idea for load "balancer" with fan-out/ voting
aotto at mosso.com
Sun Jan 10 20:41:08 CET 2010
Most applications won't handle duplicate requests gracefully, especially REST like systems that are doing operations like POST or PUT to update data. Generally, once you begin a write operation, you want it to run until the app server tells you there was a timeout.
For read-only operations you might try a short timeout from the load balancer, and upon timeout (no first-byte-response after 2 seconds for example) then mark the node as unhealthy and try a second node with no timeout. Don't be tempted to try more than two because in high load situations that only makes matters worse. You also need a concept of "fewest nodes in pool" so that when things do get slow on the whole cluster you don't end up pruning it to the point where it's too small to be able to do any useful work.... more importantly so you don't empty the cluster. The idea is just to temporarily prune a node or two that get slow, but should recover later.
This approach is better than aggressive polling because it uses less resources (on the proxy and the app server) and the end user never gets trapped on a slow/dead node in the time between the last good health check and the failure event.
Nils Goroll <slink at schokola.de> wrote:
>> so if you could duplicate an http request to multiple servers and get
>> the first response, you'd have to be pretty unlucky for all of them to
>> be garbage collecting and unresponsive.
>This issue can (and should) be solved by properly tuning your Java VMs.
>Duplicating workload on the backend to me sounds just like the opposite of what
>you usually want to achieve with an optimization tool like varnish.
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