If you've been following me for a while, you've likely heard me use the term "N-Tier Architecture" before. Today's question comes from Arno. He basically asked "What is this N-Tier Architecture thing you keep talking about?"... So, to answer his question, a N-Tier Architecture is one that uses more than one tier. Of course there's more to it than that...

He talks about the idea of a "tier" and how, despite it being somewhat subjective depending on the application and the person talking, all agree that it's a separation in the structure of your app. He shows some common setups for a PHP-based application and explains the usefulness of a tiered architecture. He also includes a more real example that involves multiple languages and data sources and explains some of the downsides to the tiered architecture approach.

As PHP applications become more and more complex, it can be easy to end up with a tangled mess of code that makes maintenance nearly impossible. Applying the concept of tiered applications can help to alleviate some of the difficulty in maintaining complex applications.

Tiered programming is a method that splits up the components of an application (like a separation between presentation and business logic). They talks about what a setup like that might gain you, the wrong way to do it and the right way that splits it out into the database tier, the business tier and the presentation tier.

If you are an architect with a team of developers, or one developer with a team of managers, here's a list of a set of modules, frameworks and strategies you should have in mind. Not all of them would necessarily apply to you, but if I were you, I would at least think about each one. So if you are starting a new project - lucky you - from scratch, or coming to an old project with a mandate for change, here's a small checklist.

The five types are:

• a good ORM (Object Relational Mapping) layer
• a tiered framework (splitting out interface, business and database logic layers)
• a event-based framework
• a caching framework (or some form of easy-to-use caching)
• an error/tracing framework

Some frameworks, like the Zend Framework, come with most of these already built in, reducing the need for more than one sort of technology stack.