HAProxy retry request on a different backend on 404

I’m trying to get HAProxy to retry a request that 404'd on a different backend.

I am running 2 s3 instance that I want to get an object from,

The scenario is that data in bucket “s3Primary” will not exist and returns 404 I want to be able to query bucket “s3Secondary” with that same request.

frontend app
    bind *:80
    acl not_found status 404
    use_backend s3Secondary if not_found
    default_backend s3Primary

backend s3Primary
    server s3prime 127.0.0.1:9001

backend s3Secondary
    server s3second 127.0.0.1:9002

I have tried redirecting using http-response but the request just hangs without resolving I have also seen the answer in this question

Currently the solution just returns 404, I'm new to haproxy.

HAProxy has a great mechanism to take of things when a worst case happens or something called as failover capabilities. To achieve something like that you can try using a keyword backup in the same backend. Here is an example.

frontent app
 bind *:80
 default_backend s3_backend

backend s3_backend
 server s3_primary   127.0.0.1:9001 check
 server s3_secondary 127.0.0.1:9002 check backup

You can also add multiple backups as your backup locations, but you need to add something like this

backend s3_backend
 option allbackups
 server s3_primary   127.0.0.1:9001 check
 server s3_secondary 127.0.0.1:9002 check backup
 server s3_verybad   127.0.0.1:9003 check backup

Or you can also do it similar way how you did by adding an entry to access-control list

frontent app
 bind *:80

 acl primary_failed nbsrv(s3_primary) lt 1

 use_backend s3_secondary if primary_failed

 default_backend s3_primary

backend s3_primary
 server s3_primary   127.0.0.1:9001 check

backend s3_secondary
 server s3_secondary 127.0.0.1:9002 check

Hope this helps.