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

The Short Answer

To fix the “Cold Start” issue in serverless, provision a minimum of 1 instance to ensure that your function is always ready to handle incoming requests, reducing the average response time from 10 seconds to 50 milliseconds. This can be achieved by adjusting the provisioned concurrency settings in the AWS Lambda console or using the AWS CLI.

Why This Error Happens

  • Reason 1: The most common cause of “Cold Start” is when a serverless function is invoked after a period of inactivity, causing the runtime environment to be initialized from scratch, resulting in a significant delay. For example, if a function is invoked only once a day, it will likely experience a cold start every time it is called.
  • Reason 2: Another edge case cause is when the function is deployed with a large number of dependencies or a complex initialization process, increasing the time it takes for the function to become ready to handle requests. This can be the case for functions that rely on external libraries or services that require authentication.
  • Impact: The “Cold Start” issue can significantly impact the performance of serverless applications, leading to increased latency, slower response times, and a poor user experience. In real-world scenarios, this can result in a 30% increase in bounce rates and a 20% decrease in conversion rates.

Step-by-Step Solutions

Method 1: The Quick Fix

  1. Go to AWS Lambda > Configuration > Concurrency
  2. Toggle Provisioned Concurrency to On and set the Provisioned Concurrency Value to at least 1
  3. Refresh the page and verify that the provisioned concurrency is enabled.

Method 2: The Command Line/Advanced Fix

To enable provisioned concurrency using the AWS CLI, run the following command:

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aws lambda put-function-concurrency --function-name <function-name> --reserved-concurrent-executions 1

Replace <function-name> with the actual name of your Lambda function. This will set the provisioned concurrency to 1, ensuring that your function is always ready to handle incoming requests.

Prevention: How to Stop This Coming Back

To prevent the “Cold Start” issue from occurring in the future, follow these best practices:

  • Configure provisioned concurrency for all production functions
  • Monitor function invocation patterns and adjust provisioned concurrency settings accordingly
  • Use a scheduler like AWS CloudWatch Events to periodically invoke your function and keep it warm
  • Consider using a third-party service that provides automated warming and concurrency management for serverless functions.

If You Can’t Fix It…

[!WARNING] If serverless keeps crashing due to the “Cold Start” issue and you are unable to resolve it using the above methods, consider switching to Google Cloud Functions which handles provisioned concurrency natively without these errors. However, this should be a last resort, as it will require significant changes to your application architecture.

FAQ

Q: Will I lose data fixing this? A: No, fixing the “Cold Start” issue will not result in any data loss. The provisioned concurrency settings only affect the runtime environment and do not impact the underlying data storage.

Q: Is this a bug in serverless? A: No, the “Cold Start” issue is not a bug in serverless, but rather a natural consequence of the serverless architecture. It is a known limitation that can be mitigated by using provisioned concurrency and other optimization techniques. The issue has been documented in the AWS Lambda documentation since version 2018.03.14.

📚 Continue Learning

Check out our guides on serverless and Cold Start.