AWS Application Load Balancer (ALB) path based routing not functioning as expected

Question

I am working on a POC to prove out AWS path based routing through an Application Load Balancer to a set of very basic "hello world" node.js applications using express. Without the path based routing in place and having multiple listeners, 1 listener for each application, each respective listener and application is working as expected. Therefore, the targets within the Target Groups have both passed health checks and are shown as healthy. However, when I switch to the path based routing implementation on 1 of the listeners (deleting the other unnecessary listener) I get the following error for both applications:

Cannot GET /expressapp
Cannot GET /expressapp2

I have gone through the following documentation to try to figure out the issue: http://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-listeners.html#path-conditions

What am I missing? Any troubleshooting ideas?

Answer 1

I believe that you are getting this error because the services in question do not expect to receive paths prefixed with /expressapp and /expressapp2. When the ALB forwards traffic to your service, the path remains intact.

Stripping off the prefix cannot be handled by ALB. If you don't have access to the source code of the apps, you will need to use some kind of reverse-proxy like nginx to rewrite the urls before sending them onto the app.

If you have access to the source code, express supports changing the base url without modifying the code. You can read a value for the url prefix in as an environment variable and configure your respective service environments accordingly.