Understanding Reserved and Provisioned Concurrency in AWS Lambda: Key Differences and Use Cases

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Introduction

In the realm of serverless computing, AWS Lambda stands out as a powerful service that allows developers to run code without managing servers. An important aspect of Lambda is managing how many concurrent instances of your functions can run. Two critical features in this context are Reserved Concurrency and Provisioned Concurrency. Though they sound similar, they serve distinct purposes and work differently. This post aims to demystify these concepts, highlighting their differences and use cases.

What is Reserved Concurrency?

Reserved Concurrency is a feature in AWS Lambda that lets you allocate a specific portion of your AWS account's total concurrency limit to a particular Lambda function.

How Does Reserved Concurrency Work?

When you set Reserved Concurrency for a function, you are ensuring that this function can always use up to the specified number of concurrent executions. This allocation means that the specified concurrency is reserved solely for that function and cannot be used by other functions in your account. It's important to note that Reserved Concurrency is also a tool for limiting a function's maximum concurrency, which can be crucial for managing costs and ensuring that a Lambda function doesn't overwhelm downstream resources.

Use Cases for Reserved Concurrency

Reserved Concurrency is particularly useful for:

  • Ensuring critical functions have the necessary resources available, especially during traffic spikes.

  • Controlling costs by limiting the maximum number of instances that a function can spawn.

  • Managing resource usage when interacting with systems that have strict rate limits.

What is Provisioned Concurrency?

Provisioned Concurrency is a feature designed to eliminate cold starts and ensure low latency for your Lambda functions.

Functioning of Provisioned Concurrency

  • With Provisioned Concurrency, AWS Lambda maintains a specified number of pre-initialized function instances. These instances are immediately ready to respond to function invocations.

  • This pre-warming of functions eradicates the initialization latency (cold starts), providing consistent and fast response times.

  • Unlike Reserved Concurrency, which just allocates a part of the concurrency pool, Provisioned Concurrency actively keeps function instances in an initialized state.

Use Cases for Provisioned Concurrency

Provisioned Concurrency is essential for:

  • Functions where low latency is a critical requirement.

  • Applications that experience irregular traffic patterns, ensuring quick responses even after long idle periods.

  • Scenarios where predictable and consistent performance is more important than cost optimization.

Key Differences

  • Purpose: Reserved Concurrency is about allocation and limitation, while Provisioned Concurrency is about maintaining readiness and eliminating cold starts.

  • Billing: There are no additional charges for setting Reserved Concurrency limits, whereas with Provisioned Concurrency, you pay for the pre-warmed instances regardless of whether they are used.

Conclusion

Understanding Reserved and Provisioned Concurrency in AWS Lambda is crucial for optimizing performance and cost. Reserved Concurrency is ideal for managing resource allocation and limiting costs, while Provisioned Concurrency is the go-to feature for ensuring low latency and eliminating cold starts. By strategically implementing these features, you can significantly enhance the efficiency and reliability of your serverless applications.