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Dynamo behaviour when adding more partitions (what happens with items that must change partition) ?

Let’s say my current DynamoDB has 2 partitions. The data is divided based on the hash of the partition key. Given that there are only 2 partitions, hashes starting from 00xx to 80xx goes to partition 1 and 81x to FFxx goes to partition 2. If I change my Read and Write capacity or storage size, let’s suppose that now I have 3 partitions.  Now we have 00xx-55xx on partition 1, 56xx-AAxx on partition 2, and ABxx-FFxx on partition 3. 

In this scenario, for instance, some of the items of partition 2 should be moved to partition 3 because now their hash will map to partition 3. How does Dynamo handles that? Does that reflects in performance while moving data?

1 Answers

Dynamo shuffles data between partitions behind the scenes.  I’m sure there is a minor performance impact but remember that Dynamo has an eventual consistency model and it can do stuff async…unlike an traditional RDBMS that would likely lock rows or tables when re-partitioning.

Keep in mind that it only scales out to more partitions automatically.   So if you bump up your RCU or WCU drastically, you’ll get lots more partitions…all taking a slice of the available RCU/WCU.   If you then reduce RCU/WCU, your partitions remain and they too each take an equal slice of the available RCU/WCU.

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