Tracking down sporadic 503 errors

Torrance torrance123 at
Tue Mar 30 21:20:25 CEST 2010

Hi Justin,

I had a similar problem with receiving fairly consistent 503 errors on
POST requests. In my case, the 503 errors were returning generally in
less than a second, despite my having set the various timeout limits to
be substantially higher. If it's the same problem, then it's possibly
not a timeout problem.

I wrote to the list earlier this year and posted some logs [1], but was
asked to post a tcpdump of all communications for further investigation
which was (is) beyond my skills.

I hope you have more success resolving this,


On 31/03/10 4:02 AM, Justin Pasher wrote:
> Hello,
> I'm currently running Varnish r4633 (I can upgrade if absolutely
> necessary), and we've been receiving very sporadic 503 errors listed
> in the log files generated by varnishncsa. It's a very small
> percentage, but nonetheless, when it happens on an important page,
> it's noticeable.
> Stats from yesterday's log file show 116 "503" errors out of about 4.1
> million hits. About 80% of the failed requests are POST requests,
> which it setup in my VCL as a "pass through". If I look in the apache
> logs (the backend server), I only see one 503 error returned by apache
> itself, so maybe there's a timeout issue somewhere. I'm trying to
> figure out the best way to troubleshoot this, since it's too
> inconsistent to just sit watching the output of varnishlog.
> Perhaps it's hitting the default value for between_bytes_timeout (60
> seconds)? If processing the data on the backend takes too long, then
> varnish would time out after 60 seconds of no data, even if the
> backend is still churning, right? I guess the question is what
> situations cause Varnish to return a 503 aside from when the backend
> itself returns a 503.
> I can post details of my VCL if needed, but it's pretty simple (mostly
> taken from examples on the site).

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