, in any major database, have both the first and second teirs within the database itself. Oracle, Microsoft SQL 6/7 and the rest of the heavyweights have built in programming languages which allow you to create SQL functions, some fairly simple configuration lets you force all incoming queries to functions rather than using SQL directly, giving you the opportunity to do the second teir authentication and abstraction using the database language.
I don't particularly recommend this in the MS-SQL case at least, the language is ugly, but it does work.
The other option is to create a good library set of objects or functions in PHP say, and use that as your second teir, using require or include to pull in the libraries that do the logic work, and constaining yourself to simple operations and layout within the website PHP themselves.
If you're really set on using a COM style system, you could try something like CORBA, PHP has bindings (http://www.php.net/manual/ref.satellite.php) which allow you to talk to CORBA objects, it is probably not, at this stage, as tightly integrated as COM is into ASP, but a few functions to wrap it up should work effectively.
Personally, for most setups, I would recommend having a logical two-teir structure within PHP, I have found this to be most effective, since most of your logic is in the same language, and it is simple to map from one to another and shift the borders of the interface a bit when the client makes that irritating little request that totally destroys your design.
Having worked heavily with a system that used 4 teirs (two within PHP and two within the database) I can say that that works very well, especially letting the second database teir handle solid permissions checks etc, making it much safer in the event of kids doing strange things with your web forms (although of course you should properly filter that stuff, but its nice to have a safety net or two).
perl is similarly capable for multi-teir design, although I personally find it less usable for web work than PHP.
You can't win a fight.
My perl CGI scripts -> your ASP (VB) scripts
My perl modules -> your COM objects
MySQL -> your MS-SQL database
I use mason as my top tier also. I call perl modules that do the database calls and other more complicated computation/processing. It's my understanding that that is pretty much what COM objects do.
I've mentioned this analogy before in one of those "you can't do 3 tiered architectures in linux" debates before, but I've always been disregarded as a loon with no explanation. If this analogy doesn't hold, why not?