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.
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