Changes in Varnish 5.2

Varnish 5.2 is mostly changes under the hood so most varnish installations will be able to upgrade with no modifications.

New VMODs in the standard distribution

We have added three new VMODs to the varnish project.

VMOD blob

We have added the variables req.hash and bereq.hash to VCL, which contain the hash value computed by Varnish for the current request, for use in cache lookup. Their data type is BLOB, which represents opaque data of any length – the new variables contain the raw binary hashes.

This is the first time that an element of standard VCL has the BLOB type (BLOBs have only been used in third-party VMODs until now). So we have added VMOD blob to facilitate their use. In particular, the VMOD implements binary-to-text encodings, for example so that you can assign the hash to a header as a base64 or hex string. It also provides some other utilities such as getting the length of a BLOB or testing BLOBs for equality.

See VMOD blob - Utilities for the VCL blob type, encoding and decoding.

VMOD purge

Before the introduction of vcl 4.0 there used to be a purge function instead of a return(purge) transition. This module works like old-style VCL purges (which should be used from both vcl_hit and vcl_miss) and provides more capabilities than regular purges, and lets you know how many objects were affected.

See VMOD purge - Varnish Purge Module.

VMOD vtc

As long as we have had VMODs, we had an internal vmod called vmod_debug which was used with varnishtest to exercise the VMOD related parts of varnishd. Over time this vmod grew other useful functions for writing test-cases.

We only distribute vmod_debug in source releases, because it has some pretty evil functionality, for instance debug.panic().

We have taken the non-suicidal test-writing goodies out of vmod_debug and put them into a new vmod_vtc, to make them available to people using varnishtest to test local configurations, VMODs etc.

The hottest trick in vmod_vtc is that VTC-barriers can be accessed from the VCL code, but there are other conveniences like workspace manipulations etc.

See VMOD vtc - Utility module for varnishtest.

News for authors of VMODs and Varnish API client applications

$ABI [strict|vrt]

VMOD authors have the option of only integrating with the blessed interface provided by varnishd or go deeper in the stack. As a general rule of thumb you are considered “on your own” if your VMOD uses more than the VRT (Varnish RunTime) and it is supposed to be built for the exact Varnish version.

Varnish was already capable of checking the major/minor VRT version a VMOD was built against, or require the exact version, but picking one or the other depended on how Varnish was built.

VMOD authors can now specify whether a module complies to the VRT and only needs to be rebuilt when breaking changes are introduced by adding $ABI vrt to their VCC descriptor. The default value is $ABI strict when omitted.

VSM/VSC API changes

The export of statistics counters via shared memory has been overhauled to get rid of limitations which made sense 11 years ago but not so much now.

A set of statistics counters are now fully defined in a .vsc file which is processed by the script into a .c and .h file, which is compiled into the relevant body of code.

This means that statistics counters are now self-describing in shared memory, and varnishstat or other VSC-API using programs no longer have a compiled in list of which counters exist or how to handle them.

This paves the way for VMODs or maybe even VCL to define custom counters, and have them show up in varnishstat and other VSC-API based programs just like the rest of the counters.

The rewrite of the VSM/VSC code simplified both APIs and made them much more robust but code calling into these APIs will have to be updated to match.

The necessary changes mostly center around detecting if the varnishd management/worker process has restarted.

In the new VSM-API once setup is done, VSM_Attach() latches on to a running varnishd master process and stays there.

VSM_Status() updates the in-memory list of VSM segments, and returns status information about the master and worker processes: Are they running? Have they been restarted? Have VSM segments been added/deleted?

Each VSM segment is now a separate piece of shared memory and the name of the segment can be much longer.

Before the actual shared memory can be accessed, the application must call VSM_Map() and when VSM_StillValid() indicates that the segment is no longer valid, VSM_Unmap() should be called to release the segment again.

All in all, this should be simpler and more robust.

VRT API changes

VRT_purge now fails a transaction instead of panicking when used outside of vcl_hit or vcl_miss. It also returns the number of purged objects.


One way to extend Varnish is to write VSM clients, programs that tap into the Varnish Shared Memory (VSM) usually via libvarnishapi or community bindings for other languages than C. Varnish already ships with VUTs (Varnish UTilities) that either process the Varnish Shared Log (VSL) like varnishlog or varnishncsa or the Varnish Shared Counters (VSC) like varnishstat.

Most of the setup for these programs is similar, and so they shared an API that is now available outside of the Varnish source tree. The VUT API has been cleaned up to remove assumptions made for our utilities. It hides most of the complexity and redundancy of setting up a log processor and helps you focus on your functionality. If you use autotools for building, a new macro in varnish.m4 removes some of the boilerplate to generate part of the documentation.

We hope that we will see new tools that take advantage of this API to extend Varnish in new ways, much like VMODs made it easy to add new functionality to VCL.