Varnish 5, serve cache objects if backend is down - varnish

Question

I'm using varnish 5.2.1.
I have configured VCL for grace mode. If TTL expire, object enter in grace mode if backend is down. It's ok.
But, this not work if I ban or purge object.
If I purge or ban, and backend is down, varnish show me the 503 error and not object saved in grace.
Can you help for use grace mode if user ban or purge objects ?
I have implemented grace mode and soft purge in varnish 5.2 with article https://www.getpagespeed.com/server-setup/varnish/varnish-5-2-grace-mode
All works great, but, for example, if I use use wordpress with Varnish HTTP Purge plugin (https://it.wordpress.org/plugins/varnish-http-purge/), when I purge specific url/article, all works, but If I request purge entire cache, not work.
I think the problem is beacuse request is a regex ... for url "/.*"
On varnishlog I see this request:
<< Request >> 163874
Begin req 163873 rxreq
Timestamp Start: 1517991657.640832 0.000000 0.000000
Timestamp Req: 1517991657.640832 0.000000 0.000000
ReqStart 10.0.1.100 37454
ReqMethod PURGE
ReqURL /.*
ReqProtocol HTTP/1.1
ReqHeader host: test.local
ReqHeader User-Agent: WordPress/4.9.4; http://test.local
ReqHeader Accept: /
ReqHeader Accept-Encoding: deflate, gzip
ReqHeader Referer: http://10.0.3.250/.*
ReqHeader X-Purge-Method: regex
ReqHeader Connection: close
ReqHeader X-Forwarded-For: 10.0.1.100
VCL_call RECV
VCL_acl MATCH purge_acl "10.0.1.100"
VCL_return hash
ReqUnset Accept-Encoding: deflate, gzip
ReqHeader Accept-Encoding: gzip
VCL_call HASH
VCL_return lookup
VCL_call MISS
ReqHeader purged: 0
VCL_return synth
Timestamp Process: 1517991657.640916 0.000084 0.000084
RespHeader Date: Wed, 07 Feb 2018 08:20:57 GMT
RespHeader Server: Varnish
RespHeader X-Varnish: 163874
RespProtocol HTTP/1.1
RespStatus 404
RespReason Not Found
RespReason Not Found
VCL_call SYNTH
RespHeader purged: 0
VCL_return deliver
RespHeader Content-Length: 0
Storage malloc Transient
RespHeader Connection: close
Timestamp Resp: 1517991657.641034 0.000201 0.000118
ReqAcct 206 0 206 148 0 148
End

Answer 1

There is no solution for keeping graced objects if you use BAN (since they are freed immediately) but for PURGE you can make use of soft purge functionality (which will keep the object in the cache despite setting TTL to 0).
Varnish 5.2 comes with purge module for that purpose (hard vs soft purges).
So you will need to implement it in your VCL:
sub vcl_recv {
if (req.method == "PURGE") {
if (client.ip !~ purge_acl) {
return (synth(405));
}
return (hash);
}
}
sub my_purge {
set req.http.purged = purge.soft();
if (req.http.purged == "0") {
return (synth(404));
}
else {
return (synth(200));
}
}
sub vcl_hit {
if (req.method == "PURGE") {
call my_purge;
}
}
sub vcl_miss {
if (req.method == "PURGE") {
call my_purge;
}
}
sub vcl_synth {
if (req.method == "PURGE") {
if (req.http.purged) {
set resp.http.purged = req.http.purged;
}
return (deliver);
}
}
More information can be found here: Implementing grace mode in Varnish 5.2.