AWS Lambda Concurrency and Throttling Management

Understanding and managing concurrency and throttling is crucial for building robust and scalable serverless applications with AWS Lambda. This module explores these concepts, their impact, and how to effectively control them.

What is Lambda Concurrency?

Concurrency refers to the number of requests your Lambda function is serving at any given time. Each concurrent request runs in its own execution environment. AWS Lambda automatically scales by creating new execution environments to handle incoming requests, up to your account's concurrency limits.

Concurrency is the number of simultaneous executions of your Lambda function.

Think of concurrency as the number of doors open for your function to process requests at the same time. More doors mean more requests can be handled simultaneously.

When a Lambda function is invoked, AWS Lambda checks if an execution environment is available. If one is available and not currently processing a request, it uses that environment. If all available environments are busy, Lambda provisions a new execution environment to handle the request. This process continues until the function reaches its concurrency limit.

Types of Concurrency

Concurrency Type	Description	Management
Unreserved Concurrency	The default concurrency allocated to your account. It's shared across all Lambda functions in the region. The default is 1000 executions per region.	Managed by AWS, but can be reallocated to specific functions.
Reserved Concurrency	A specific amount of concurrency allocated exclusively to a single Lambda function. This guarantees that the function will always have that many concurrent executions available, even if other functions in the account are experiencing high traffic.	Configured per function in the Lambda console or via IaC.

Reserved Concurrency for a function cannot exceed the Unreserved Concurrency for your account.

What is Lambda Throttling?

Throttling occurs when Lambda cannot execute your function due to concurrency limits being reached. If your function is invoked more times concurrently than its allocated concurrency limit (either account-level unreserved or function-level reserved), Lambda will reject subsequent requests.

Throttling is Lambda's way of saying 'I'm too busy right now' when concurrency limits are hit.

Imagine a popular restaurant with a limited number of tables. Once all tables are occupied, new customers are turned away until a table becomes free. Throttling is similar for Lambda functions.

When throttling happens, the invoking service receives an error. For synchronous invocations (like API Gateway), this error is typically returned to the client. For asynchronous invocations (like SQS or SNS), Lambda retries the invocation based on its retry policy. Understanding throttling is key to preventing service disruptions and ensuring predictable performance.

Managing Concurrency and Throttling

Effective management involves setting appropriate concurrency limits and understanding how different invocation types interact with these limits.

Setting Reserved Concurrency

For critical functions that require guaranteed capacity, set Reserved Concurrency. This prevents other functions from consuming its potential execution environments. Be mindful that setting Reserved Concurrency for one function reduces the Unreserved Concurrency available for others.

Provisioned Concurrency

Provisioned Concurrency keeps a specified number of execution environments initialized and ready to respond instantly to requests. This is useful for functions with latency-sensitive workloads, as it eliminates cold starts. It's a more advanced feature that incurs costs for the provisioned environments.

Asynchronous Invocations and Retries

For asynchronous invocations, Lambda automatically retries failed invocations (including those due to throttling) up to two times. The default retry behavior can be configured. Understanding this retry mechanism is important to avoid duplicate processing or overwhelming downstream systems.

Monitoring and Alarming

Utilize Amazon CloudWatch metrics like

code

ConcurrentExecutions

and

code

Throttles

to monitor your function's concurrency and identify throttling events. Set up alarms to be notified when these metrics exceed predefined thresholds.

Visualizing Lambda concurrency: Imagine a pipeline with multiple parallel processing units. Each unit represents an execution environment. Concurrency is the number of units actively processing requests. Throttling occurs when all units are busy and a new request arrives, causing it to be temporarily blocked or rejected.

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What is the primary difference between Unreserved and Reserved Concurrency?

Unreserved Concurrency is shared across all functions in an account, while Reserved Concurrency is dedicated to a single function.

What happens when a Lambda function is throttled?

Subsequent requests are rejected because the function has reached its concurrency limit.

Best Practices for Concurrency and Throttling

To ensure optimal performance and prevent unexpected behavior, follow these best practices:

Allocate Reserved Concurrency wisely: Assign it to functions that are critical or have predictable high traffic. Avoid over-allocating, which can starve other functions.

Monitor CloudWatch metrics: Regularly check
code
```
ConcurrentExecutions
```
and
code
```
Throttles
```
for your functions and set up alarms.

Understand invocation types: Differentiate between synchronous and asynchronous invocations, as their retry and throttling behaviors differ.

Use Provisioned Concurrency for latency-sensitive applications: If cold starts are unacceptable, provisioned concurrency is the solution, but be mindful of costs.

Design for idempotency: Especially with asynchronous invocations and retries, ensure your functions can handle being invoked multiple times with the same input without adverse effects.

Request account concurrency limit increases: If your application consistently requires more concurrency than the default account limit, submit a request to AWS Support.

Learning Resources

AWS Lambda Concurrency(documentation)

Official AWS documentation detailing Lambda concurrency configurations, including reserved and provisioned concurrency.

AWS Lambda Throttling(documentation)

AWS documentation explaining common causes of Lambda throttling and how to troubleshoot them.

AWS Lambda Provisioned Concurrency(documentation)

Learn about Provisioned Concurrency, a feature to eliminate cold starts and manage latency for Lambda functions.

Monitoring AWS Lambda with Amazon CloudWatch(documentation)

Guide on using CloudWatch metrics and alarms to monitor Lambda function performance, including concurrency and throttles.

AWS Lambda Best Practices for Performance(blog)

A blog post from AWS that covers performance optimization, including aspects of concurrency and memory management.

Understanding AWS Lambda Concurrency Limits(video)

A YouTube video explaining AWS Lambda concurrency limits and how they affect your applications.

Serverless Patterns: Concurrency and Throttling(blog)

An architectural blog post discussing common serverless patterns related to managing concurrency and throttling.

AWS Lambda Concurrency and Throttling Explained(blog)

A detailed explanation of Lambda concurrency and throttling concepts with practical examples.

AWS Lambda Best Practices(documentation)

Best practices for developing serverless applications on AWS, including sections on performance and scaling.

AWS Lambda Pricing(documentation)

Understand the pricing model for AWS Lambda, which is influenced by execution duration, memory, and provisioned concurrency.