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My online whiteboard application has been working previously, but for whatever reason, it no longer works. Now it shows a 503 Varnish cache error as seen here: http://grab.by/eFHG
Do you happen to know where I should start to look to try to resolve this issue?
Thanks!
Donny
This error means that Varnish had no response (even not an HTTP error) from the backend within the timeout limit.
You can troubleshoot that in many ways :
On the backend : do you see requests from Varnish in your webserver
log ?
On Varnish server : Run varnishlog and check the request process. You should have events in this order : RxRequest > TxRequest > RxResponse > TxResponse. Your problem is between TxRequest (request sent to backend) and RxResponse (response received from backend).
On your Varnish server try connecting on the backend using telnet (telnet ). Does it connect ? If it does, try sending a request (e.g. “GET / “). Do you receive a response ?
Probable causes could be : firewall/selinux blocking between varnish & backend, bad varnish or backend web server config (are backend address & port sync ?), webserver stopped, …