Distributed in-memory caching has become one of the primary techniques for scaling up a Web application. Open-source and commercial caching products, such as memcached, JBossCache, or Oracle Coherence, have enabled some of the largest Web sites to service lots of incoming requests.

Microsoft entered the fray of distributed caching with Velocity, a new in-memory cache for serializable .NET objects. The project provides a downloadable preview as well as code samples and documentation, describing Velocity as:

A distributed in-memory application cache platform for developing scalable, available, and high-performance applications. [It] fuses memory across multiple computers to give a single unified cache view to applications...

Applications can store any serializable CLR object without worrying about where the object gets stored. Scalability can be achieved by simply adding more computers on demand. Velocity also allows for copies of data to be stored across the cluster, thus protecting data against failures...

Velocity can be configured to run as a service accessed over the network or can be run embedded with the distributed application. Velocity includes an ASP.NET session provider object that enables ASP.NET session objects to be stored in the distributed cache without having to write to databases. This increases the performance and scalability of ASP.NET applications

Microsoft's Dare Obasanjo writes about Velocity in a blog post:

Like memcached, you can think of Velocity as a giant hash table that can run on multiple servers which automatically handles maintaining the balance of objects hashed to each server and transparently fetches/removes objects from over the network if they aren't on the same machine that is accessing an object in the hash table. In addition, you can add and remove servers from the cluster and the cache automatically rebalances itself.

Microsoft white paper on Velocity lists the following features:

• Caches any serializable CLR object and provides access through simple cache APIs.
• Supports enterprise scale: tens to hundreds of computers.
• Configurable to run as a service accessed over the network or run embedded with the application.
• Supports common cache configurations.
• Supports dynamic scaling by adding new nodes.
• Configurable number of backup copies to provide high availability.
• Automatic load balancing.
• Integration with administration and monitoring tools such as ETW, System Center, etc.
• Provides tight integration with ASP.NET to be able to cache ASP.NET session data in the cache without having to write it to source databases. It can also be used as a cache for application data to be able to cache application data across the entire Web farm.
• Follows the cache-aside architecture (also known as Explicit Caching) for V1. That is, you must decide explicitly which objects to put/remove in your applications and “Velocity” does not synchronize with any source database automatically.
• Support for multiple client languages (for example, PHP, C#, C++, etc.).

What do you think of Velocity as a distributed cache for Web sites? What caching solutions do you currently use to scale .NET-based sites?