I spent a bit more time searching this list and found the answer: No, retrying another server in the backend upon receipt of a 503 is not supported.
https://www.mail-archive.com/haproxy%40formilux.org/msg17281.html On Wed, Nov 25, 2015 at 1:45 PM, Mike Sample <[email protected]> wrote: > Thanks Jeff, > > The 'backend' HAProxies do have a purpose - they: > * allow stripping global routing information (e.g. from the http path) > * allow rate limiting at the worker node (even with many 'client > haproxies') > * allow for real backends of different types on one node while just > having one entry host/port per node (e.g. use global routing info before > stripping) > * provide a highly tunable HTTP connection terminator to ensure keepalive > etc work. > * simplify network ACLs by sharing a host/port pair for many different > real backends on a node > > The configuration challenges of managing all these haproxies is avoided by > another system; they would not be configured by hand. > > However, the one challenge is that this indirect access to the real > backend confuses retry logic. > > /Mike > > On Wed, Nov 25, 2015 at 12:28 PM, Jeff Palmer <[email protected]> wrote: > >> Mike, >> >> Help me understand: >> >> if haproxy 1 is just frontending haproxy 2 and 3, which each only have >> 1 backend.. what is the purpose of haproxy2 and 3, other than to just >> complicate the architecture? are they serving some other purpose >> that isn't being communicated? >> >> If not, I'd say follow the KISS principle. have haproxy1 utilize the >> 2 backends directly. and let it detect backend availability as it's >> designed to do. >> >> >> >> >> On Wed, Nov 25, 2015 at 3:00 PM, Mike Sample <[email protected]> >> wrote: >> > In a setup like this where HAProxy1 LBs over N other HAProxies (servers >> in a >> > backend of HAProxy1): >> > >> > HAProxy1 >> > | --> HAProxy2 -> 1 real backend server >> > | --> HAProxy3 -> 1 real backend server >> > >> > Is there a way to trigger a retry by HAProxy to use HAProxy3 if it tried >> > HAProxy2 and just its real backend server was down? i.e. HAProxy1 >> > successfully connected to HAProxy2 but HAProxy2 returned a 503 because >> its >> > one backend server was down. >> > >> > The goal is that the initial call into HAProxy1 succeeds by using the >> > HAProxy3, even though it tried HAProxy2. >> > >> > I looked at the 'observe' server option but my read of the config >> manual is >> > that it won't trigger a retry (just a failed health check). I also >> realize >> > that, in general, retrying on a 503 has idempotency issues so using a >> custom >> > 5xx code might be a better signal to indicate no-work was done. >> > >> > Thanks >> >> >> >> -- >> Jeff Palmer >> https://PalmerIT.net >> > >
