Why does ** Swagger UI work locally ==--** but not in AWS Lambda?

In modern web development, API documentation is an essential part of building and maintaining services. Swagger UI has emerged as a popular tool for automatically generating interactive API documentation. When integrating Swagger UI into a Spring Boot application, the process is often smooth in development environments, but issues can arise when deploying to cloud environments like AWS Lambda. This is particularly true when trying to access the Swagger UI behind an API Gateway, where certain deployment nuances can lead to a blank page or other errors.

This guide will discuss a specific issue related to deploying a Spring Boot application with Swagger UI on AWS Lambda, provide a thorough explanation of the problem, and offer solutions and troubleshooting steps. Additionally, we will cover common FAQs around Swagger UI in AWS Lambda deployments.

Overview of the Problem

When integrating Swagger UI into a Spring Boot application, everything works fine in a local environment, but the Swagger UI page appears blank when deployed in AWS Lambda. The console logs show the following error messages:

javascript

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Uncaught SyntaxError: Invalid or unexpected token (at swagger-ui-bundle.js:2:867116) swagger-ui-standalone-preset.js:2 Uncaught SyntaxError: Invalid or unexpected token (at swagger-ui-standalone-preset.js:2:138206) swagger-initializer.js:5 Uncaught ReferenceError: SwaggerUIBundle is not defined at window.onload (swagger-initializer.js:5:3)

Upon inspection, the swagger-ui-bundle.js file contains garbled or corrupted characters, likely due to an encoding issue, which is causing the JavaScript files to fail when the Swagger UI attempts to load.

1. Understanding the Root Cause

The root cause of this issue lies in how AWS Lambda and API Gateway serve static files, such as the JavaScript files required by Swagger UI. Specifically, these files are not being served with the correct encoding or content type, leading to issues with loading or interpreting the files in the browser.

There are several potential factors contributing to this issue:

Incorrect MIME Type: AWS API Gateway may not be sending the correct Content-Type header for JavaScript or other static files.

File Encoding Issues: The JavaScript files may have been corrupted or encoded incorrectly during the build or deployment process, leading to unreadable content being served.

Lambda Function Limitations: AWS Lambda functions have size and performance constraints that might cause issues when serving larger files, such as Swagger UI resources.

2. Solutions to Fix the Swagger UI Issue

Solution 1: Ensure Proper Static File Handling

In AWS Lambda, static files like Swagger UI assets (e.g., JavaScript, CSS) need to be served correctly. One option is to place these files in Amazon S3 and configure API Gateway to point to this S3 bucket for serving static content.

Steps:

Upload Swagger UI Files to S3:

Download the Swagger UI distribution package from the Swagger UI GitHub repository.

Upload the required files (such as swagger-ui-bundle.js, swagger-ui-standalone-preset.js, and index.html) to an S3 bucket.

Configure API Gateway to Serve Static Content from S3:

In the API Gateway console, create a new resource or use an existing one to serve the Swagger UI.

Set up an S3 integration to serve the Swagger UI files directly from your S3 bucket.

Configure the API Gateway to return the appropriate MIME type (e.g., application/javascript for JavaScript files).

Update the Swagger UI URL:

In your Lambda function, modify the URL in your application configuration to point to the correct endpoint in API Gateway where the Swagger UI files are hosted.

Solution 2: Set Correct MIME Type in API Gateway

Sometimes the issue arises because the JavaScript files are being served with the wrong MIME type. You can enforce the correct MIME type in the API Gateway settings:

Go to the API Gateway Console.

Select the relevant API that you are using to serve Swagger UI.

Enable binary support for the JavaScript files: Navigate to the "Binary Media Types" section.

Add application/javascript to ensure that API Gateway correctly serves JavaScript files with the correct content type.

Solution 3: Check for File Corruption During Deployment

Ensure that the Swagger UI assets have not been corrupted during the build or deployment process. You can do this by comparing the files in your local environment with those deployed to AWS Lambda. If there is any discrepancy, you may want to:

Rebuild the application with proper encoding settings.

Ensure that all files are correctly packaged and uploaded to the Lambda deployment package or S3 bucket.

Solution 4: Use a Custom Lambda Authorizer for Swagger UI

If you need to control access to the Swagger UI or ensure it is only accessible to authorized users, you can implement a custom Lambda authorizer for API Gateway. This allows you to intercept requests to the Swagger UI and enforce authentication or other access rules before returning the Swagger UI content.

Solution 5: Use an External Hosting Service

Instead of hosting the Swagger UI directly through AWS Lambda, you can consider using an external service (like Netlify or GitHub Pages) to host the static assets (Swagger UI files) and then point your API Gateway to this hosted URL. This simplifies the static file hosting process and reduces the risk of file corruption or misconfiguration in AWS Lambda.

3. Debugging Tips

If you are still facing issues after applying the above solutions, here are some additional debugging tips:

Check Network Logs:

Use browser developer tools (F12) to inspect network requests. Ensure that all Swagger UI files are being fetched successfully and with the correct response headers.

Verify S3 Bucket Permissions:

Ensure that your S3 bucket is publicly accessible or that API Gateway has the necessary permissions to access the files.

Check API Gateway Logs:

Review the CloudWatch logs associated with your API Gateway to ensure that requests to fetch Swagger UI files are being handled correctly.

Clear Browser Cache:

Sometimes, browsers cache old versions of JavaScript files. Clear your browser cache or try accessing the Swagger UI in an incognito window to avoid issues with cached files.

4. Example Code for AWS Lambda with Swagger UI

Below is an example of how you can configure your Spring Boot application to serve Swagger UI in an AWS Lambda environment:

application.properties:

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# Configure the base URL for Swagger UI springdoc.api-docs.enabled=true springdoc.swagger-ui.enabled=true springdoc.swagger-ui.path=/swagger-ui.html

LambdaHandler.java:

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import com.amazonaws.serverless.exceptions.ContainerInitializationException; import com.amazonaws.serverless.proxy.spring.SpringBootProxyHandlerBuilder; import com.amazonaws.serverless.proxy.spring.SpringLambdaContainerHandler; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; @SpringBootApplication public class LambdaHandler { public static void main(String[] args) { SpringApplication.run(LambdaHandler.class, args); } public SpringLambdaContainerHandler getHandler() throws ContainerInitializationException { return new SpringBootProxyHandlerBuilder() .defaultProxy() .springBootApplication(LambdaHandler.class) .build(); } }

In the above code, we initialize the Spring Boot application to work with AWS Lambda and set up the Swagger UI path for easier access in both local and cloud environments.

5. FAQs

Q1: Why does Swagger UI work locally but not in AWS Lambda?

The issue usually arises due to file serving problems in the cloud environment. AWS Lambda and API Gateway require proper configurations to serve static assets like Swagger UI files, including setting the correct MIME type, handling encoding issues, and ensuring the files are accessible.

Q2: How do I serve static files like Swagger UI in AWS Lambda?

One of the best practices is to host the Swagger UI assets in an S3 bucket and use API Gateway to serve them. This avoids potential issues related to serving static files directly from Lambda.

Q3: How can I ensure the correct encoding for Swagger UI files?

Ensure that Swagger UI files are correctly packaged and uploaded to the deployment environment. If using S3, ensure that the files are uploaded with the correct encoding and MIME type.

Q4: Can I use a custom domain for Swagger UI?

Yes, you can configure a custom domain for the Swagger UI by mapping an API Gateway endpoint to your domain using AWS API Gateway Custom Domain feature.

Q5: How can I secure access to Swagger UI in production?

To secure access to Swagger UI in production, you can implement API Gateway authentication (e.g., using AWS Cognito or a custom Lambda authorizer) or deploy Swagger UI behind a VPN.

Conclusion

Deploying Swagger UI on AWS Lambda can be challenging due to the need to serve static files properly. By following the steps outlined above, including setting up S3 for static file hosting, configuring API Gateway to serve the files with the correct MIME type, and ensuring proper file encoding, you can resolve the blank page issue and get your Swagger UI working smoothly on AWS Lambda. Additionally, the provided FAQs and debugging tips can help you troubleshoot any remaining issues during the deployment process.