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From Walter Underwood <wun...@wunderwood.org>
Subject Re: Limiting the number of queries/updates to Solr
Date Thu, 03 Aug 2017 04:45:15 GMT

> On Aug 2, 2017, at 8:33 PM, Shawn Heisey <apache@elyograg.org> wrote:
> 
> IMHO, intentionally causing connections to fail when a limit is exceeded
> would not be a very good idea.  When the rate gets too high, the first
> thing that happens is all the requests slow down.  The slowdown could be
> dramatic.  As the rate continues to increase, some of the requests
> probably would begin to fail.

No, this is a very good idea. It is called “load shedding” or “fail fast”. Gracefully
dealing with overload is an essential part of system design.

At Netflix, with a pre-Jetty Solr (war file running under Tomcat), we took down 40 front end
servers with slow response times from the Solr server farm. We tied up all the front end threads
waiting on responses from the Solr servers. That left no front end threads available to respond
to incoming HTTP requests. It was not a fun evening.

To fix this, we configured the Citrix load balancer to overflow to a different server when
the outstanding back-end requests hit a limit. The overflow server was a virtual server that
immediately returned a 503. That would free up front end connections and threads in an overload
condition. The users would get a “search unavailable” page, but the rest of the site would
continue to work.

Unfortunately, the AWS load balancers don’t offer anything like this, ten years later.

The worst case version of this is a stable congested state. It is pretty easy to put requests
into a queue (connection/server) that are guaranteed to time out before they are serviced.
If you have 35 requests in the queue, a 1 second service time, and a 30 second timeout, those
requests are already dead when you put them on the queue.

I learned about this when I worked with John Nagle at Ford Aerospace. I recommend his note
“On Packet Switches with Infinite Storage” (1985) for the full story. It is only eight
pages long, but packed with goodness.

https://tools.ietf.org/html/rfc970

wunder
Walter Underwood
wunder@wunderwood.org
http://observer.wunderwood.org/  (my blog)



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