When do we have web server and an application server as two separate physical tiers?

Question

Typically when an application is deployed on a server farm, it has a central database, one or more application servers and one or more web servers.

Since all the services in the application tier are going to be accessed via the web server, why not deploy the web server on the same physical tier as that of the application server?

The specific query is - what are the reasons of deploying the web server as a separate tier?

On a broader perspective, what are the general considerations while deciding on the deployment scenario?

Castillo

Answer 1

If your application server is more CPU intensive than the Web server (which is the case most of the time), you may scale out the application server while keeping the number of Web servers as it is.

Put a NLB between the web farm and the application farm and you can have (for example) 2 web servers and 5 or more (or whatever) app servers to process the requests. It makes it easier to manage the infrastructure costs.

Meggy

Can you give more specific recomendations when to move web server to another physical tier? I frankly don't understand what are benefits of having this 2 web servers. We can combine web application and application server into one application, put a NLB between internet clients and this farm. If we have not enough power, we just add more servers and move on. And we don't need to deal with SOAP (or what else we use for communication between two tiers) because all communications are made inprocess. – Fumy

@Artyom: If the user interface is very lightweight but a lot a business processing happens, it may be a good idea to have one web server dedicated to user interface and one or more dedicated to business processing. In that case their is no need to load-balance the Web UI and can still benefit from in-proc sessions. – Meggy