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How does saint mode work when director is being used?

 

 

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Jeff.Stroomer at disney

Dec 27, 2011, 10:22 AM

Post #1 of 2 (320 views)
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How does saint mode work when director is being used?

Varnish folks,

How does saint mode work when a director is being used? The discussion here

https://www.varnish-cache.org/trac/wiki/VCLExampleSaintMode

mentions this case briefly, but doesn't have much detail.

To make this more concrete, let's say I am using a round-robin director to distribute traffic across three back ends named A, B, and C, and suppose that in vcl_fetch I see a 500 response that came from A. If I set saint mode to 20 seconds on the response, does that mean that the object is blacklisted on A for 20 seconds, and therefore subsequent requests for that object go to B and C? And if saint mode subsequently also blacklists the object on B and C as well, does that mean that grace mode kicks in, and a cached copy is returned?

Assuming I have this right, then it sounds like the backend health checks used by the director are a large-scale way to blacklist all requests for a back end, whereas saint mode is a small-scale object-by-object way to blacklist a back end.

Thanks in advance for the help,

Jeff


kristian at varnish-software

Dec 29, 2011, 2:01 AM

Post #2 of 2 (309 views)
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Re: How does saint mode work when director is being used? [In reply to]

Greetings,

On Tue, Dec 27, 2011 at 10:22:46AM -0800, Stroomer, Jeff wrote:
> To make this more concrete, let's say I am using a round-robin
> director to distribute traffic across three back ends named A, B, and
> C, and suppose that in vcl_fetch I see a 500 response that came from
> A. If I set saint mode to 20 seconds on the response, does that mean
> that the object is blacklisted on A for 20 seconds, and therefore
> subsequent requests for that object go to B and C? And if saint
> mode subsequently also blacklists the object on B and C as well, does
> that mean that grace mode kicks in, and a cached copy is returned?

Correct.

> Assuming I have this right, then it sounds like the backend health
> checks used by the director are a large-scale way to blacklist all
> requests for a back end, whereas saint mode is a small-scale
> object-by-object way to blacklist a back end.

That's precisely it. I couldn't have said it better myself.

- Kristian

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