Example of 4-Tier (for N-Tier) Architecture?

Question

Recently a friend of mine asked me about N-Tier architectures and I was able to explain to him about 1, 2 and 3 tier architectures with examples. But I was stuck when I wanted t

Answer 1

1. Fundamental Services : e.g. Database, Directory Services, File & Print Services, Hardware abstraction. This tier is increasingly called the platform.
2. Business Domain Tier : An Application Server such JavaEE including EJB, DCOM or CORBA Service Objects. Provide business functionality, increasing using SOA and Micro-services.
3. Presentation Tier : e.g. Java Servlets/JSP, ASP, PHP. This tier will increasingly include WebServices as proxies and adaptors for business tier services.
4. Client Tier : Thin clients like HTML Pages on Browsers and Rich Clients like Java WebStart & Flash.

• In Java EE it is common to divide the Business Domain tier into Data-Access (Entity Beans) & Business Services (Session Beans).
• In an Enterprise SOA (Service Oriented Architecture) the ESB (Enterprise Service Bus) would typically exist as an additional tier between tiers 1 & 2. It may be part of the platform provision.
• In Mashups you could have an aggregation tier between tier 3 & 4.

The move to being called N-Tier is a reflection of the move to increasingly componentised architectures from the older client-server to first 3-Tier then 4-Tier. The defining characteristic of a tier is a clearly defined interface with a separation of concerns.