3-tier architecture v. 3-server architecture

Question

I'm building a traditional .NET MVC site, so I've got a natural 3-tier software architecture setup (presentation in the form of Views, business layer in the controller, and data layer in the models and data access layer).

When I've deployed such sites, it usually goes either on one server (where the web site and db live), or two servers (a web server and a separate db server).

How does one go about a 3-server architecture (WEB, APP, and DB)? Would the web server just have the presentation (e.g. the physical View/aspx pages), the app server would hold the config file and bin folder, and the db server would remain as is?

My question is essentially, can you simply move the /bin and all app logic onto a separate server from the presentation views? If so, how do you configure the servers to know where to look? If there's a good primer somewhere or someone can give me the lowdown, I'd be forever indebted.

Answer 1

In some circles, I have seen this discussion phrased as the difference between n-tier and n-layer where a "layer" in this context potentially represents another machine. In order to have a middle layer using this definition, it must be hosted. For example, if you had a service layer which the presentation layer called to get its data, then the service layer could be on a different machine than the presentation or database. However, that service layer is hosted either as a windows service or as a web service. I.e., there is a process listening for requests on that machine. Thus, you cannot simply move the bin folder to different machine and hope to have this work. I would look at WCF (Windows Communication Foundation) for creating these types services.

Answer 2

Next logical scale up would be two web servers and one database server.

Eventually after adding many web servers it might be worth adding a service layer.

You might also want to add a distributed cache, session state server, email server, and other specialized servers at some point too as you scale.

Answer 3

ASP.NET MVC does not help you in setting up a 3tier system. This is realy only a frontend pattern.

The main issue you have to solve implementing a multi tier system is the transport of objects from one server to another. You have to find a way to serialize all objects depending on the transport channel. This gets slow and development gets more complicated.

There are reasons to have a separate app-server: You might have logic in it that other application need or the app-server might have different permissions than the Webserver. But its hard to imagine a high traffic website, where all requests lead to a call to a remote app - server.

Answer 4

So your questions seems to be ...

"can you simply move the /bin and all app logic onto a separate server from the presentation views?"

If I am understanding correctly, I believe the files in your bin folder will be the compiled code behinds for your asp.net pages. If that is the case then, no, I believe they need to be on the same machine as the asp pages.

If you want to have your business logic on a seperate machine from the presentation layer you would need to wrap that code into a seperate dll and expose it via soap or some other protocol .. and then call those SOAP exposed dlls on the other server from the code in your presentation layer.

Answer 5

MVC is not a 3-tier architecture. Not every solution needs to be 3-tier or n-tier, but it is still important to understand the distinction. MVC happens to have 3 main elements, but those elements do not work in a "tiered" fashion, they are interdependent:

Model <----- Controller
         \       |
          \      v
           ---- View

The View depends on the Model. The Controller depends on the View and Model. These multiple dependency paths therefore do not function as tiers.

Typically a 3-tier solution looks like:

Data Access <--- [Mapper] ---> Domain Model <--- [Presenter/Controller] ---> UI

Presenter/Controller is somewhat optional - in Windows Forms development, for example, you usually don't see it, instead you have a "smart client" UI, which is OK too.

This is a 3-tier architecture because each of the 3 main tiers (Data, Domain, UI) has only one dependency. Classically, the UI depends on the Domain Model (or "Business" model) and the Domain Model depends on the DAL. In more modern implementations, the Domain Model does not depend the DAL; instead, the relationship is inverted and an abstract Mapping layer is injected later on using an IoC container. In either case, each tier only depends on the previous tier.

In an MVC architecture, C is the Controller, V is the UI (Views), and M is the Domain Model. Therefore, MVC is a presentation architecture, not a system architecture. It does not encapsulate the data access. It may not necessarily fully encapsulate the Domain Model, which can be treated as an external dependency. It is not tiered.

If you wanted to physically separate the tiers then it is usually done by exposing the Domain Model as a Web Service (i.e. WCF). This gives you improved scalability and a cleaner separation of concerns - the Domain Model is literally reusable anywhere and can be deployed across many machines - but comes with a significant up-front development cost as well as an ongoing maintenance cost.

The server architecture mirrors the 3-tier diagram above:

Database Server <----- Web Services <----- Application

The "Application" is your MVC application, which shares a Domain Model with the Web Services (through SOAP or REST). Web Services run on a dedicated server (or servers), and the database is, obviously, hosted on its own server. This is a 3-tier, 3-server architecture.