Stefan Caunter

working example vcl for a system with multiple iis7 backends
if the webapp[s] run slowly at times you will need several backends to improve client’s chances of hitting a fast server first.
we always bail out to a long wait for an answer so we don’t show 503s.
the patient and impatient backends are the same webservers, we just poll impatiently and take out slow ones
confirm restarts are working by checking in varnishlog. make sure you have hosts file entries so the hostname entry resolves for the polling check.
client should either get a cache hit if we have the object, or a fast back end on requests that we don’t send directly to the patient director, and if it’s a specific url where we need to wait for a database or some query, we are happy to wait, by using return(lookup), so the object enters the cache for next time.
I have logic for asp sessions and facebook logic too (next time)

# patient backends, no polling, long timeout
backend search1 {
.host = “192.168.0.1”;
.port = “80”;
.first_byte_timeout = 60s;
.between_bytes_timeout = 10s;
}
backend search2 {
.host = “192.168.0.2”;
.port = “80”;
.first_byte_timeout = 90s;
.between_bytes_timeout = 90s;
}
backend my1 {
.host = “web1.example.com”;
.port = “80”;
.first_byte_timeout = 100ms;
.between_bytes_timeout = 500ms;
.probe = {
.url = “/checking”;
.timeout = 100 ms;
.interval = 5s;
.window = 5;
.threshold = 3;
}
}
backend my2 {
.host = “web2.example.com”;
.port = “80”;
.first_byte_timeout = 100ms;
.between_bytes_timeout = 500ms;
.probe = {
.url = “/checking”;
.timeout = 100 ms;
.interval = 5s;
.window = 5;
.threshold = 3;
}
}
# patient director, patience is a property of the backend definition
director patient random {
{ .backend = search1; .weight = 3; }
{ .backend = search2; .weight = 3; }
}
# impatient director, impatience is a property of the backend polling
# you need several backends for best effect, we only show two for example
director impatient random {
{ .backend = my1; .weight = 3; }
{ .backend = my2; .weight = 3; }
}

sub vcl_recv {
# omitting commonly available settings
if (req.restarts == 0) {
set req.backend = impatient;
} else if (req.restarts == 1) {
set req.backend = patient;
} else if (req.restarts == 2) {
set req.backend = patient;
} else if (req.restarts == 3) {
set req.backend = patient;
} else if (req.restarts == 4) {
set req.backend = search1;
}
if (req.url ~ “/slowthing”) {
set req.backend = patient;
return(lookup);
}
if (req.url ~ “/slowthing2”) {
set req.backend = patient;
return(lookup);
}
if (req.url ~ “/control-update”) {
set req.backend = impatient;
return(pipe);
}

}
sub vcl_fetch {
# keep trying if iis does not answer, use vcl_recv restart logic
if (beresp.status != 200 && beresp.status != 403 && beresp.status != 404 && beresp.status != 301 && beresp.status != 302) {
restart;
}
# set things here on objects we send back to clients
if (req.url ~ “^/$” && req.http.host ~ “www.example.com”){
set beresp.ttl = 300s;
set beresp.http.cache-control = “max-age = 300”;
set beresp.http.X-MyCacheReason = “MyHomePage”;
set beresp.grace = 1d;
return(deliver);
}
return(deliver);
}
sub vcl_error {
# avoid the dreaded 503, increase restart number as necessary
if (obj.status == 503 && req.restarts < 5) {
restart;
}
}