Varnish Grace Mode

default.vcl

# /etc/varnish/default.vcl
# Define the list of backends (web servers).
# Port 80 Backend Servers
backend yourapp_webserver1 {
  .host = "000.000.000.000";
  .port = "80";
  .max_connections = 250;
  .connect_timeout = 5s;
  .first_byte_timeout = 5s;
  .between_bytes_timeout = 5s;
  .probe = {
    .url = "/";
    .interval = 3s;
    .timeout = 2s;
    .window = 3;
    .threshold = 2;
  }
}
# Define the director that determines how to distribute incoming requests.
director your_app round-robin {
  { .backend = your_app_webserver1; }
}

# VCL_RECV FUNCTION
if (req.http.host ~ "your\.host\.name\.com\.br" || req.http.host == "origin.your.host.name.com.br") {
  set req.backend = your_app;
 
  # defines how long an overdue an object can be for Varnish to still consider it for grace mode
  set req.grace = 24h;
}

# VCL_FETCH FUNCTION
if (req.http.host ~ "your\.host\.name\.com\.br" || req.http.host == "origin.your.host.name.com.br") {
  # defines how long past the beresp.ttl-time Varnish will keep an object
  set beresp.grace = 24h;
 
  # only caches the backend response when its status is lower than 500
  if (beresp.status >= 500) {
    set beresp.ttl = 0s;
    set beresp.cacheable = false;
  }
}