CODEWITHSUNDEEP
amazon-dynamodb
Mike Howard
Mike Howard

Reputation: 333

Read throughput in DynamoDB

Ok, so my understanding of read units is that it costs 1 read unit per item, unless the item exceeds 4KB in which case read units = ceiling(item size/4).

However when I submit a scan asking for 80 items (provisioned throughput is 100), the response returns a ConsumedCapacity of either 2.5 or 3 read units. This is frustrating because 97% of the provisioned hardware is not being used. Any idea why this might be the case?

Upvotes: 2

Views: 458

Answers (1)

Daniela Miao
Daniela Miao

Reputation: 1373

What is your item size for the 80 items? Looking at the documentation here: http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/ProvisionedThroughputIntro.html

You can use the Query and Scan operations in DynamoDB to retrieve multiple consecutive items from a table in a single request. With these operations, DynamoDB uses the cumulative size of the processed items to calculate provisioned throughput. For example, if a Query operation retrieves 100 items that are 1 KB each, the read capacity calculation is not (100 × 4 KB) = 100 read capacity units, as if those items were retrieved individually using GetItem or BatchGetItem. Instead, the total would be only 25 read capacity units ((100 * 1024 bytes) = 100 KB, which is then divided by 4 KB).

So if your items are small, that would explain why Scan is not consuming as much capacity as you would expect. Also, note Scan uses eventually consistent reads, which consume half of the read capacity units.

Upvotes: 1

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