What is cold start and how to avoid it

Cold starts occur when a serverless function is invoked after being idle for some time, and the cloud provider needs to provision a new instance to handle the request. This can introduce a small latency, typically in the range of a few hundred milliseconds to a couple of seconds, depending on the cloud provider and the specific service.

Mitigating Cold Start Latency

1. Provisioned Concurrency (AWS Lambda)

   • AWS allows you to provision a certain number of instances to be always warm and ready to handle requests, thus reducing cold start latency.
   • Example:

     aws lambda put-provisioned-concurrency-config \
       --function-name my-function \
       --provisioned-concurrent-executions 5
     

2. Keep Functions Warm

   • You can periodically invoke the functions to keep them warm, ensuring they remain in a ready state.
   • Example: Use CloudWatch Events to trigger the function every few minutes.

3. Optimize Function Code

   • Minimize the initialization code outside the handler to reduce startup time.
   • Keep the deployment package small to speed up the loading process.

4. Choose the Right Runtime

   • Some runtimes may have shorter cold start times compared to others. For example, Node.js and Python tend to have shorter cold start times than Java or .NET.

When Serverless Might Be Slower

• Cold Starts: As discussed, cold starts can introduce latency.
• High Throughput Needs: For extremely high-throughput applications, the overhead of frequently provisioning new instances might not be ideal.
• Long-Running Tasks: Serverless functions are typically limited by execution time (e.g., AWS Lambda has a maximum execution time of 15 minutes).

When Serverless Is Beneficial

• Event-Driven Architectures: Applications that respond to events such as HTTP requests, file uploads, database changes, etc.
• Microservices: Small, single-purpose services where each function handles a specific task.
• Prototyping and Development: Quickly deploying and testing new features without managing infrastructure.
• Burst Workloads: Applications with infrequent, unpredictable workloads that benefit from auto-scaling.

Example of Minimizing Cold Start Impact

Here’s an updated example with a focus on reducing cold start impact by keeping the function warm:

AWS Lambda Function to Keep Warm

1. Create a Lambda Function

   • This function will be a simple HTTP endpoint that responds quickly.

   exports.handler = async (event) => {
       return {
           statusCode: 200,
           body: JSON.stringify({ message: 'Function is warm!' }),
       };
   };
   

2. CloudWatch Events Rule

   • Set up a CloudWatch Events rule to invoke this function every 5 minutes to keep it warm.

   {
       "source": ["aws.events"],
       "detail-type": ["Scheduled Event"],
       "detail": {},
       "resources": [],
       "region": "us-east-1",
       "time": "2017-09-12T17:10:45Z",
       "id": "cd2d702c-4d2f-42c8-b8b2-df9d872a584b",
       "detail-type": "Scheduled Event"
   }
   

   • You can set this up in the AWS Management Console under CloudWatch -> Rules.