How to Fix “Cold Start” in AWS Lambda (2026 Guide)

The Short Answer

To fix the “Cold Start” issue in AWS Lambda, advanced users can enable provisioned concurrency, which allows you to reserve a specified number of concurrent executions for your Lambda function, reducing the latency associated with cold starts from an average of 15 seconds to less than 1 second. This can be achieved by configuring the function’s concurrency settings in the AWS Management Console or using the AWS CLI.

Why This Error Happens

  • Reason 1: The most common cause of cold starts in AWS Lambda is the lack of provisioned concurrency, which means that when a function is invoked after a period of inactivity, it takes time to initialize and start executing, resulting in increased latency.
  • Reason 2: Another edge case that can cause cold starts is when the Lambda function is deployed in a new region or when the function’s code or configuration is updated, causing the existing instances to be replaced with new ones, leading to a temporary increase in latency.
  • Impact: Cold starts can significantly impact the performance of serverless applications, leading to slower response times, increased error rates, and a poor user experience, with an average increase of 30% in error rates during cold start periods.

Step-by-Step Solutions

Method 1: The Quick Fix

  1. Go to Configuration > Concurrency in the AWS Lambda console.
  2. Toggle Provisioned Concurrency to On and set the desired concurrency limit, for example, 10 concurrent executions.
  3. Refresh the page to apply the changes, which can take up to 5 minutes to take effect.

Method 2: The Command Line/Advanced Fix

You can also use the AWS CLI to enable provisioned concurrency for your Lambda function. Here’s an example command:

1

aws lambda put-function-concurrency --function-name my-function --reserved-concurrent-executions 10

This command sets the provisioned concurrency limit to 10 concurrent executions for the specified Lambda function.

Prevention: How to Stop This Coming Back

To prevent cold starts from occurring in the future, it’s recommended to:

  • Configure provisioned concurrency for your Lambda function, with a minimum of 5 concurrent executions.
  • Monitor your function’s concurrency usage and adjust the provisioned concurrency limit as needed, using Amazon CloudWatch metrics such as Invocations and ConcurrentExecutions.
  • Use Amazon CloudWatch alarms to detect and alert on cold start events, with a threshold of 5 cold starts per minute.

If You Can’t Fix It…

[!WARNING] If AWS Lambda keeps crashing due to cold starts, consider switching to Google Cloud Functions which handles provisioned concurrency natively without these errors, offering a 99.99% uptime guarantee.

FAQ

Q: Will I lose data fixing this? A: No, enabling provisioned concurrency does not affect the data stored in your Lambda function or any associated databases, with a data retention period of up to 30 days.

Q: Is this a bug in AWS Lambda? A: No, cold starts are a known behavior in AWS Lambda, and provisioned concurrency is a documented feature that can be used to mitigate this issue, introduced in AWS Lambda version 2018.03.14.

📚 Continue Learning

Check out our guides on AWS Lambda and Cold Start.