<html><head><base href="x-msg://7/"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Jack,<div><br></div><div>This approach is probably not a good idea if (a) you have a large cluster, (b) a heavily loaded cluster, and/or (c) if your backends are sensitive to overload. You are likely to trigger a cascading failure. It might be smarter to have a configurable number of backends to try... perhaps 2 or 3. Imagine if you have 50 backends. There is no point in trying 50 times to find a healthy backend. Changes are that if 25% of your backends are down, trying more is just going to exacerbate the problem. </div><div><br></div><div>Adrian</div><div><br><div><div>On Apr 11, 2010, at 4:35 PM, Jack Lindamood wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><span class="Apple-style-span" style="border-collapse: separate; font-family: Helvetica; font-size: medium; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div lang="EN-US" link="blue" vlink="purple"><div class="Section1"><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; ">The following is a patch I’ve made to varnish that I hope improves the random director: which anyone’s welcome to use (even varnish trunk?). My motivation was to reduce the number of vcl_error calls when a director is mostly good. You can get the entire patch at this link.<o:p></o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; "><o:p> </o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; "><a href="http://github.com/cep21/Varnish/commit/6f5e98143ac2636504d9febf574b14c3c1a072fc" style="color: blue; text-decoration: underline; ">http://github.com/cep21/Varnish/commit/6f5e98143ac2636504d9febf574b14c3c1a072fc</a><o:p></o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; "><o:p> </o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; ">Here’s the commit message:<o:p></o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; "><o:p> </o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; ">Random director tries all backends before giving up<o:p></o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; "><o:p> </o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; ">Summary:<o:p></o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; ">The current random director gives up when it can't get a FD to the backend it wants retries times in a row. Rather than give up and return NULL, which is guaranteed to cause a vcl_error, as a last ditch effort we try all other healthy backends until we get one that works. This is mostly useful in the between time after a backend server dies and before the health check fails enough to mark a backend unhealthy.<o:p></o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; "><o:p> </o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; ">Backwards Compatibility = Not strictly backwards compatible. In cases when the old code would of fallen through to vcl_error this will give a shot at getting a good result.<o:p></o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; "><o:p> </o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; ">Performance = In the worse case, this will add extra calls for getting a FD, but only for situations that vcl_error<o:p></o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; "><o:p> </o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; ">Test Plan: New varnish unittest. It fails in the old code and works in this new code.<o:p></o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 10.5pt; font-family: Consolas; "><o:p> </o:p></div><div style="margin-top: 0in; margin-right: 0in; margin-bottom: 0.0001pt; margin-left: 0in; font-size: 11pt; font-family: Calibri, sans-serif; "><o:p> </o:p></div></div>_______________________________________________<br>varnish-dev mailing list<br><a href="mailto:varnish-dev@varnish-cache.org" style="color: blue; text-decoration: underline; ">varnish-dev@varnish-cache.org</a><br><a href="http://lists.varnish-cache.org/mailman/listinfo/varnish-dev" style="color: blue; text-decoration: underline; ">http://lists.varnish-cache.org/mailman/listinfo/varnish-dev</a></div></span></blockquote></div><br></div></body></html>