# https://www.varnish-cache.org/docs/2.1/tutorial/vcl.html
# https://www.varnish-cache.org/trac/wiki/VCLExamples

# Summary
# 1. Varnish will poll the backend at /health_check to make sure it is
#    healthy. If the backend goes down, varnish will server stale content
#    from the cache for up to 1 hour.
# 2. Varnish will pass X-Forwarded-For headers through to the backend
# 3. Varnish will remove cookies from urls that match static content file
#    extensions (jpg, gif, ...)
# 4. Varnish will normalize the Accept-Encoding header
# 5. Varnish will respect the Cache-Control header, even when a Set-Cookie
#    header accompanies it from the backend. For example,
#    `Cache-Control: no-cache` will not get stored in Varnish and
#    `Cache-Control: max-age=600` will.
# 6. Varnish will add a X-Varnish-Cache header to aid in debugging

backend default {
  .host = "127.0.0.1";
  .port = "8080";

  # https://www.varnish-cache.org/trac/wiki/BackendPolling
  .probe = {
    .url       = "/health_check";
    .timeout   = 2 s;
    .interval  = 1s;
    .window    = 10;
    .threshold = 8;
  }
}

# NOTE: vcl_recv is called at the beginning of a request, after the complete
# request has been received and parsed. Its purpose is to decide whether or not
# to serve the request, how to do it, and, if applicable, which backend to use.
sub vcl_recv {
  # If the backend is healthy, only accept objects that are 30 seconds old,
  # but if the backend is sick, accept objects that are up to an hour old.
  # https://www.varnish-cache.org/trac/wiki/VCLExampleGrace
  if (req.backend.healthy) {
    set req.grace = 30s;
  } else {
    set req.grace = 1h;
  }

  # Normalize Accept-Encoding to prevent duplicates in the cache
  # https://www.varnish-cache.org/trac/wiki/VCLExampleNormalizeAcceptEncoding
  if (req.http.Accept-Encoding) {
    if (req.http.Accept-Encoding ~ "gzip") {
      set req.http.Accept-Encoding = "gzip";
    } elsif (req.http.Accept-Encoding ~ "deflate") {
      set req.http.Accept-Encoding = "deflate";
    } else {
      # unkown algorithm
      remove req.http.Accept-Encoding;
    }
  }

  # This rule is to insert the client's ip address into the request header
  if (req.restarts == 0) {
    if (req.http.x-forwarded-for) {
      set req.http.X-Forwarded-For = req.http.X-Forwarded-For ", " client.ip;
    } else {
      set req.http.X-Forwarded-For = client.ip;
    }
  }

  # Force lookup if the request is a no-cache request from the client
  # IE - shift-reload causes cache refresh - We may not want this in 
  # production but useful during initial deployment and testing
  if (req.http.Cache-Control ~ "no-cache") {
    purge_url(req.url);
  }

  # Don't cache POST, PUT, or DELETE requests
  if (req.request == "POST" || req.request == "PUT" || req.request == "DELETE") {
    return(pass);
  }

  # Strip cookies from static content
  if (req.request == "GET" && req.url ~ "\.(png|gif|jpg|swf|css|js)$") {
    unset req.http.cookie;
  }

  # We will try to retrieve every request from the cache. There will be no
  # intelligence on the varnish side to determine whether to look or not look
  # at the cache.
  return(lookup);
}

sub vcl_pipe {
  # Note that only the first request to the backend will have
  # X-Forwarded-For set.  If you use X-Forwarded-For and want to
  # have it set for all requests, make sure to have:
  # set bereq.http.connection = "close";
  # here.  It is not set by default as it might break some broken web
  # applications, like IIS with NTLM authentication.
  set bereq.http.connection = "close";
  return(pipe);
}

# NOTE: vcl_fetch is called after a document has been successfully retrieved
# from the backend. Normal tasks her are to alter the response headers, trigger
# ESI processing, try alternate backend servers in case the request failed.
sub vcl_fetch {
  # Keep objects 1 hour in cache past their expiry time. This allows varnish
  # to server stale content if the backend is sick.
  set beresp.grace = 1h;

  # If header specifies "no-cache", don't cache.
  if (
    beresp.http.Pragma        ~ "no-cache" ||
    beresp.http.Cache-Control ~ "no-cache" ||
    beresp.http.Cache-Control ~ "private"
  ) {
    return(pass);
  }

  # If header specifies "max-age", remove any cookie and deliver into the cache.
  # The idea here is to trust the backend. If the backend set a max-age in
  # the Cache-Control header, then the response should be cached even if there
  # is a Set-Cookie header. The cleaner way to handle this is the not set a
  # Set-Cookie header in the backend, but unfortunately Rails always sets one.
  if (beresp.http.Cache-Control ~ "max-age") {
     unset beresp.http.Set-Cookie;
     return(deliver);
  }

  # Do not deliver into cache otherwise.
  return(pass);
}

sub vcl_deliver {
  # The below provides custom headers to indicate whether the response came from
  # varnish cache or directly from the app.
  if (obj.hits > 0) {
    set resp.http.X-Varnish-Cache = "HIT";
  } else {
    set resp.http.X-Varnish-Cache = "MISS";
  }
}