December 29, 2008 12:20 PM PST
The Cloud and eventual consistency
Amazon CTO Werner Vogels recently revisited a post titled 'Eventually Consistent' , about building consistency models for globally distributed systems and the trade-offs required to process trillions of transactions. It's a little heady, but definitely worth a read if you are trying to figure out how to architect applications for the Cloud or other large system architectures.
Whether or not inconsistencies are acceptable depends on the client application. In all cases the developer needs to be aware that consistency guarantees are provided by the storage systems and need to be taken into account when developing applications. There are a number of practical improvements to the eventual consistency model, such as session-level consistency and monotonic reads, which provide better tools for the developer. Many times the application is capable of handling the eventual consistency guarantees of the storage system without any problem. A specific popular case is a Web site in which we can have the notion of user-perceived consistency. In this scenario the inconsistency window needs to be smaller than the time expected for the customer to return for the next page load. This allows for updates to propagate through the system before the next read is expected.
Dave Rosenberg dishes up "Software, Interrupted" with nearly 15 years of technology and marketing experience that spans from Bell Labs to multiple start-up IPOs to open-source enterprise software companies. He is co-founder of MuleSource and currently serves as the general manager of Hardy Way. He is a member of the CNET Blog Network and is not an employee of CNET. Disclosure. You can contact Dave via e-mail at softwareinterrupted@gmail.com or follow him on Twitter @daveofdoom. 
