AWS SDK for Java Client Warm-Up: Reducing First-Request Latency

An application can start successfully, pass its health checks, and still respond slowly to its first request. This is common in Java applications that use cloud SDKs, where class loading, Just-In-Time (JIT) compilation, HTTP client initialization, DNS resolution, and TLS negotiation can add latency before the application reaches its normal operating state.

For applications built with the AWS SDK for Java 2.x, this delay can affect serverless functions, containerized services, and applications that create new JVM processes frequently.

AWS introduced the SDK client warm-up feature to address this problem. Available in AWS SDK for Java 2.x version 2.54.0 and later, it provides an API that initializes the SDK request path before the application handles its first real service request.

Instead of allowing the first user request to trigger all initialization work, developers can move that work into application startup. The result can be a faster first request, particularly when combined with AWS Lambda SnapStart or other techniques that reduce application startup latency.

Why Does the First AWS SDK Request Take Longer?

The first request made through an SDK client can follow a different execution path from subsequent requests.

When a Java application calls an AWS service for the first time, several operations may need to happen:

1. The JVM loads and initializes classes required by the SDK.

2. The SDK prepares request serialization and response deserialization logic.

3. The JIT compiler begins optimizing frequently executed code paths.

4. The HTTP client initializes its connection-management components.

5. The application resolves the service endpoint and establishes a network connection.

6. TLS negotiation and certificate validation take place where applicable.

7. The SDK sends the request and processes the response.

Not every operation occurs on every request. For example, an HTTP connection can often be reused, and compiled code remains available while the JVM continues running.

The first request is frequently slower because initialization costs have not yet been paid.

Warm-up does not eliminate every cold-start cost. It moves selected initialization work to an earlier point in the lifecycle.

What Is the AWS SDK Client Warm-Up API?

The AWS SDK for Java 2.x provides the SdkWarmUp utility in the sdk-core module.

The API supports two main approaches:

• Warm all supported SDK service clients discovered on the classpath.

• Warm only the specific service clients used by the application.

The SDK warms the service-client request path and the associated synchronous or asynchronous HTTP client path.

It does this without invoking an actual business operation against an AWS service. The service-client portion uses a local response to exercise request processing, while the HTTP-client portion makes a network request to an AWS endpoint to initialize connection-related code.

According to AWS documentation, the warm-up network request is unsigned and does not invoke a service operation. It therefore does not require AWS credentials or service-operation permissions and does not incur AWS service-operation charges.

The network request still requires appropriate network connectivity, and the overall application can incur costs from its other startup activities.

Step 1: Add the Required AWS SDK Dependency

The warm-up API is available in AWS SDK for Java 2.x version 2.54.0 and later.

For a Maven project that uses Amazon S3, the dependency can be configured as follows:

<dependency>
    <groupId>software.amazon.awssdk</groupId>
    <artifactId>s3</artifactId>
    <version>2.54.0</version>
</dependency>

Use a current, compatible SDK version in a real project and keep service modules aligned through your dependency-management strategy.

Because SdkWarmUp is part of sdk-core, you do not need to add a separate warm-up dependency when the required SDK version is already present.

Step 2: Warm Up All SDK Clients

The simplest option warms the supported SDK service clients discovered on the application's classpath.

import software.amazon.awssdk.core.warmup.SdkWarmUp;

public final class AwsSdkInitializer {

    private AwsSdkInitializer() {
    }

    public static void initialize() {
        SdkWarmUp.warmUp();
    }
}

Call AwsSdkInitializer.initialize() during application startup, before the application begins processing real requests.

For example, a service can invoke the initializer before registering itself as healthy with a load balancer.

This approach is convenient when the application uses most of the SDK service clients available on its classpath.

However, it can perform unnecessary work if transitive dependencies introduce service modules that the application never uses.

Step 3: Warm Up Only the Clients You Need

For applications that use only a few AWS services, targeted warm-up is often the better choice.

Consider a Java application that uses Amazon S3 to retrieve documents and Amazon DynamoDB to read application metadata.

import software.amazon.awssdk.core.warmup.SdkWarmUp;
import software.amazon.awssdk.services.dynamodb.DynamoDbClient;
import software.amazon.awssdk.services.s3.S3Client;

public final class AwsSdkInitializer {

    private AwsSdkInitializer() {
    }

    public static void initialize() {
        SdkWarmUp.warmUp(
            S3Client.class,
            DynamoDbClient.class
        );
    }
}

The targeted overload accepts service-client classes and warms the corresponding request paths.

For synchronous clients, it warms the synchronous path. For asynchronous clients, pass the corresponding asynchronous client class to warm the asynchronous path.

For example, an application using S3AsyncClient should pass S3AsyncClient.class when it wants to warm the asynchronous S3 request path.

Targeted warm-up is useful when an application has a large dependency graph but only a small number of AWS service clients are used in latency-sensitive requests.

Where Should Warm-Up Run?

Warm-up is most useful when it runs before the application begins serving requests.

The exact initialization point depends on the deployment environment.

A common mistake is to call warm-up inside a request handler. Although the method can be called from application code, placing it in the first request path defeats much of the latency benefit.

Warm-up should be part of a deliberate lifecycle strategy, not a request-time optimization.

Using SDK Warm-Up With AWS Lambda SnapStart

AWS Lambda SnapStart reduces startup latency by creating a snapshot of an initialized execution environment and using it to start additional function environments.

The SDK client warm-up feature complements this approach.

Without warm-up, a snapshot may capture an environment in which the SDK has not yet exercised all the request paths required by the application. The first request after restoration can then encounter initialization work that was not completed before the snapshot.

With warm-up, the application exercises the relevant SDK paths before the snapshot is created. The initialized state can then be included in the snapshot, allowing restored environments to benefit from that preparation.

A simplified lifecycle looks like this:

Lambda Initialization
        |
        v
Initialize Application
        |
        v
Warm Up Required SDK Clients
        |
        v
Create SnapStart Snapshot
        |
        v
Restore Execution Environment
        |
        v
Handle First Real Request

This is a conceptual lifecycle. Actual initialization behavior depends on the Lambda configuration and the application's implementation.

Warm-up does not guarantee that every connection will remain reusable after restoration. Network connections and other resources may have lifecycle constraints, so applications must follow SnapStart guidance for resources that require reinitialization after restore.

Understanding the Trade-Off: Faster Requests vs. Startup Time

Warm-up moves initialization work earlier. It does not make that work disappear.

If an application warms ten service clients but only uses two, startup can become unnecessarily expensive. This is why the choice between warming every client and warming specific clients matters.

Consider the following trade-offs.

The appropriate strategy depends on the application.

For a service that uses several AWS APIs during almost every request, warming those clients may be worthwhile. For a small Lambda function that uses only S3, warming unrelated clients provides little practical benefit.

How to Measure the Actual Improvement

Warm-up should be evaluated using application measurements rather than assumptions.

Record the baseline latency before enabling warm-up, then compare it with the latency after the change.

Useful measurements include:

• Application initialization duration

• Time taken by the first AWS SDK request

• Median and tail request latency

• Lambda initialization duration and post-restore behavior, where applicable

• Warm-up duration for all clients versus selected clients

• Error rates during initialization

• Resource consumption during startup

Run repeated tests under comparable conditions. Separate application initialization time from the latency experienced by the first user request.

For Lambda functions, distinguish cold starts from warm invocations and compare behavior under the same deployment and concurrency conditions.

Avoid assuming that warm-up produces a fixed percentage improvement. The result depends on the Java runtime, SDK modules, HTTP implementation, deployment environment, and amount of initialization already performed.

Common Mistakes to Avoid

Warming Every Client Without Checking Dependencies

The all-clients overload can initialize clients that the application never uses. Inspect the dependency graph and prefer targeted warm-up when unnecessary clients would add startup overhead.

Running Warm-Up After Traffic Begins

If the first request triggers warm-up, the request may still pay the initialization cost. Run the warm-up during the appropriate startup phase.

Ignoring Asynchronous Clients

Warming a synchronous client does not mean the asynchronous request path has been warmed. Pass the appropriate asynchronous client classes when the application relies on asynchronous operations.

Assuming Warm-Up Solves Every Cold-Start Problem

Warm-up addresses selected SDK initialization costs. It does not eliminate all JVM startup time, application initialization, network latency, database connection costs, or downstream service delays.

Skipping Measurement

A longer startup can be an acceptable trade-off for a faster first request, but that decision should be based on the application's latency objectives and deployment model.

Best Practices for Production Applications

Follow these practices when introducing AWS SDK client warm-up:

1. Use a supported SDK version. Confirm that the application uses AWS SDK for Java 2.x version 2.54.0 or later.

2. Warm clients before serving traffic. Integrate the call into the appropriate application initialization phase.

3. Prefer targeted warm-up when appropriate. Avoid spending startup time on service clients that the application does not use.

4. Warm the correct execution path. Distinguish between synchronous and asynchronous service clients.

5. Evaluate SnapStart behavior. Warm clients before checkpoint creation and follow AWS guidance for resources that must be refreshed after restoration.

6. Measure startup and request latency separately. Determine whether the optimization improves the metric that matters to the application.

7. Handle failures deliberately. Decide whether warm-up failures should prevent the application from becoming healthy, based on the required availability and latency guarantees.

8. Review HTTP client configuration. Warm-up complements appropriate HTTP client selection, connection management, and dependency optimization.

These practices help teams improve first-request behavior without adding unnecessary startup work.

Conclusion

The AWS SDK for Java 2.x client warm-up feature provides a straightforward way to exercise SDK request paths before an application handles real traffic.

The SdkWarmUp API supports warming all discovered clients or selecting only the synchronous and asynchronous clients the application actually uses. When combined with AWS Lambda SnapStart, it can also help capture initialized SDK paths in a snapshot.

The main benefit is better control over where initialization costs occur. Rather than allowing the first user request to trigger SDK setup, developers can move that work into application startup or checkpoint preparation.

For production applications, start by warming only the clients used by latency-sensitive paths, measure the effect on both startup and first-request latency, and verify the behavior under the actual deployment model. That approach provides a practical way to reduce avoidable cold-start overhead without assuming warm-up will solve every performance problem.