Changes in Varnish 5.0

Varnish 5.0 changes some (mostly) internal APIs and adds some major new features over Varnish 4.1.

Separate VCL files and VCL labels

Varnish 5.0 supports jumping from the active VCL’s vcl_recv{} to another VCL via a VCL label.

The major use of this will probably be to have a separate VCL for each domain/vhost, in order to untangle complex VCL files, but it is not limited to this criteria, it would also be possible to send all POSTs, all JPEG images or all traffic from a certain IP range to a separate VCL file.

VCL labels can also be used to give symbolic names to loaded VCL configurations, so that operations personnel only need to know about “normal”, “weekend” and “emergency”, and web developers can update these as usual, without having to tell ops what the new weekend VCL is called.

Very Experimental HTTP/2 support

We are in the process of adding HTTP/2 support to Varnish, but the code is very green still - life happened.

But you can actually get a bit of traffic though it already, and we hope to have it production ready for the next major release (2017-03-15).

Varnish supports HTTP/1 -> 2 upgrade. For political reasons, no browsers support that, but tools like curl does.

For encrypted HTTP/2 traffic, put a SSL proxy in front of Varnish.

HTTP/2 support is disabled by default, to enable, set the http2 feature bit.

The Shard Director

We have added to the directors VMOD an overhauled version of a director which was available as an out-of-tree VMOD under the name VSLP for a couple of years: It’s basically a better hash director, which uses consistent hashing to provide improved stability of backend node selection when the configuration and/or health state of backends changes. There are several options to provide the shard key. The rampup feature allows to take just-gone-healthy backends in production smoothly, while the prewarm feature allows to prepare backends for traffic which they would see if the primary backend for a certain key went down.

It can be reconfigured dynamically (outside vcl_init{}), but different to our other directors, configuration is transactional: Any series of backend changes must be concluded by a reconfigure call for activation.

Hit-For-Pass is now actually Hit-For-Miss

Almost since the beginning of time (2008), varnish has hit-for-pass: It is basically a negative caching feature, putting into the cache objects as markers saying “when you hit this, your request should be a pass”. The purpose is to selectively avoid the request coalescing (waitinglist) feature, which is useful for cacheable content, but not for uncacheable objects. If we did not have hit-for-pass, without additional configuration in vcl_recv, requests to uncacheable content would be sent to the backend serialized (one after the other).

As useful as this feature is, it has caused a lot of headaches to varnish administrators along the lines of “why the beep doesn’t Varnish cache this”: A hit-for-pass object stayed in cache for however long its ttl dictated and prevented caching whenever it got hit (“for that url” in most cases). In particular, as a pass object can not be turned into something cacheable retrospectively (beresp.uncacheable can be changed from false to true, but not the other way around), even responses which would have been cacheable were not cached. So, when a hit-for-pass object got into cache unintentionally, it had to be removed explicitly (using a ban or purge).

We’ve changed this now:

A hit-for-pass object (we still call it like this in the docs, logging and statistics) will now cause a cache-miss for all subsequent requests, so if any backend response qualifies for caching, it will get cached and subsequent requests will be hits.

In short: We’ve changed from “the uncacheable case wins” to “the cacheable case wins” or from hit-for-pass to hit-for-miss.

The primary consequence which we are aware of at the time of this release is caused be the fact that, to create cacheable objects, we need to make backend requests unconditional (that is, remove the If-Modified-Since and If-None-Match headers): For conditional client requests on hit-for-pass objects, Varnish will now issue an unconditional backend fetch and, for 200 responses, send a 304 or 200 response to the client as appropriate.

As of the time of this release we cannot say if this will remain the final word on this topic, but we hope that it will mean an improvement for most users of Varnish.

Ban Lurker Improvements

We have made the ban lurker even more efficient by example of some real live situations with tens of thousands of bans using inefficient regular expressions.

The new parameter ban_lurker_holdoff tells the ban lurker for how long it should get out of the way when it could potentially slow down lookups due to lock contention. Previously this was the same as ban_lurker_sleep.

Request Body sent always / “cacheable POST”

Previously, we would only send a request body for passed requests (and for pipe mode, but this is special anyway and should be avoided).

Not so any more, but the default behaviour has not changed:

Whenever a request has a body, it will get sent to the backend for a cache miss (and pass, as before). This can be prevented by an unset bereq.body and the builtin.vcl removes the body for GET requests because it is questionable if GET with a body is valid anyway (but some applications use it).

So the often-requested ability to cache POST/PATCH/… is now available, but not out-of-the-box:

  • The builtin.vcl still contains a return(pass) for anything but a GET or HEAD because other HTTP methods, by definition, may cause state changes / side effects on backends. The application at hand should be understood well before caching of non-GET/non-HEAD is considered.

  • For misses, core code still calls the equivalent of set bereq.method = "GET" before calling vcl_backend_fetch, so to make a backend request with the original request method, it needs to be saved in vcl_recv and restored in vcl_backend_fetch.

  • Care should be taken to choose an appropriate cache key and/or Vary criteria. Adding the request body to the cache key is not possible with core varnish, but through a VMOD

To summarize: You should know what you are doing when caching anything but a GET or HEAD and without creating an appropriate cache key doing so is almost guaranteed to be wrong.

ESI and Backend Request Coalescing (“waitinglist”) Improvement

Previously, ESI subrequests depending on objects being fetched from the backed used polling, which typically added some ~5ms of processing time to such subrequests and could lead to starvation effects in extreme corner cases.

The waitinglist logic for ESI subrequests now uses condition variables to trigger immediate continuation of ESI processing when an object being waited for becomes available.

Backend PROXY protocol requests

Are now supported through the .proxy_header attribute of the backend definition.

Default VCL search path

For default builds, vcl files are now also being looked for under /usr/share/varnish/vcl if not found in /etc/varnish.

For custom builds, the actual search path is ${varnishconfdir}:${datarootdir}/varnish/vcl


The basic device detection vcl is now bundled with varnish.


  • resp.msg renamed to resp.reason for consistency with vcl

  • HTTP2 testing capabilities added

  • default search path for executables and vmods added

  • sema mechanism replaced by barrier

  • support for PROXY requests


Brief notes on other changes

  • Added separate thread for object expiry

  • The ESI parser is now more tolerant to some syntactic corner cases

  • Reduced needless rushing of requests on the waitinglist

  • varnishhist can now process backend requests and offers a timebend function to control the processing speed

  • std.integer() can now also parse real numbers and truncates them

  • std.log() now also works correctly during vcl_init{}

  • further improved stability when handling workspace overflows

  • numerous vcl compiler improvements

News for VMOD authors

  • It is now mandatory to have a description in the $Module line of a vcc file.

  • vcl cli events (in particular, vcl_init{} /vcl_fini{}) now have a workspace and PRIV_TASK available for VMODs.

  • PRIV_* now also work for object methods with unchanged scope. In particular, they are per VMOD and not per object - e.g. the same PRIV_TASK gets passed to object methods as to functions during a VCL task.

  • varnish now provides a random number api, see vrnd.h

  • vbm (variable size bitmaps) improved

  • for translating vcc files has been largely rewritten, there may still exist regressions which remained unnoticed

  • now requires at least Python 2.6

  • New autoconf macros are available, they should greatly simplify build systems of out-of-tree VMODs. They are implemented and documented in varnish.m4, and the previous macros now live in varnish-legacy.m4 so existing VMODs should still build fine.