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5 Common Causes of Lambda Cold Starts

What Are Lambda Cold Starts?

Lambda cold starts occur when your AWS Lambda function is invoked after sitting idle for a while. Imagine your Lambda function as a car that's been parked overnight. In the morning, it requires a little more effort to start the engine --- this initial effort is similar to a Lambda cold start.

The term 'cold start' originates from the state in which your function exists before it is invoked. If it's idle, it's considered 'cold'. When a request comes in, AWS needs to create a container for your function, allocate necessary resources, load your function into the new container, and then execute it.

However, if your function is invoked again within a short period, AWS can reuse the existing container, avoiding the need for another cold start. This is known as a 'warm' invocation. Therefore, the primary difference between a cold and warm start is the additional time and resources needed for the former.

Why Are Lambda Cold Starts Problematic?

Lambda cold starts are an inherent part of serverless architecture, and they can't be completely eliminated. However, they can create issues in certain scenarios. Let's explore some of the reasons why Lambda cold starts can be problematic.


The most prominent issue with cold starts is latency. A cold start can add significant delay to your function's execution time, potentially leading to a poor user experience. This latency can vary based on several factors, including your function's runtime, the size of your deployment package, and whether your function is connected to a VPC.

Unpredictable Performance

Cold starts also introduce a level of unpredictability to your Lambda functions. The frequency and duration of cold starts can be inconsistent and hard to predict. This unpredictability can make it difficult to guarantee consistent performance for your application, particularly if you're working with latency-sensitive workloads.

Resource Utilization

Resource utilization is another concern with Lambda cold starts. Cold starts consume more CPU and memory resources than warm starts, which can lead to higher costs. This is particularly problematic if your function experiences frequent cold starts, as it means you're paying for resources that aren't being fully utilized.

Complexity in Debugging

Finally, debugging Lambda functions can be more complex due to cold starts. The additional steps involved in a cold start can make it harder to identify and troubleshoot issues. Plus, cold starts introduce more variables into your function's execution environment, which can complicate the debugging process.

5 Common Causes of Lambda Cold Starts and How to Mitigate Them

Now that you have a basic understanding of what a Lambda cold start is, let's get into its common causes and how you can mitigate them.

Initialization Overhead

One of the primary causes of Lambda cold starts is initialization overhead. Every time a Lambda function is invoked after being idle, AWS needs to set up a new runtime environment or 'container'. This process includes loading and initializing any external dependencies your function might have, which can take up valuable time and cause a cold start.

To mitigate this issue, make sure to minimize your function's initialization code. This means avoiding unnecessary dependencies and keeping your initialization code lightweight. Also, consider using global variables to store any data that doesn't change across invocations.

Another way to mitigate initialization overhead is by using provisioned concurrency. Provisioned concurrency keeps your function initialized and ready to respond in double-quick time, thereby reducing the likelihood of cold starts.

VPC Configuration

Your VPC (Virtual Private Cloud) configuration can also be a cause of Lambda cold starts. When your Lambda function accesses resources within a VPC, AWS has to set up an ENI (Elastic Network Interface), which adds to the cold start time.

To mitigate this, consider whether your Lambda function needs to run inside a VPC. If it doesn't, running it outside the VPC can eliminate this source of cold starts. If running inside a VPC is necessary, make sure to use provisioned concurrency to keep a pool of initialized functions ready.

Code Size and Dependencies

The size of your codebase and the number of dependencies in your Lambda function can significantly affect the cold start time. The larger the deployment package, the longer it takes for AWS to initialize the function.

To mitigate this issue, keep your codebase and dependencies as small as possible. Only include the necessary dependencies in your deployment package. Also, consider using a tool like Webpack or Parcel to bundle your code and remove any unused dependencies.

Inadequate Provisioned Concurrency

Provisioned concurrency allows you to keep a set number of Lambda functions initialized and ready to respond, thereby reducing cold starts. However, if you don't have enough provisioned concurrency, you may still experience cold starts during periods of high demand.

To mitigate this, monitor your application's usage patterns and adjust your provisioned concurrency accordingly. If your application experiences regular periods of high demand, make sure to increase your provisioned concurrency during these times.

Inefficient Language/Runtime

The language or runtime you choose for your Lambda function can also affect the cold start time. Some languages, like Java, have a longer cold start time compared to others, like Node.js or Python. In general, dynamically typed languages can be started up faster, reducing the cold start phenomenon. If you're building a new serverless application, consider choosing a language/runtime that has a shorter cold start time.

In conclusion, understanding the phenomenon of Lambda cold starts and their causes is the first step towards optimizing your serverless applications. By taking steps to mitigate these issues, you can improve the performance and user experience of your application. Always remember to monitor your application's performance and make adjustments as necessary to keep your application running smoothly.

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