Varnish Glossary

backend

The HTTP server varnishd is caching for. This can be any sort of device that handles HTTP requests, including, but not limited to: a webserver, a CMS, a load-balancer another varnishd, etc.

backend response

The response specifically served from a backend to varnishd. The backend response may be manipulated in vcl_fetch.

body

The bytes that make up the contents of the object, varnishd does not care if they are in HTML, XML, JPEG or even EBCDIC, to varnishd they are just bytes.

client

The program which sends varnishd a HTTP request, typically a browser, but do not forget to think about spiders, robots script-kiddies and criminals.

header

A HTTP protocol header, like “Accept-Encoding:”.

hit

An object Varnish delivers from cache.

master (process)

One of the two processes in the varnishd program. The master proces is a manager/nanny process which handles configuration, parameters, compilation of :term:VCL etc. but it does never get near the actual HTTP traffic.

miss

An object Varnish fetches from the backend before it is served to the client. The object may or may not be put in the cache, that depends.

object

The (possibly) cached version of a backend response. Varnishd receives a reponse from the backend and creates an object, from which it may deliver cached responses to clients. If the object is created as a result of a request which is passed, it will not be stored for caching.

pass

An object Varnish does not try to cache, but simply fetches from the backend and hands to the client.

pipe

Varnish just moves the bytes between client and backend, it does not try to understand what they mean.

request

What the client sends to varnishd and varnishd sends to the backend.

response

What the backend returns to varnishd and varnishd returns to the client. When the response is stored in varnishd’s cache, we call it an object.

varnishd (NB: with ‘d’)

This is the actual Varnish cache program. There is only one program, but when you run it, you will get two processes: The “master” and the “worker” (or “child”).

varnishhist

Eye-candy program showing responsetime histogram in 1980ies ASCII-art style.

varnishlog

Program which presents varnish transaction log in native format.

varnishncsa

Program which presents varnish transaction log in “NCSA” format.

varnishstat

Program which presents varnish statistics counters.

varnishtest

Program to test varnishd’s behaviour with, simulates backend and client according to test-scripts.

varnishtop

Program which gives real-time “top-X” list view of transaction log.

VCL

Varnish Configuration Language, a small specialized language for instructing Varnish how to behave.

worker (process)

The worker process is started and configured by the master process. This is the process that does all the work you actually want varnish to do. If the worker dies, the master will try start it again, to keep your website alive.