How does partition capacity limit relate to table's total capacity in DynamoDB?

Question

In the Dynamodb table, each partition is subject to a hard limit of 1,000 write capacity units and 3,000 read capacity units. What I don't understand is that how these limits relate to the table's total RCU/WCU?

For example, if I configure a table's RCU to 6000 and WCU to 3000. Is this capacity evenly used by all partitions in the table? Or do all partitions fight for the total capacity?

I can't find a way to know how many partitions the DynamoDB table is using. Is there a metric to tell me that?

Answer 1

The single-partition limit will only matter if your workload is so terribly imbalanced that a significant percentager of requests go to the same partition. In a better designed data model, you have a large number of different partition keys, which allows DynamoDB to use a large number of different partitions, so you never see a significant percentage of your requests going to the same partition.

That does not mean, however, that the load on all partitions is equal. It might very well be that one partition sees twice the number of requests as another partition. A few years ago, this meant your performance suffered: DynamoDB split the provisioned capacity (RCU/WCU) equally between partitions, so as the busier partition got throttled sooner, the total capacity you got from DynamoDB was less than what you paid for. However, they fixed this a few years ago with what they call adaptive capacity: DynamoDB now detects when your workloads total capacity is under what you paid for, and increase the capacity limits on individual partitions.

For example if you provision 10,000 RCU capacity and DynamoDB divides your data into 10 partitions, each of those start out with 1,000 RCU. However, it one partition gets double the requests as other, this will lead the workload to doing only 1000+9*500 = 5,500 RCU, significanltly less than the 10,000 you are paying for. So DynamoDB quickly recognizes this, and increases the busy partition's limit from 1,000 to 1,818 RCU - and now the total performance is 1,818 + 9*909 = 9,999 RCU. DynamoDB does this automatically for you - you don't need to do anything special. All you need to is to make sure that your workload has enough different partition keys, and no significant percentage of requests go to one specific partition keys - otherwise DynamoDB will not be able to achieve high total RCU - it will always be limited by that single-partition limit of 3,000.

Regarding your last question, I don't know if there is such a metric (maybe another responder will know), but the important thing to check is that you have a lot of partition keys. If that's the case, and your workload doesn't access one specific key for a large percentage of the requests, you should be safe.