Cost savings with DynamoDB On-Demand: Lessons learned
One of my favorite features announced during re:Invent 2018 is DynamoDB On-Demand. With DynamoDB On-Demand, we can use DynamoDB without provisioning capacity. Instead, we pay per request. Sounds amazing? I was excited and re-configured all DynamoDB tables of our SaaS product marbot: cloud-native alerting for CloudWatch via Slack. The result is stunning but misleading.
I shared my excitement on Twitter and today I add what we learned in the following weeks.
Level up, strengthen your AWS skills.
Our book Amazon Web Services in Action is a comprehensive introduction to computing, storing, and networking in the AWS cloud. You'll find clear, relevant coverage of all the essential AWS services, emphasizing best practices for security, high availability, and scalability. Get the first chapter for free!
Behind the marketing term
What does DynamoDB On-Demand mean? When compared with the provisioned DynamoDB model, on-demand is:
- A table that scales automatically.
- A new cost model where you pay per request.
Scaling takes time if you hit a new peak
DynamoDB On-Demand provisions capacity to handle two times the past peak traffic. Let’s say your previous peak in January 2019 was 10,000 requests/sec. After that new peak, you can go from zero to 20,000 requests/sec at any time without being throttled. However, if all of a sudden you send 30,000 requests/second it takes around 30 minutes for DynamoDB On-Demand to scale up. You see throttles in the meantime! The best way to avoid throttling is to prescale an on-demand table by simulating a new traffic peak before you go live.
Economics
There are cases where on-demand is significantly more expensive compared to provisioned with Auto Scaling. My rule of thumb: The spikier your workload, the higher the savings with on-demand. Workloads with zero requests also benefit. The reason why we saw such significant savings with marbot is our workload that goes down to almost zero requests/second for half of the day in production and is mostly zero for 24/7 in our test environment. My suggestion is to switch to on-demand for one day and compare your costs with the day before.
