Retry for idempotent backends

[Edward Hibbert]

This is partly a FAQ, but hopefully it goes beyond that…into a more 
subtle FAQ, perhaps.

I understand that in general it is dangerous for haproxy to retry a 
request once it has been sent to a server, for example after a timeout 
or error, because the server may not be idempotent. I get this 
objection.

However there are some backends that are designed to be idempotent, or 
where the consequences of retrying an operation are less bad than the 
consequences of clients timing out. I have servers supporting an AJAX 
API over HTTP which are designed in this way, partly because HTTP client 
retries can result in duplicate POST requests anyway.

It would be lovely if there was an option to retry in such situations, 
with suitable caveats.

Edward

(Originally posted at 
https://discourse.haproxy.org/t/retry-for-idempotent-backends/2896/2 but 
it was suggested I bring it here.)