We have an ALB load balancing 2 targets. But we are experiencing 502 responses from ALB quite often, that interrupts UI to throw an error. Our application is UI based running on node js and uses socket.io.
Sample log from ALB access log.
https 2019-06-10T09:29:46.987095Z app/DES-G1-USE4-ext-elb/7c8fddfc050d66f6 184.51.199.55:55418 10.72.72.155:8888 0.000 8.697 -1 502 - 876 485 "GET https://designer-use4.genesyscloud.com:443/socket.io/?EIO=3&transport=polling&t=Mj0kC0w HTTP/1.1" "Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:65.0) Gecko/20100101 Firefox/65.0" ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 arn:aws:elasticloadbalancing:us-east-1:361190373704:targetgroup/DES-G1-USE4-tg-desg/4cc8feabb9ee8f89 "Root=1-5cfe2302-b19c6d5059c2fc6096e048e0" "-" "session-reused" 0 2019-06-10T09:29:38.289000Z "forward" "-" "-"
Here -1 502 - means that request is forwarded to the backend by ELB, but the target is not responded. And the connection is b/w ELB and target is closed somehow. As per https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-access-logs.html.
In our application log, there is no trace of this request reached our application.
Please help us to debug this issue from ALB.