Do DynamoDB consumed capacity units in on-demand mode compare to provisioned capacity?

Question

Right now I am using on-demand mode for my DynamoDB tables, as I didn't know how much data to expect. But now that the application has run a while, I can see the metrics for ConsumedReadCapacityUnits and ConsumedWriteCapacityUnits for my tables in CloudWatch.

In on-demand mode I pay per request, whereas in provisioned capacity mode I have to pay for the provisioned capacity. If I simply take the metrics for (max) consumed capacity units and compare the prices of those in provisioned capacity mode to my current costs, I believe provisioned capacity mode would be a lot cheaper for me.

My question is, can I simply take the metrics and take the max (plus some buffer) of the consumed capacity units and configure them as provisioned capacity, or is that an error in reasoning on my part?

Answer 1

There are two other things you need to consider:

1. How 'bursty' is your throughput?
2. Are you using SDKs to connect to your database?

Setting your provision to the maximum throughput you ever see will ensure you don't get throttled requests, however you will probably be setting the provision too high. Dynamodb can actually consume more provision than you have set using Burst Capacity. This will accomodate short bursts of high throughput over the space of 5 minutes. If you see sustained peaks, for example your database is busy in the day but not the night, you might consider setting your tables to Autoscale. In this case you can set the provisioned throughput lower, and Dyanmodb will automatically scale up provision as required. Note that autoscale is good for workloads that vary over the course of hours (e.g. for handling daily peak hours). It's not good for reacting to events that occur in less than about 30 minutes.

If you are using official SDKs, they will handle throttle responses, and retry any failed requests. This gives Dynamodb some time to scale without your application failing requests.