502's show up when you have more connections from the front end to the back end that it can service. The proxy server has a timeout [1], and if it doesn't get a response within that time frame you get a 502.

You can create a custom error document on your front end for 502's to make them a little less jarring.

[1] http://httpd.apache.org/docs/2.2/mod/mod_proxy.html#proxytimeout

Quoting John ORourke <[EMAIL PROTECTED]>:

Hi folks,

Slightly OT but hopefully someone on here has had similar experience.
I've got a site with fairly heavy traffic and a light/heavy apache
setup.  Occasionally the back-end servers seem to get swamped and
suddenly every request from the front end starts getting a 502 proxy
timeout until I restart both apache servers.

I'm about to start digging through logs to try and track down the issue
but wondered if anyone who's seen this before could give me a head
start.

cheers
John



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