What does a web-based framework scalable?

thanks you very much in advance.

First of all, I conceive scalability as the ability to design a system that doest not change when the demand of its services, whatever they are, increases considerably. May you need more hardware (vertically or horizontally0? Fine, add it at your leisure because the system is prepared and has been designed to cope with it.

My question is simple to ask but presumably very complex to answer. I would like to know what you I look at in a framework to make sure it will scale accordingly, both in number of hits and number of sessions running simultaneously.

This question is not about technology nor a particular framework at all, it is more a theoretical question.

I know that depend very much on having a good database design and a proper hardware behind with replication, etc... Let's assume that this all exists, however yet my framework must meet some criteria, what?

- Provide a memcache?
- Ability to run across multiple machines (at the web server level) and use many replicated databases? But what is in the software that makes that possible?
- etc...

Please, let's not relate the answers with any particular programming language or technology behind.

Thanks again,

D.

I think scalability depends most of all on the use case: do you expect huge amounts of data, then you should focus on the database, if it's about traffic, focus on the server, is it about adding new features, focus on your data-model and the framework you are using...

Comparing a microposts-service like Twitter to a university website or a webservice like GoogleDocs you will find quite different requirements.

First of all the common notion of scalability is the ability of a software to improve in throughput or capacity if more hardware resources are added (CPUs, memory, bandwidth etc).
Software that does not improve in increased resources is not scalable.
Getting out of the definitions, I think your question is related to evaluation of frameworks you are planning to introduce to your implementation that may affect your software's ability to scale.
IMHO the most important factor to evaluate when introducing a framework is to see if there is hidden serialization in it (that serialization in effects transfers to/affects your software)
So if you introduce a framework that introduces serialization in your application that can affect your ability to scale.
How to evaluate?

1. Careful source code inspection (if open source)
2. Are there any performance guarantees offered by those that build the
framework?
3. Do measurements yourself to see how introducing this framework
affects your performance and replace if not satisfied