DynamoDB Capacity Modes: Provisioned vs. On-Demand
When designing serverless applications with AWS Lambda and DynamoDB, understanding how to manage your database's capacity is crucial for performance, cost-effectiveness, and scalability. DynamoDB offers two primary capacity modes: Provisioned Throughput and On-Demand. Each mode has distinct characteristics that make it suitable for different workload patterns.
Provisioned Throughput
Provisioned Throughput mode requires you to specify the read and write capacity units (RCUs and WCUs) your application needs. You pay for the throughput you provision, regardless of whether you use it. This mode is ideal for applications with predictable traffic patterns where you can accurately estimate your capacity requirements.
Provisioned Throughput: Predictable costs for predictable workloads.
In Provisioned Throughput mode, you set specific RCU and WCU values. If your application exceeds these limits, you'll experience throttling. Conversely, if you provision more than you need, you'll incur unnecessary costs.
Read Capacity Units (RCUs) and Write Capacity Units (WCUs) are the fundamental units of throughput in DynamoDB. An RCU can perform one strongly consistent read per second for an item up to 4 KB in size. An RCU can perform two eventually consistent reads per second for an item up to 4 KB. A WCU can perform one write per second for an item up to 1 KB in size. You can adjust provisioned capacity manually or use Auto Scaling to automatically adjust it based on defined metrics.
On-Demand Capacity
On-Demand Capacity mode is designed for workloads with unpredictable or variable traffic patterns. DynamoDB automatically scales read and write capacity up or down to meet your application's needs. You pay for the read and write requests your application actually serves, making it a pay-per-request model.
On-Demand Capacity: Flexibility and cost savings for unpredictable workloads.
With On-Demand, you don't need to provision capacity in advance. DynamoDB handles the scaling, ensuring your application can handle traffic spikes without throttling. This is particularly useful for new applications or those with spiky traffic.
This mode is ideal for development and testing, or for applications with infrequent or unpredictable traffic. While it offers great flexibility, it can be more expensive than Provisioned Throughput for consistently high-traffic applications. DynamoDB automatically scales to handle sudden bursts of traffic, but there's a brief ramp-up period for new tables or when traffic significantly increases.
Key Differences and Use Cases
| Feature | Provisioned Throughput | On-Demand Capacity |
|---|---|---|
| Pricing Model | Pay for provisioned capacity (RCUs/WCUs) | Pay for actual requests |
| Workload Suitability | Predictable, consistent traffic | Unpredictable, variable, or spiky traffic |
| Scalability | Manual adjustment or Auto Scaling | Automatic, seamless scaling |
| Throttling Risk | Higher if capacity is underestimated | Lower, but potential for brief ramp-up |
| Cost Efficiency | Potentially lower for stable, high-throughput | Potentially lower for low or variable throughput |
Choosing the right capacity mode is a trade-off between cost predictability and operational flexibility. For serverless applications with Lambda, consider the expected traffic patterns of your functions when making this decision.
Considerations for Serverless Architectures
When integrating DynamoDB with AWS Lambda, the event-driven nature of Lambda can lead to unpredictable read/write patterns. For new projects or those with uncertain usage, On-Demand is often a safer starting point. As your application matures and traffic patterns become clearer, you can evaluate migrating to Provisioned Throughput with Auto Scaling for potential cost optimization.
Provisioned Throughput and On-Demand Capacity.
Provisioned Throughput.
You pay only for the read and write requests your application actually serves.
Learning Resources
Official AWS pricing page detailing costs for Provisioned Throughput and On-Demand capacity, including RCU and WCU costs.
AWS Developer Guide explaining the differences between Provisioned and On-Demand capacity modes and when to use each.
Learn how to configure Auto Scaling for DynamoDB tables in Provisioned Throughput mode to automatically adjust capacity.
A blog post from AWS covering best practices for integrating Lambda with DynamoDB, including capacity management.
A video tutorial explaining the concepts of Read Capacity Units (RCUs) and Write Capacity Units (WCUs) for Provisioned Throughput.
A comparative video that breaks down the pros and cons of DynamoDB's On-Demand and Provisioned capacity modes.
A Medium article discussing practical advice on selecting between On-Demand and Provisioned throughput based on workload characteristics.
A tutorial from Tutorialspoint offering a clear explanation and comparison of the two DynamoDB capacity modes.
Wikipedia entry providing a general overview of Amazon DynamoDB, its features, and use cases.
A comprehensive deep dive into DynamoDB features and best practices presented at AWS re:Invent, including capacity management.