I'm currently writing a large scale ASP.Net web app.
One of the thngs I can't find out about is how to justify when to use the cloud. E.g. when should I use google app engine/azure?
Also, when would I want to use bigtable over a standard dbms such as Sql Server?
Thanks
Cloud computing is all about scalability. It allows you to scale up AND scale down without having to rework your designs.
It works well for small sites, since you are only paying for resources used, but if you need to scale up, it just happens automatically (provided your application was designed for the cloud).
Also, there are theoretically much better tools in place for maintaining uptime and reliability in the cloud. For example, a system upgrade can happen without stopping your service, since the cloud computing platforms can automatically take on or off servers to service your application.
There's been a lot of talk about that from the Azure devs.
Also, there can be a financial motivation for using the cloud. Using a hosted cloud architecture can be less expensive than managing the multiple servers (DB, web, etc) that would be required for a traditional site, at least up front. As your usage goes up, the cost follows, but in theory, it can be more cost effective.
This question is very closely related to another question asked today: "When shouldnt-you-use-a-relational-database?"
Relational databases and non-relational databases (like BigTable) address different needs. Not only in scale and performance, but in the structure and usage of the data.
The "Cloud" as I understand it is about scalability primarily. That is, the architecture refers to a capability to increase capacity in a scalable way.
Also, the Cloud is used frequently in reference to the Software-as-a-Service (SaaS) model, where someone else takes care of the servers, but that's an independent issue from the Cloud architecture. I.e. you could operate your own set of servers in a Cloud architecture.
So the justification for using the Cloud architecture is that you have an application that has a variable need for computing capacity. So it would be overkill to have N servers dedicated to match your peak level of activity. The Cloud allows you to vary your usage of the servers as your level of activity grows (and diminishes) over time.
The justification for using a SaaS model is that you don't want to be in the business of operating a data center. You're willing to relinquish some control and pay for the service, so that you can leave operation details to the experts in that technology. They handle backups, hardware failures, upgrades, 24x7 operation, etc. You handle your application and your business.
I'm not too familiar with anything else except app engine and EC2.
I'll try to add something to the previous answers:
The best thing about app engine is it's free until you attract a certain amount of users and you are charged for what your application uses, idle time is not charged.
Big table may differ from an rdbms architecturaly but from a perspective of a developer using it it's not that different.
Another good thing is python is supported. The bad thing is the standard library is crippled.
Also, you don't have full control over your data on the cloud (appengine), what I mean is you can't completely restrict the people from google from taking a peek in what you store there.
I recommend you subscribe to and read the High Scalability blog, especially some of the most visited posts such as those about the architecture of various large sites, as you will learn a lot from it that may help you make a decision. There is no hard rule as to when you should or should not use a cloud service or move from a relational database to a keyvalue system like BigTable.
One upside of cloud services in any case is that if you build your application with them, it will be immediately scalable and require much less rework later on if you require that kind of performance. However, in view of premature optimisation, it would be wise to be sure that you need that kind of scalability before you decide to build your app on such a platform.
There are several concepts to wrap your head around when using a datastore system like BigTable as well, such as not being able to just slam out writes like you would in a relational database, and having to precalculate a lot of your data rather than just doing that based on info from the database.
Although again, you can learn a lot from reading the abovementioned blog and related posts about Youtube, Plentyoffish, Google, etc.
You say you are "currently writing a large scale ASP.NET app". If you have made significant progress on it, you are already pass the point where you can justify using Google app engine or Azure. Both require significantly different architectures than you have build with a traditional application due to language support, database differences, and maturity.
Google App Engine is Python only so switching to it would require a complete rewrite
Big table is not a relational database and requires very different coding patters. SQL Data Services originally announced to be non-relational as well, but is moving to be more relational. I have not seen how close to a standard MSSQL database it currently is.
I would consider Google app engine to be a relatively immature platform so far. Database functionality is limited, you cannot run background processes, profiling and performance tuning tools are limited at best. Azure is currently in limited community preview, and so is not even available to ship a product on today.
While there are many very valid reasons to use a cloud architecture, moving to it will require significantly different architectures. Think about what effect changing that architecture (and possibly waiting for platform availability) will do to your release date.
If you are early in your project, cloud vs. not cloud is a great question to ask. If you have well on your way, I think that the importance of getting to shipping code and leveraging the work you have already put in should trump any benefits to the cloud you may see.