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cqrs for dummies - an interlude - eventual consistency at Amazon

I'm sure I've posted this before, but there's a really good presentation available of the theoretical underpinnings of Eventual Consistency and how Amazon tackled it.

Some teases:

" Given the worldwide scope of these systems, we use replication techniques ubiquitously to guarantee consistent performance and high availability. Although replication brings us closer to our goals, it cannot achieve them in a perfectly transparent manner."

"This means that there are two choices on what to drop: relaxing consistency will allow the system to remain highly available under the partitionable conditions, whereas making consistency a priority means that under certain conditions the system will not be available. Both options require the client developer to be aware of what the system is offering. If the system emphasizes consistency, the developer has to deal with the fact that the system may not be available to take, for example, a write. If this write fails because of system unavailability, then the developer will have to deal with what to do with the data to be written. If the system emphasizes availability, it may always accept the write, but under certain conditions a read will not reflect the result of a recently completed write."

"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."

Good stuff.

posted on Friday, February 19, 2010 10:39 AM Print
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