Ops on Varnish Cache

VSL cheatsheet

Wed, 29 May 2024 00:00:00 +0000

This page explains a few options common to VSL tools and provides ready-to-use VSL queries to simplify the first steps around the Varnish Shared Log.

Concepts

For performance reasons, Varnish doesn’t write log files. Instead, it outputs them in an in-memory circular buffer. Other processes can then read this buffer and decide what to do with the information.

This translates into a few points:

logging is extremely fast and doesn’t slow down the main process
this allows you to log a great deal of data for each transaction
another tool is needed to collect/process the logs

varnishlog

varnishlog is not the tool you want to collect logs to store. However, it is the perfect choice to obtain debugging data thanks to the vast amount of data it produces.

Mixed content and ERR_TOO_MANY_REDIRECTS errors in WordPress when using Varnish

Tue, 05 Dec 2023 00:00:00 +0000

If you’re running a WordPress site and use Varnish without TLS support, there are 2 typical issues you may encounter when it comes to offloading TLS: mixed content and the dreaded ERR_TOO_MANY_REDIRECTS error. Both issues are related to a lack of TLS awareness in the stack.

This tutorial will show you how to tackle both issues and how to create TLS awareness in a situation where both WordPress and Varnish might be lacking that awareness.

Hands-on varnishtest

Fri, 09 May 2025 00:00:00 +0000

This tutorial will help you dive into varnishtest using self-contained and concrete examples that you can run on your own computer. Each test case adds more layers and is duly commented so you can explore on your own.

The supporting files can be found in this repository and are linked to at the beginning of each section.

1. The most basic test case

Let’s start with basic-001.vtc:

# lines starting with a # are comment, they are ignored by varnishtest,
# same as empty lines

# every vtc MUST start with a "varnishtest DESCRIPTION" line

varnishtest "This does nothing"

We can run this with varnishtest basic-001.vtc:

Generating Varnish access logs with varnishncsa

Tue, 18 Jun 2024 00:00:00 +0000

Because Varnish sits in front of the web server and caches its output, the access logs of the web server no longer give the full picture of your site’s traffic. Only cache misses and bypassed requests will end up in the log.

To ensure that these logs still get generated in spite of the cache, the varnishncsa program can be used.

The Varnish Shared Memory Log

The varnishd process reserves shared memory for logging by creating and mapping files in a tmpfs memory file system. The files are subsequently mapped by log consumers like varnishlog, varnishncsa, and varnishstat in read-only mode, allowing them to get information and logs from varnishd without affecting the caching process. These files are collectively know as Varnish Shared Memory (VSM). The part of the VSM responsible for communicating information about the traffic flowing through Varnish is called the Varnish Shared Memory Log (VSL).

Configuring Varnish systemd services

Fri, 17 May 2024 00:00:00 +0000

Introduction

On modern Linux OS distributions, systemd is the default init system, replacing SysV. The following tutorial aims to show some methods to manage configuration of systemd-based services. Newer packages supplied by Varnish Software have moved away from external files containing startup parameters to the systemd best practice of keeping the parameters in the system .service file. The guide shows how to manage the Varnish systemd service including how to configure startup parameters. Even though the Varnish service is the focus of this guide, the methods are applicable to any other similar service.

Varnishlog: measure your Varnish cache performance

Tue, 08 Aug 2023 00:00:00 +0000

Varnish sits in front of the web server or application server and caches its output. As a result, the logs of the web server no longer paint a complete picture of your site’s traffic.

Although cache misses and cache bypass still appear in the logs, there is a need for logging on the edge. And that’s where varnishlog comes into play. Not just as a replacement for your web server logs, but also as a powertool to perform in-depth analysis of your requests, responses, backend fetches, backend health and caching behavior.

Troubleshooting Varnish

Tue, 04 Jan 2022 00:00:00 +0000

Varnish is incredibly stable, but that doesn’t mean troubleshooting doesn’t have to take place. On rare occassions crashes may occur. Based on the available server resources and traffic patterns, misconfigured runtime parameters may cause unwanted behavior.

Crashes rarely occur, but when they do, you need to figure out what’s going on.

Even when Varnish is stable, the hit rate of the cache might be too low for your liking.

There are many ways to perform troubleshooting: from panic inspection, to Varnish Shared Memory Logs (VSL), to monitoring metrics.

Varnish monitoring

Mon, 01 Jan 0001 00:00:00 +0000

There are multiple tools and metrics available to monitor a Varnish installation. This tutorial provides information on important counters that will assist in monitoring vital aspects of a Varnish installation.

There are various commands that can be used to access additional information. Each option will be explained the first time it’s encountered, but you can find a more complete description in the man page of each tool.

varnishlog

varnishlog is in charge of presenting transaction logs in the most verbose way possible. This is a prime source of information for debugging.