How many per second request can my provisioned Dynamodb throughput support

0
votes

After reading several documentations and blogs about Dynamodb, I'm still struggling to understand how does RCU and WCU works.

I deployed a table with 5 RCU. The request on this table is mostly a GetItem request with an average response payload of 7KB. I know it's not the size of the payload that will be measured but the consumed data size during a read operation. But let's just hypothetically say it consumed similar data size.

From Dynamodb docs, each provisioned RCU can support up to 4KB of consistent read so if I understand it, my provisioned throughput can support up to 20KB per second without throttling.

Based on this, if my average payload is 7KB, am I correct in saying that, my table can support like ~ 2 request/sec?

Would appreciate if anyone can shed some lights on this.

1
amazon-dynamodb

1 Answers

1
votes

TD;LR

Yes you're correct

More details

You probably already know some of this, but for sake of completeness and to help others, I'm going to detail a bit:

  • RCU is Read Capacity Unit

  • WCU is Write Capacity Unit

  • Eventually consistent read: You may get stale data if the latest writes didn't replicate everywhere yet.

  • Strongly consistent read: You always get the latest data.
  • Transactional read: You are reading as part of a DynamoDB transaction.

Units are not equivalent to request per seconds. Based on what and how you read, there are different performance profiles. I'll explain.

Reads performance

There are 3 kinds of reads: Strongly consistent, transactional and eventually consistent. It goes like this:

Per 4Kb blocks of data returned (which may be multiple rows from the DB):

  • Eventually consistent: 1 RCU can read 2 blocks per second
  • Strongly consistent: 1 RCU can read 1 block per second
  • Transactional: 2 RCUs can read 1 block per second

So for your example where your items are about 7Kb and you want to do strongly consistent reads, it'll require 2 RCUs to read 1 item per second. Since you've provisioned 5 RCUs, you are correct and you can expect about 2 reads per second.

If you want to squeeze more juice out of your DB, be mindful how much data you are returning. I think if you're average item size is 7Kb, that's a little concerning. You may want to consider an object store instead and store references to the payloads in dynamodb instead of the payloads themselves. I guess it depends what you are storing and your use case. It might be legit, but it sounds a bit suspicious.