Varnish Server-Timing

RUMvision can collect Varnish timing and cache data when your website exposes it through the Server-Timing response header.

This helps you understand whether slow server response times are related to Varnish cache behavior or to your origin server.

Varnish does not expose standard Server-Timing metrics by default. If your site is not exposing these headers yet, you can add cache information in VCL and preserve origin timings sent by your origin or reverse proxy.

Add Server-Timing via Varnish

Below are the recommended fields to expose. Once these fields are exposed and Varnish is selected in your tech stack settings and enabled the Varnish checkbox, we will automatically collect them.

ValueType/sourceMeaning
varnish_cacheFilter
VCL cache handling		Shows whether Varnish handled the request as HIT, MISS, or PASS.
originnot collected
implement yourselves		Time spent at the origin.
This should be emitted by your application, NGINX, Apache, or another origin layer before Varnish receives the response.

Where to implement

Implement this in your Varnish VCL configuration.

/etc/varnish/default.vcl

Rule to implement

Apply this to all HTML document requests. These are the main page requests RUMvision uses to understand document TTFB and backend-related delays.

This usually means the main document request of your pages, not images, scripts, stylesheets, or API calls.

If needed, you can also expose these timings on other request types, but HTML document requests are the recommended starting point.

Add the following VCL:

sub vcl_hit {
  set req.http.X-RUM-Varnish-Cache = "HIT";
}
sub vcl_miss {
  set req.http.X-RUM-Varnish-Cache = "MISS";
}
sub vcl_pass {
  set req.http.X-RUM-Varnish-Cache = "PASS";
}

sub vcl_deliver {
  if (!req.http.X-RUM-Varnish-Cache) {
    set req.http.X-RUM-Varnish-Cache = "UNKNOWN";
  }

  if (resp.http.Server-Timing) {
    # Append to existing Server-Timing
    set resp.http.Server-Timing = resp.http.Server-Timing + ", varnish_cache;desc=" + req.http.X-RUM-Varnish-Cache;
  } else {
    # Set Server-Timing http header
    set resp.http.Server-Timing = "varnish_cache;desc=" + req.http.X-RUM-Varnish-Cache;
  }

  unset req.http.X-RUM-Varnish-Cache;
}

This adds Varnish cache information to Server-Timing. If your origin already emits an origin timing, this configuration keeps it and appends the Varnish cache status.

Testing the outcome

Next step is testing the outcome. You could either wait for data to arrive in your RUMvision dashboard, or proactively test the outcome using the DevTools of your preferred browser.

Expected outcome

For a cache miss where the origin also exposes timing data, your HTML document may return a header similar to this:

Server-Timing: origin;dur=248, varnishCache;desc=MISS

For a cached response, the header may look like this:

Server-Timing: varnishCache;desc=HIT

Check the response headers

To verify the setup:

  1. Open your website in Chrome or Edge.
  2. Open DevTools.
  3. Go to the Network tab.
  4. Reload the page.
  5. Click the main HTML document request.
  6. Check the Response Headers section.
  7. Look for server-timing.

Test in DevTools Console

As the browser exposes Server-Timing values through the PerformanceServerTiming interface, you can also test it in the browser console by running the following JavaScript:

const navEntries = window.performance.getEntriesByType('navigation');
console.table( navEntries[0].serverTiming );

Important

In general, be sure to:

  • Always test on staging environment before deploying these steps to a production environment.
  • Avoid exposing sensitive internal details. Server-Timing data is visible in the browser, so only expose metrics that are safe to share with visitors and third-party scripts.

Varnish notes

This VCL only adds varnishCache to the Server-Timing header. It does not calculate or add origin timing itself.

If you also want origin;dur=..., your origin application, NGINX, Apache, or another backend layer must emit a Server-Timing header before the response reaches Varnish. Varnish will then preserve that existing value and append varnishCache.

If your origin already sends a Server-Timing header, make sure your Varnish configuration preserves it and appends its own cache information instead of replacing it.

Table of Contents