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From Graham Leggett <>
Subject Re: The Case for a Universal Web Server Load Value
Date Wed, 14 Nov 2012 23:32:41 GMT
On 15 Nov 2012, at 12:48 AM, Tim Bannister <> wrote:

> This only makes sense for idempotent requests. What about a POST or PUT?
> For a plausible example that mixes POST and GET: a cluster of N webservers providing
SPARQL HTTP access to a triplestore. Most queries will use GET but some might use POST, either
because they are too long for GET or because the query is an update.
> The reverse proxy / balancer manager might want to:
> • balance query workload across the active set of webservers
> • spin up an extra backend as required by load
> • skew load onto the minimum number of webservers (and suspend any spares)
> SPARQL is an example of a varying workload where none of httpd's existing lbmethods is
perfect. One complex query can punish a backend whilst its peers are idle handling multiple
concurrent requests. SPARQL sometimes means POST requests; a subset of these are safely repeatable
but determining which ones is too complex for any HTTP proxy.

There is no reason why a load balancer can't take into account existing requests in addition
to new requests when making load balancing decisions. When there are a number of connections
to a backend that are in flight but taking a while to complete, this is a sign the backend
may be busy and should be avoided.

That said if you have pathologically expensive requests coming into your backends, no load
balancer is going to help you.


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