A. You are using iSCI for block volume attachment to the compute instances in your backed HTTP service. TCP/IP configuration of your block volume attachment is not configured correctly, leading to issues in your backend service.

B. You are NOT using regional subnets in your Virtual Cloud Network. With Availability Domain (AD) specific subnet. the compute instances of the backend service running in the subnet have issues when the AD is down.

C. You are using OCI Domain Name System. You have misconfigured the 'A' record with the wrong IP address leading to requests not getting routed correctly.

D. You are running a TCP-level health check against your HTTP service. The TCP handshake can succeed and indicate that the service is up even when the HTTP service has issues.