What are cold starts in serverless computing?

Cold starts in serverless computing refer to the delay that occurs when a serverless function is invoked after being idle or scaled down. In serverless architectures, cloud providers automatically manage resources, shutting down inactive instances to save costs. When a new request arrives for a function that isn’t already running, the provider must allocate compute resources, load the function’s code, and initialize its runtime environment. This setup process adds latency before the function can process the request, resulting in a “cold start.” For example, AWS Lambda, Azure Functions, and Google Cloud Functions all experience this behavior, though the exact duration varies by platform and configuration.

Several factors influence the severity of cold starts. The programming language and runtime environment play a role: languages like Python or Node.js typically initialize faster than Java or .NET, which may require more time for JIT compilation or framework setup. The size of the function’s deployment package also matters. Functions with large dependencies or complex initialization logic—such as loading machine learning models or connecting to databases—extend cold start times. Additionally, configurations like Virtual Private Cloud (VPC) access can introduce extra latency due to network setup. For instance, a Python function with minimal dependencies might cold-start in 200ms, while a Java function in a VPC could take several seconds.

Developers can mitigate cold starts through optimization and platform-specific strategies. Keeping functions “warm” by periodically invoking them prevents shutdowns, though this may increase costs. Reducing deployment package size by trimming unused dependencies or splitting large functions into smaller ones minimizes initialization work. Some cloud providers offer features like AWS Lambda’s Provisioned Concurrency, which pre-initializes instances to serve requests immediately. Choosing lightweight runtimes (e.g., switching from Java to Go) or reusing connections and resources across invocations also helps. For example, a team handling real-time APIs might use Provisioned Concurrency during peak hours while optimizing their Node.js code to load only essential libraries, balancing performance and cost effectively.