Urgent Help Needed: Application Load Balancer Health Check Issue Across AWS Accounts

Hello all,

I hope this message finds you well. I am currently facing a challenging issue with an AWS architecture setup involving a load balancer and ECS Fargate clusters across different AWS accounts. I'm reaching out to the community for insights and assistance in resolving the problem.

Problem Description:

I have a load balancer deployed in VPCA (publicly accessible) within AWS Account A. Simultaneously, an ECS Fargate cluster is deployed into VPCB (private subnet) within AWS Account B. The connection between these VPCs is established through a transit gateway.

Initially, everything was working seamlessly, but suddenly the targets in the application load balancer's target group started showing as unhealthy with the reason "request timed out."

Troubleshooting Steps Taken:

VPC Reachability Analyzer: I ran VPC reachability analysis from the Application Load Balancer's Network Interface to the ECS Fargate container, and the result indicated that the connection is reachable. Seeking Community Input:

I'm reaching out to the community to inquire if anyone has encountered similar issues or has insights into potential causes and solutions for this "request timed out" problem. Your experiences and suggestions will be immensely helpful in troubleshooting and resolving this issue.

Questions:

Have you faced a similar situation where targets in an application load balancer's target group suddenly became unhealthy with a "request timed out" reason? Are there any specific considerations or configurations that might lead to such issues in a multi-account setup with AWS ECS Fargate and an Application Load Balancer? Your guidance and input are highly appreciated. Thank you in advance for your time and assistance!