
Logging in Varnish
------------------

One of the really nice features in Varnish is how logging
works. Instead of logging to normal log file Varnish logs to a shared
memory segment. When the end of the segment is reached we start over,
overwriting old data. This is much, much faster then logging to a file
and it doesn't require disk space.

The flip side is that is you forget to have program actually write the
logs to disk they will disappear.

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 as well, we'll show you
these later.

In the terminal window you started varnish now type *varnishlog* and
press enter.

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

    0 CLI          - Rd ping
    0 CLI          - Wr 200 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 SessionOpen  c 127.0.0.1 58912 0.0.0.0:8080
   11 ReqStart     c 127.0.0.1 58912 595005213
   11 RxRequest    c GET
   11 RxURL        c /
   11 RxProtocol   c HTTP/1.1
   11 RxHeader     c Host: localhost:8080
   11 RxHeader     c Connection: keep-alive

The first column is an arbitrary number, it defines the request. Lines
with the same number are part of the same HTTP 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 beeing logged. Tags
starting with Rx indicate Varnish is recieving data and Tx indicates
sending data.

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

Now, you can filter quite a bit with varnishlog. The basics option 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.

-i tag
 Only show lines with a certain tag. "varnishlog -i SessionOpen" 
 will only give you new sessions. Note that the tags are case 
 insensitive. 

-I Regex
 Filter the data through a regex and only show the matching lines. To
 show all cookie headers coming from the clients:
 ``$ varnishlog -c -i RxHeader -I Cookie``

-o 
 Group log entries by request ID.


Now that Varnish seem to work OK its time to put Varnish on port 80
while we tune it.
