AWS ELB unhealthy 4xx

Question

I am using express.js and graphql for backend server which uses port 4000. I am trying to connect my server with ELB but the status returns unhealthy with error code 400 or 404. These are my settings about ELB.

• Target group

• Target groups registered targets

• Target groups health checks

If I change the path to /graphql it returns 400, and just / returns 404. It is still working with no error when I call the API but it seems like I should fix it. Could anyone please tell me what can I do?

Answer 1

The standard way to handle this case is to define the path in your express application for example /health or /ping which should return HTTP status code 200.

If /graphql does not return HTTP status code 200 the LB will mark the target unhealthy.

you have two option

• Update Success codes to 400, the LB will mark the target healhty
• Define path like / or /health and return status code 200.

In express for example

app.get('/health',function(req,res){
res.status(200).send("instance is healthy");
})

Then setting for LB will

Or if you want to go without changing in the application the below will work.

Answer 2

Apollo supports a simple health check by default at this URL /.well-known/apollo/server-health (source), so you can add change the path in the target group from / to the URL