Question

This is a pretty open question. I was wondering: how can you handle concurrency if you have a web application that is hosted on many computers on the server side? For example, how can you manage a simple stack if you have an ASP .NET application running on different machines with each one having its own memory? Is there something I’m missing?

Thanks

Answer 1

If I understand the question correctly: how do multiple Web servers co-ordinate to return responses?

In general a farm of Web servers will serve up exactly the same HTML to clients when a particular request arrives. If there’s a need to store state – suppose the user posts back a form with data, or session state is used to keep track of a shopping cart – then you’ll need some shared mechanism to do that.

There’s plenty of ways to share state between multiple instances of a Web server:

• A database can store data that needs to be persisted indefinitely, such as user profile information
• Similarly, a key-value store (like the Windows Azure table storage) can be used for permanent storage, with advantages in performance over less flexibility when querying
• Shared session state (using a database or central session state server) can store moderately-persistent data that’s local to the current session

Things can get complex, though:

• If data is cached, you may need to have a mechanism to expire the cache on each Web server
• Eventually, contention over shared session storage or database storage can negate the advantages of multiple Web servers

To get around this some systems use the principle of “eventual consistency”, where, for instance, a profile update might not be replicated immediately to every server in a cluster. Not-quite-fresh data is sometimes regarded as good enough.

As you can tell this is a complex subject and I’ve only really skated around the edges of it.

The good news is that generally you won’t need to bother as unless you’re creating a very large or complex site most modern server hardware can handle many tens of requests per second without breaking into a sweat. It’s only when you start to move above a few dozen simultaneous requests per second that Web farms start to be necessary.