Re: Middle Tire in PHP MySQL

From: Tony Marston (tony_at_NOSPAM.demon.co.uk)
Date: 08/26/04 

Date: Thu, 26 Aug 2004 17:35:57 +0100

"Michael Austin" <maustin@firstdbasource.com> wrote in message
news:bPmXc.13434$SS5.3168@newssvr22.news.prodigy.com...
> Kumar Saurabh wrote:
>
>> Hi Devs,
>>
>> I was thinking of implementing a middle tire (most probably in XML)
>> for a PHP MySQL web app but couldn't make a head way. can any one
>> suggest how can i proceed with this.
>>
>> Thanks in advance.
>
> I think you mean Tier :)
>
> In a multi-tier application each tier performs certain tasks. Let's take
> a 3-tier system and break them down.
>
> Tier-1: User Interface. Browser or PC app that communicates with an
> Application server.
>
> Tier-2: Application Server: Each application server process is
> generally, but not always multi-threaded/re-entrant such that multiple
> User Interface connections can be handled by one Server process. This
> process is responsible for handling business logic and other validation
> routines as well as communicating with the next Tier.

The 3-Tier architecture does NOT necessarily mean having an application
server between the Presentation layer and the Data Access layer. You are
confusing a 'logical' 3 tier structure with a 'physical' 3 tier structure.

In a 'logical' 3 tier structure the middle tier is known as the Business
layer and is where all the business rules are processed. It is possible to
implement all 3 tiers (layers) on the same machine without even a hint of an
application server. I know because I have done so in two different
languages. This is a purely software thing.

When an application reaches a size where all three layers overload a single
device then one of the options is to put each of the layers onto a different
machine. This then becomes a 'physical' 3 tier structure, and it may be
necessary to have the capabilities of an application server to handle all
the business layer components.

You should note that it is not possible to implement a physical 3 tier
structure unless your software has already been split into logical 3 tier.

> Tier-3: Further processes the data and is responsible for storing it in a
> data store (flat-file or database).
>
> Now if you understand this very basic definition of a multi-tier
> architecture, this means that you would add a layer of unnecessary
> complexity by introducing a level of difficulty that you may not need nor
> completely understand. As a learning exercise this could be valuable.
>
> Write a front-end (Windows or Linux-based) app in GCC, VCC, VB that
> generates XML output that then communicates with an application server
> running a JAVA app

It is not necessary to restrict the communication between the Presentation
and Business layers to the passing of XML files. I use XML, but only to have
it transformed into XHTML from a series of XSL files.

> that then communicates with your PHP app to store the data in MySQL. The
> levels of data obfuscation are virtually unlimited.

But if you have too many levels your application may become so unwieldy that
it is virtually useless. I have seen it done. 

-- 
Tony Marston
http://www.tonymarston.net

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