Varnish Custom Statistics provides granular and detailed performance metrics on your cache contents and its associated traffic. It produces real-time aggregated statistics from a group of Varnish servers. Then these statistics are grouped according to keys defined in VCL and presented in time series.
VCS is split into two components:
vcs server: This is the main database, UI, and API server for VCS and it should be installed on a separate server.
vcs-agent: They connect and push data to the
vcs server and are installed on each of your Varnish servers.
Please refer to our VCS 4.0 documentation for more information.