Reserved Concurrency in AWS Lambda: What It Is and How It Works

In AWS Lambda, reserved concurrency is a setting that guarantees a fixed number of function instances are always available to run your Lambda function. It limits the maximum concurrent executions for that function, helping control costs and prevent other functions from being starved of resources.

How It Works

Think of AWS Lambda as a busy kitchen where chefs (function instances) prepare meals (run your code). Reserved concurrency is like reserving a certain number of chefs just for your dish, so they are always ready to cook when orders come in.

This means your Lambda function can scale up to the reserved number of concurrent executions without waiting. If more requests come in beyond this limit, they wait in line or get throttled.

At the same time, reserved concurrency also protects other Lambda functions by limiting how many chefs your function can use, so it doesn't take all the kitchen resources.

Example

This example shows how to set reserved concurrency for a Lambda function using AWS CLI. It reserves 5 concurrent executions for the function named MyFunction.

bash

aws lambda put-function-concurrency --function-name MyFunction --reserved-concurrent-executions 5

Output

{
    "ReservedConcurrentExecutions": 5
}

When to Use

Use reserved concurrency when you want to guarantee your Lambda function always has capacity to run, especially for critical workloads that must not be delayed.

It is also useful to control costs by capping how many instances can run at once, preventing unexpected spikes.

For example, if you have a payment processing function, reserving concurrency ensures it can handle requests promptly without being slowed by other functions.

Another use case is to protect shared resources like databases by limiting how many Lambda instances can access them simultaneously.

Key Points

• Reserved concurrency sets a fixed limit on how many Lambda instances can run at the same time for a function.
• It guarantees capacity for your function and prevents throttling due to lack of resources.
• It also protects other Lambda functions by limiting resource usage.
• Setting reserved concurrency helps control costs and manage resource sharing.

Key Takeaways

Reserved concurrency guarantees a fixed number of Lambda instances for your function.

It limits maximum concurrent executions to control costs and resource use.

Use it to ensure critical functions always have capacity and avoid throttling.

It protects other functions by reserving resources and preventing overload.

You can set reserved concurrency easily via AWS CLI or console.