W.I.P.
There are many ways to run a tiered application such that is is scalable (where "scalable" means that it can be set up to run more parallel sessions for users, digest more service calls etc...).
Scalability in tiered applications means, e.g.
- a single web server directing traffic to multiple application servers
- many servers talking to a single database
- servers talking to many databases
- etc.
The need for scalability is not only driven by sheer capacity demand (mostly going along with requests to use low cost hardware to run services). It is very much linked to other non-functional requirements such as (high-) availability and maintainability.
This is the reason why understanding the various architecture patterns which are supported by a given application stack is crucial to deliver the right solution for a given business demand aka. requirement. This HOWTO guide talks about various options to "lay out" the physical server landscape in order to achieve certain non-functional requirements.
Most of the examples are taken from various threads on the community mailing list and replicated here in an abstract way.