Thank you for the detailed description.
To diagnose the problem, let's first confirm that the 504 error is being generated by the ALB. We can do this by checking if the
server: awselb/2.0 header is present in the response or by reviewing the
HTTPCode_ELB_504_Count metrics in CloudWatch.
If the 504 error is indeed being generated by the ALB, we can refer to this AWS document, which lists all possible causes of the error and provides guidance on how to resolve them. Additionally, this Knowledge Center article provides further guidance on fixing the issue. For instance, we should verify that all ALB nodes can connect to the targets, which may reside in different subnets with specific security groups and subnet ACLs.
As a side note, it may be helpful to temporarily enable ALB access logs, which can provide more information about the 504 requests for deeper analysis.
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