You can reload the Varnish VCL configuration without actually restarting Varnish. A restart would stop the varnishd process and start it anew, clearing all the cache it has built up in the meantime. But you can also reload the varnish configurations, to load your new VCL without losing the cache.
Beware though, there are times when you want to clear your cache on VCL changes: for instance, healthchecks on backend definitions can get pretty funky when a reload of the VCL would modify their IPs, and there are situations where when you change the vcl_hash routine, a restart is advised since the data in memory would never be used again (because of a hash-change). Having said that, there are plenty of reasons to reload a Varnish VCL cache without losing the data in memory.
Via init.d scripts
Not all init.d scripts have a reload option, and it can be disabled with the sysconfig settings if the RELOAD_VCL option is turned off, but if it’s enabled, this by far the easiest way.
$ /etc/init.d/varnish reload
This will reload the VCL, compile it and make Varnish use the new version.
Via the Varnish Reload VCL script
Varnish ships with a command called varnish_reload_vcl (if you use the official RPM/Deb repos). You can use this via the CLI to make Varnish load the default.vcl file again into memory (assuming that is the VARNISH_VCL_CONF defined in the /etc/sysconfig/varnish file).
$ varnish_reload_vcl Loading vcl from /etc/varnish/default.vcl Current running config name is boot Using new config name reload_2014-12-14T20:19:16 VCL compiled. available 11 boot active 0 reload_2014-12-14T20:19:16 Done
The new Varnish VCL is loaded, without losing in-memory data.
Via a custom script
The varnish_reload_vcl is essentially a Bash-script, you can view the contents by looking into /usr/bin/varnish_reload_vcl. If you want to write something similar into your own scripts, here’s a stripped/easier version of that script. It uses varnishadm to load the file and eventually switch the config to use it.
#!/bin/bash # Generate a unique timestamp ID for this version of the VCL TIME=$(date +%s) # Load the file into memory varnishadm -S /etc/varnish/secret -T 127.0.0.1:6082 vcl.load varnish_$TIME /etc/varnish/default.vcl # Active this Varnish config varnishadm -S /etc/varnish/secret -T 127.0.0.1:6082 vcl.use varnish_$TIME
At any time, you can view the in-memory available Varnish VCL files using the vcl.list command. You can load/active one with the vcl.use command and you can compile a new one with vcl.load.
To view the available ones, run the following command.
$ varnishadm -S /etc/varnish/secret -T 127.0.0.1:6082 vcl.list available 8 boot available 0 varnish_1418585083
Eeach of those names can be activated with vcl.use.
$ varnishadm -S /etc/varnish/secret -T 127.0.0.1:6082 vcl.use varnish_1418585083