Amazon DynamoDB stands out as a scalable and flexible service that requires minimal infrastructure management. However, to optimize costs, understanding the capacity modes of DynamoDB and selecting the right one for your workload is essential. This article breaks down the two capacity modes available in DynamoDB and offers strategies for making informed decisions based on your application needs and cost considerations.
DynamoDB Capacity Modes
DynamoDB offers two capacity modes: Provisioned Capacity Mode and On-Demand Mode. Each mode has its own pricing model, with distinct advantages depending on the nature of your workload.
- Provisioned Capacity Mode:
- In this mode, you specify the number of read capacity units (RCUs) and write capacity units (WCUs) that your application will use. You’re billed based on the capacity you provision, regardless of whether you use the full allocation.
- For instance, if you set 100 RCUs, your application can handle 100 reads per second without experiencing throttling. The billing occurs based on the capacity you’ve reserved, even if your actual usage is less.
- To make the most of provisioned capacity mode, AWS Auto Scaling can be enabled to automatically adjust throughput capacity based on usage. This reduces the manual effort of adjusting capacity, though it may not respond instantly to rapid traffic spikes.
- On-Demand Mode:
- On-demand mode simplifies management, as you only pay for what you use, with no need to pre-define capacity. In this mode, DynamoDB charges $1.25 for every million write requests (in the us-east-1 region; pricing may vary across regions). The best part? If your table isn’t used, you don’t pay for any capacity.
- On-demand is ideal for workloads with unpredictable traffic or when you’re in the early development stages, as there are no upfront costs. You’re only billed when actual traffic hits your application.
When to Use Each Mode
The choice between provisioned capacity and on-demand mode depends on the predictability of your workload:
- Provisioned Capacity Mode is ideal for workloads with stable and predictable traffic. If your application consistently uses similar levels of resources, provisioned mode can save money, especially when auto-scaling is enabled to adapt to changing demand.
- On-Demand Mode is best for workloads with unpredictable traffic patterns. If your application’s traffic spikes at irregular intervals, or if you’re still in the development stage, on-demand mode provides flexibility without the need for upfront capacity planning.
Handling Traffic Spikes
For applications experiencing spiky traffic, where sudden bursts of requests occur, provisioned capacity mode with auto-scaling might not adjust quickly enough to prevent throttling. Auto-scaling responds to increased demand only after the table has consumed capacity beyond the threshold for two consecutive minutes. If the increase in demand is steep and rapid, the auto-scaling response may not kick in fast enough, potentially causing performance issues.
In contrast, on-demand mode handles spiky traffic more gracefully. DynamoDB dynamically adjusts capacity to accommodate sudden peaks. For new tables, the initial peak capacity is set at 4,000 write request units (WRUs) or 12,000 read request units (RRUs) per second, but this automatically scales up based on usage, doubling with each peak. This adaptive approach makes on-demand mode well-suited for unpredictable traffic spikes.
Cost Considerations: Provisioned vs. On-Demand
While on-demand mode might seem more expensive at first glance, the actual cost depends on your usage pattern. A general rule of thumb is that if your workload consistently consumes more than 18% of your provisioned capacity, then provisioned capacity mode will likely be cheaper. Conversely, if you utilize less than 18% of the provisioned capacity or have erratic traffic patterns, on-demand mode will provide better cost savings.
Example:
Imagine a table configured with 44,000 capacity units in provisioned mode. If the traffic only utilizes 3% of the available capacity, then you’re essentially overpaying for unused resources. In this scenario, on-demand mode would be more cost-effective, as you would only pay for the capacity consumed.
However, if the utilization of the provisioned capacity consistently exceeds 50%, provisioned capacity mode becomes more cost-effective, especially when paired with auto-scaling.
Strategies for Spiky and Controlled Workloads
For workloads with scheduled spikes (e.g., batch jobs or backfill processes), provisioned capacity mode can still be useful. By using the AWS Application Auto Scaling API or directly interacting with the DynamoDB API, you can schedule changes to provisioned capacity ahead of time to meet these spikes. This ensures that your workload can handle increased traffic without incurring unnecessary costs during periods of lower traffic.
On-demand mode, on the other hand, is particularly effective when spikes are unpredictable or if you’re running an application with low but inconsistent traffic. It prevents throttling during spikes and ensures that you only pay for actual usage.
Testing and Comparison
For cost optimization, a good approach is to experiment with both capacity modes. Try on-demand mode for one to two weeks to assess how well it handles your traffic, and then switch to provisioned capacity to compare costs. By reviewing actual usage data and costs during this trial period, you can make an informed decision about which mode offers better savings.
AWS also offers a Table Capacity Mode Optimization Tool, which can analyze your DynamoDB table usage, simulate auto-scaling behavior, and recommend the best capacity mode for your workload. Although this tool provides recommendations, always validate the findings to ensure they align with your workload’s unique needs.
Optimizing DynamoDB Costs
Cost optimization in DynamoDB boils down to understanding your workload patterns and selecting the appropriate capacity mode—whether it’s provisioned capacity with auto-scaling or on-demand mode. For predictable and stable traffic, provisioned capacity mode can help save costs, especially when paired with auto-scaling. For unpredictable or spiky traffic, on-demand mode provides the flexibility needed to avoid throttling and reduce costs during low-usage periods.
By carefully monitoring your workload with tools like AWS Lambda Insights and experimenting with capacity modes, you can strike the perfect balance between performance and cost-efficiency for your DynamoDB workloads.
For businesses looking to optimize their DynamoDB usage, ZirconTech provides expert advice and strategies to streamline your costs while ensuring maximum performance. Reach out today for a consultation.