# Logging in Varnish

One of the really nice features in Varnish is the way logging works. Instead of logging to a normal log file Varnish logs to a shared memory segment, called the VSL - the Varnish Shared Log. When the end of the segment is reached we start over, overwriting old data.

This is much, much faster than logging to a file and it doesn’t require disk space. Besides it gives you much, much more information when you need it.

The flip side is that if you forget to have a program actually write the logs to disk they will be overwritten.

varnishlog is one of the programs you can use to look at what Varnish is logging. varnishlog gives you the raw logs, everything that is written to the logs. There are other clients that can access the logs as well, we’ll show you these later.

In the terminal window you started Varnish now type `varnishlog -g raw` and press enter.

You’ll see lines like these scrolling slowly by.:

    0 CLI            - Rd ping
    0 CLI            - Wr 200 19 PONG 1273698726 1.0

These is the Varnish master process checking up on the caching process to see that everything is OK.

Now go to the browser and reload the page displaying your web app.

You’ll see lines like these.:

    11 SessOpen       c 127.0.0.1 58912 :8080 0.0.0.0 8080 1273698726.933590 14
    11 ReqStart       c 127.0.0.1 58912
    11 ReqMethod      c GET
    11 ReqURL         c /
    11 ReqProtocol    c HTTP/1.1
    11 ReqHeader      c Host: localhost:8080
    11 ReqHeader      c Connection: keep-alive

The first column is an arbitrary number, it identifies the transaction. Lines with the same number are coming from the same transaction. The second column is the tag of the log message. All log entries are tagged with a tag indicating what sort of activity is being logged.

The third column tell us whether this is is data coming from or going to the client (‘c’), or the backend (‘b’). The forth column is the data being logged.

Now, you can filter quite a bit with varnishlog. The basic options we think you want to know are:

‘-b’
  Only show log lines from traffic going between Varnish and the backend servers. This will be useful when we want to optimize cache hit rates.
‘-c’
  Same as ‘-b’ but for client side traffic.
‘-g request’
  Group transactions by request.
‘-q query’
  Only list transactions matching this query.

For more information on this topic please see varnishlog.