AWS App Runner: Fix Deployment Failures During Traffic Routing

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If you’re experiencing an issue where updating your existing AWS App Runner service fails during traffic routing, but creating a new service with the same image works fine, you’re not alone. Here’s a straightforward way to troubleshoot and resolve this problem.

First, understand that when deploying a new image to your existing service, the process usually succeeds up until the traffic routing phase. This is where AWS App Runner tries to redirect traffic from the old version to the new one. If this step fails, it can be frustrating because the deployment pulls the image, performs health checks, and verifies resources successfully — yet still fails at the traffic switch.

The good news is that the problem often resides in details that may seem minor but are crucial. Here’s what you should check:

Make sure your app is listening on the correct port. The Dockerfile should expose port 3000, and your app should bind to 0.0.0.0, not localhost. You can test this locally with docker run -p 3003:3000 <image>, confirming the app starts properly and listens as expected.

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Verify that your health checks are passing before the routing step. AWS App Runner uses health checks to make sure the app is ready before directing traffic. Since your old service passes health checks, this isn’t the issue but double-check that the health check path and port are configured correctly.
Check resource usage. Low CPU and memory mean resource exhaustion isn’t the cause of failure.
Confirm that your Dockerfile correctly exposes the right port and binds to all network interfaces.

Now, why does the new service work but the existing one doesn’t during routing? Sometimes, configuration differences—such as environment variables, network settings, or outdated routing rules—can cause the traffic routing to fail during updates. It’s possible that the deployment process for the existing service isn’t syncing these configurations properly.

Here’s what you can do to fix this:

Try deleting and recreating the existing service, ensuring that the setup matches the new service. Sometimes, lingering configurations or old settings cause routing issues during updates.
Check if there are any environment variables, IAM roles, or network security groups that differ between the services.
If you’re using custom domain names or TLS certificates, verify that these are properly associated with your existing service.
Consider manually updating the service configuration by removing and re-adding network and routing settings, making sure they align with the working new service.
Look into the deployment logs carefully; they might reveal specific errors or mismatches in configuration causing the traffic routing failure.

By following these steps, you should be able to identify what’s blocking traffic routing in your old App Runner service during updates. Creating a new service often works because it starts with a clean configuration, avoiding any settings conflicts that might have accumulated over time. Reapplying or resetting configurations on the existing service usually solves the problem and ensures smooth deployments in the future.