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layers vs tiers

 
JigaR Parekh
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while reading book Humphrey Sheil i was confused with difference between layers and tiers, if any body can help
 
Marut pandey
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As per my understanding Tiers are physical separation of system and layers is logical like

We do call Business layer and DAO layer because they are separated on the basis of logic. On the other hand when you talk about tier it will be referred as physical entity like

Presentation Tier>>Logic Tier>>Database Tier
 
JigaR Parekh
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Marut Thanks for reply, i was thinking this before reading SCEA book but after reading i got confused,

In book definition
Tiers
A Tier can be logical or physical organization of components into an ordered chain of service providers and consumers.


Examples : Client, Web, Business, Integration, Resource

Layers
A layer is the hardware and software stack that hosts services within a given tier.


Examples: Application, Virtual Platform (Components), Containers, OS etc.

so it looks like other way around. But not sure.
 
Marut pandey
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Read this clears all. I got confused too when read that book, I had to ignore their concept.

http://blogs.msdn.com/jmeier/archive/2008/09/06/layers-and-tiers.aspx
 
Keith Rieck
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I think this terminology is left-over from Sun's Suntone architecture. In this concept, Sun organized a design on three dimensions: Layers, Tiers, and Systemic Qualities.

Systemic qualities are the non-functional requirements, such as the big seven topics tested on the SCEA exam: scalability, maintainability, reliability, availability, extensibility, performance, manageability, and security.

Tiers are major logical and physical components through which processing passes, such as client, web server, app server, database, etc.

Layers are design components grouped by level of abstraction. The bottom layer might be hardware. The operating system might be a layer on hardware. Sharable libraries might be a layer on that, then the app server on that, then EJBs on top.
 
JigaR Parekh
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Thanks Keith, now i am much more clear with tiers and layers,
just one question any example of logical tiers as i understand client, web server, app server, database are physical tiers.
 
Keith Rieck
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Well, a full JEE5 app server would contain a servlet container and also an EJB container. Physically they'll be in the same box, but it may be useful to consider them separate tiers.

There's no official list of tiers, so you can decompose your design in any way that would be useful. Maybe it's important for a design to have a cache between the application code and the database. It might be useful to consider the cache as a separate tier, whose physical location is determined later.
 
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