Cold starts in Serverless JavaScript: what are they and how to minimize them

Anton Ioffe - November 8th 2023 - 9 minutes read

Welcome to a deep dive into one of the cryptic challenges haunting serverless JavaScript deployments: the dreaded cold start. As you architect elegant, event-driven solutions that scale with ease, you might find yourself wrestling with this unforeseen specter that can undermine the responsiveness and efficiency of your applications. From dissecting the celadon belly of performance impacts to wielding strategic patterns and design principles, we invite you on a journey to outmaneuver cold starts. Along the way, we'll unravel the intricacies of function design, diagnose common development pitfalls, and arm you with solutions to keep your serverless functions in peak form. Prepare to elevate your expertise and ensure that your serverless JavaScript applications are not just resilient, but also consistently swift on the draw.

Dissecting Cold Starts in Serverless JavaScript Environments

In serverless JavaScript environments, a cold start occurs when a serverless function is invoked after having been idle, resulting in the function's runtime and associated resources being initialized from scratch. This phenomenon directly corresponds to the delay a user might experience when interacting with a serverless application that hasn't been invoked for a while. Upon an incoming request, if no existing execution context is available or 'warm', the serverless platform must allocate a fresh environment, load the function's code from storage, and execute any bootstrap logic necessary for the runtime and the function itself.

The function lifecycle is thus intimately tied to the occurrence of cold starts. Initially, the platform creates an execution environment with the specified memory allocation and runtime configuration. It then proceeds to initialize the environment: downloading the function's code, preparing the runtime, and executing any declared global variables or initialization code that reside outside the event handler. This process introduces additional latency before the event handler is eventually called to process the event, which collectively encompasses the cold start duration.

The overhead introduced by this initialization phase is non-negligible, particularly within the context of JavaScript, a language that can be sensitive to such startup times due to patterns in dependency management and module loading. It's imperative to recognize that the impact on user experience can be significant because, in the modern web, end-users often expect instantaneous feedback. When cold starts result in delays of even a few hundred milliseconds, this can disrupt the perceived performance of an application, potentially affecting user engagement and satisfaction.

Despite the fact that cold starts typically occur in a minority of invocations—often less than 1% in the case of popular cloud services like AWS Lambda—their unpredictability poses challenges. The varying duration of cold starts, which can stretch from milliseconds to several seconds depending on the factors at play, demands optimized application design to ensure that when they do arise, their impact on the user experience is reduced to the bare minimum. Such optimization is a vital consideration in serverless architecture planning, where strategies are essential to keep not just the functions but also the users 'warm'.

Performance Toll: Assessing Cold Start Impact

When assessing the performance cost of cold starts in serverless computing, it's imperative to consider their impact on response times. A cold start can introduce a delay ranging from less than a hundred milliseconds to several seconds, depending on various factors such as runtime language, dependency size, and codebase complexity. This latency penalty can be particularly detrimental when it affects synchronous user interactions, where even marginal increases in response time can lead to decreased user satisfaction and potentially increased bounce rates. As serverless functions scale to accommodate increasing loads, the cold start latency can manifest unexpectedly, adding a layer of uncertainty to the application performance profile.

Resource efficiency is another critical consideration in the serverless model, with cold starts having a measurable impact on the utilization of cloud resources. Upon invoking a serverless function experiencing a cold start, additional CPU cycles and memory are expended to establish the new execution environment. This overhead not only increases operational costs but also can lead to underutilization of provisioned resources during the ramp-up time. Moreover, since the billing model of serverless architectures typically revolves around execution time and memory usage, the additional time spent during cold starts contributes directly to the cost incurred by the deploying organization.

For systems that require high availability and low latency, the scalability challenges posed by cold starts are non-negligible. As load increases and new instances are required to handle the influx of requests, the onset of cold starts can create bottlenecks, limiting the system's ability to scale efficiently. With significant cold start delays, the accumulation of executing functions can lead to a cascade effect, exacerbating response times, and potentially creating a poor user experience. The ability of the serverless platform to rapidly allocate resources in response to demand is partially compromised by cold starts, thus challenging one of the key value propositions of serverless computing—elastically matching resource allocation with load.

In sum, the penalty of cold starts in serverless computing is multifaceted, affecting performance via increased latency, resource utilization through operational overhead, and scalability by impeding efficient load handling. These aspects underline the importance of mitigating cold starts, as the consequences can span from marginally higher costs to critically diminished user experiences. Understanding and addressing the cold start phenomenon is thus pivotal for developers and architects aiming to leverage the full potential of serverless while maintaining a responsive and cost-effective infrastructure.

Strategic Warmer Patterns and Provisioned Concurrency Models

Serverless architectures often compel developers to navigate the delicate balance between function responsiveness and resource optimization. Strategic Warmer Patterns such as self-ping scripts are one method to mitigate cold starts. These scripts invoke functions at regular intervals to maintain a warm state, thereby reducing latency. However, they can also generate unnecessary invocations, leading to increased costs. When scaled up or when encountering multiple concurrent requests, these scripts may fall short, provisioning resources during peak traffic but not aligning optimally with actual demand.

Dedicated Warmer Functions present a more nuanced approach. They intentionally invoke serverless functions in accordance with expected traffic patterns, ensuring a requisite number of containers are warmed to match demand. This method reduces cold starts by intelligently gauging usage patterns, which could theoretically prove cost-effective. Yet, complexity is the trade-off, as the development and management of these warmer functions necessitate additional investment in time and resources.

Provisioned Concurrency stands out as an AWS-native solution, countering the challenges of cold starts. This feature maintains a specified number of function instances in an initialized state, offering rapid response times and consistent latency. The allure of Provisioned Concurrency lies in its capacity to scale and performance predictability. The catch is in calibrating the exact number of pre-warmed instances to maintain readiness without incurring unnecessary expense.

Assessing cold start mitigation strategies entwines improving performance with management simplicity. While self-ping scripts can be adequate for low-concurrency situations, Dedicated Warmer Functions serve high and fluctuating concurrent requests, despite introducing complexity. On the other hand, for situations requiring high throughput and low latency, Provisioned Concurrency stands as a strong choice, provided it is judiciously configured. The strategic objective remains to embrace an approach that merges operational effectiveness with performance optimization, ensuring serverless functions adeptly respond to user interactions and sidestep the latency introduced by cold starts.

// Example of a basic self-ping script to keep the serverless function warm
const aws = require('aws-sdk');
const lambda = new aws.Lambda();

function selfPing() {
    // Replace 'FunctionName' with your Lambda function's name
    const params = {
        FunctionName: 'YOUR_FUNCTION_NAME', 
        InvocationType: 'RequestResponse',
        LogType: 'None',
        Payload: JSON.stringify({ source: 'selfPing' })

    lambda.invoke(params, function(err, data) {
        if (err) {
            console.error(`Error self-pinging Lambda: ${err}`);
        } else {
            console.log('Successfully invoked self-ping on Lambda function');

// Schedule the selfPing to run at regular intervals (e.g., every 5 minutes)
// This could be managed with AWS CloudWatch Events or any other scheduling mechanism
setInterval(selfPing, 300000);

Function Design for Minimizing Initialization Overhead

In serverless architectures, JavaScript function design plays a critical role in minimizing the initialization overhead associated with cold starts. One best practice is lazy loading of dependencies—that is, loading modules only when they are needed rather than when the application starts. This can dramatically reduce the time required for a function to become 'warm'. When writing JavaScript for serverless functions, consider structuring your code to only import essential libraries during the initial execution and defer other imports until they are required within specific function calls. While lazy loading can complicate error handling if a required module fails to load on demand, it offers a significant performance benefit by reducing the amount of code that must be parsed and executed during start-up.

Another technique is to embrace modularity in your function design. Design each function to perform a single, focused task, making it easier to reason about and test. Modular functions are also typically lightweight and have fewer dependencies, which translates to faster cold starts. However, this can lead to a proliferation of functions, which may increase management overhead. Careful naming conventions and organization strategies are necessary to maintain clarity in a highly modular architecture.

To avoid bloating your functions, regularly audit your codebase to remove dead code and extraneous dependencies. Shrinking the size of your deployment package by stripping out unnecessary files and minimizing dependencies not only speeds up cold starts but also enhances overall performance by reducing the function’s memory footprint. Take advantage of tree-shaking tools and other module bundlers that can eliminate unused code and minify the remaining JavaScript, but be aware of the risk of over-optimization, which might accidentally trim necessary code and cause runtime errors.

Lastly, employ strategic caching of data and connections, so subsequent invocations of your function can benefit from work done during the initial cold start. Caching can significantly boost performance, but it can introduce state-related bugs if not managed properly. It's crucial to ensure that the cached data or connections remain valid and secure, and that the cache invalidation strategy aligns with the function's expected behavior and data freshness requirements. Balancing these considerations with the application's workflow can yield a well-tuned function that minimizes initialization overhead and maintains predictable performance.

Diagnosing and Debugging: Common Cold Start Pitfalls and Solutions

In the landscape of serverless, a recurring challenge is the excessive initialization time owing to improper dependency management. Overzealous inclusion of packages bloats the application and contributes significantly to the cold start times. A common misstep involves the indiscriminate import of entire libraries when only specific modules are needed. Consider the following real-world example:

const aws = require('aws-sdk'); // Imports entire SDK
const express = require('express'); // Bulky HTTP server framework not always necessary

These imports can be optimized by cherry-picking only the necessary modules and functions:

const { S3 } = require('aws-sdk'); // Import only the S3 module

Furthermore, forgoing the reliance on heavy frameworks in favor of lighter alternatives, or even native Node.js modules for HTTP handling, can reduce loading times:

const http = require('http');

Another pitfall is the lack of attention to asynchronous operations and their proper handling. If the setup phase is crammed with synchronous calls to external services or databases, it may elongate the time taken to get the function up and running. Asynchronous initialization patterns, like the following, should be used to avoid blocking the event loop:

async function initializeDependencies() {
    // Asynchronous resource setup can go here
exports.handler = async (event) => {
    await initializeDependencies();
    // Function logic follows

Speaking of initialization, failing to decouple the initial setup from the core functionality can lead to unnecessary repetition of setup tasks. This is especially true for actions that only need to be carried out once, such as establishing database connections or integrating third-party APIs. A solution is to implement global and static variables for storages that will persist across function invocations:

let dbConnection; // Static variable outside the handler scope

exports.handler = async (event) => {
    if (!dbConnection) {
        dbConnection = await initializeDbConnection();
    // Function logic using dbConnection follows

Here are some thought-provoking questions to muse over: Are all the dependencies and modules you're using essential for your function's operation? Have you audited your code for synchronous blocks that could be refactored into non-blocking asynchronous calls? And finally, how often do you review your serverless function's initialization routine for optimizations that keep your setup lean? These reflections can steer developers toward more efficient serverless function designs, mitigating the dreaded cold start delays.


In this article, the author delves into the concept of cold starts in serverless JavaScript environments and the impact they have on application performance. They discuss the challenges posed by cold starts, such as increased latency, resource utilization, and scalability, and provide strategies to minimize their effects. The article highlights the importance of function design, including lazy loading dependencies and embracing modularity, as well as utilizing caching techniques. It also mentions common pitfalls to avoid, such as improper dependency management and blocking the event loop with synchronous calls. The key takeaway is the need for developers to optimize their serverless functions to ensure responsiveness and minimize cold start delays. The challenging technical task for the reader is to review their own serverless function's initialization routine for optimizations that keep the setup lean and efficient.

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