How to use varnish to pass data when the backend is ok && return the last normal beresq when the backend is down?

[JackDrogon]

Hi， All:
	I need varnish to return data directly and update cache at every time when the backend is ok. I also need varnish to return the last normal beresq data from the cache when the backend is down. How shuold I do with varnish?

Thanks.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://www.varnish-cache.org/lists/pipermail/varnish-misc/attachments/20161107/2c29376b/attachment.html>