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WARNING: Not tested in production

This is a collection of configuration files, documentation and
proof-of-concept scripts for selective web request debugging, aimed
specifically at web developers, devops and operations. The idea is
simply to write as much debugging information as possible to syslog as
long as the client has specified two HTTP variables:

  X-Debug-Level: n
  X-Debug-Key: mydebugkey

The debug level corresponds to the standard syslog levels (0 to 7),
and the key is used as a simple authorization mechanism, as extensive
debugging is likely to have an impact on performance.

Any node in the request path would then typically log details to
a central syslog server, and the developer/admin would then be able to
tail the request in realtime at a single point, as it passes through
any node, proxy or application on the way. This of course depends on
having a properly configured central syslog server available.

A web server like Apache would typically map the request debug headers
to environment variables, for further use in applications. See the
included Apache configuration.

TODO:
* F5 BIG-IP v10 and v11 configuration/documentation.
* A10 Networks AX configuration/documentation.
* Other web servers, such as lighttpd, cherokee.
* Appropriate use of log levels.
* More thought-through use of syslog facilities and levels.
* Tips on cgi/application logging, including SQL.
* Recommendations on syslog server configuration / interface.

Here's an example of what is possible with a combination of Varnish,
Apache and a simple cgi script that runs some trivial sql. The local
syslogd (rsyslog in this case) is set up with microsecond timestamps
to provide further insight into timing. The number in parathesis is
the Varnish request ID, but this ID could be derived from anything,
for example a rng at the first node in the path.

Comments are added for clarity.

# Client fires off a request with debug headers set.
client:~$ wget -q --header "X-Debug-Level: 7" \
--header "X-Debug-Key: mydebugkey" \
http://t.example.net:6081/~sveniu/debug/cgitest.py

# Varnish receives the request. It misses the cache and is passed to
# the backend Apache.
2012-06-04T21:56:45.629274+02:00 varnishd[2542]: vcl_recv(126139968) Debugging request 127.0.0.1 -> 127.0.1.1:6081, debuglevel=7
2012-06-04T21:56:45.629295+02:00 varnishd[2542]: vcl_recv(126139968) Req: GET http://t.example.net:6081/~sveniu/debug/test.cgi HTTP/1.1
2012-06-04T21:56:45.629300+02:00 varnishd[2542]: vcl_recv(126139968) User-Agent: Wget/1.13.4 (linux-gnu)
2012-06-04T21:56:45.629304+02:00 varnishd[2542]: vcl_recv(126139968) Backend: default (healthy=true)
2012-06-04T21:56:45.630139+02:00 varnishd[2542]: vcl_miss(126139968) Cache miss: /~sveniu/debug/test.cgi
2012-06-04T21:56:45.630151+02:00 varnishd[2542]: vcl_miss(126139968) Backend will see X-Forwarded-For: 127.0.0.1

# Apache's input filter processes the request first.
2012-06-04T21:56:45.657822+02:00 a2filter[2632]: req(126139968): Debugging request 127.0.0.1:42791 -> 127.0.0.1:6081, debuglevel=7, pppid=2605
2012-06-04T21:56:45.657892+02:00 a2filter[2632]: req(126139968): GET http://t.example.net/~sveniu/debug/test.cgi HTTP/1.1, script_filename=/home/sveniu/public_html/debug/test.cgi
2012-06-04T21:56:45.657946+02:00 a2filter[2632]: req(126139968): HTTP header: User-Agent = Wget/1.13.4 (linux-gnu)
2012-06-04T21:56:45.657981+02:00 a2filter[2632]: req(126139968): HTTP header: Host = t.example.net:6081
2012-06-04T21:56:45.658010+02:00 a2filter[2632]: req(126139968): HTTP header: X-Varnish = 126139968
2012-06-04T21:56:45.658039+02:00 a2filter[2632]: req(126139968): HTTP header: Accept = */*
2012-06-04T21:56:45.658069+02:00 a2filter[2632]: req(126139968): HTTP header: X-Forwarded-For = 127.0.0.1
2012-06-04T21:56:45.658102+02:00 a2filter[2632]: req(126139968): HTTP header: X-Debug-Key = mydebugkey
2012-06-04T21:56:45.658129+02:00 a2filter[2632]: req(126139968): HTTP header: X-Debug-Level = 7
2012-06-04T21:56:45.658156+02:00 a2filter[2632]: req(126139968): HTTP header: Accept-Encoding = gzip

# The cgi program then runs, executes some sql, logs 'EXPLAIN ANALYZE'
# results, resource usage, etc.
2012-06-04T21:56:45.681706+02:00 cgi[2631]: req(126139968): 127.0.0.1:42791 -> 127.0.0.1:6081, debuglevel=7, pppid=2605
2012-06-04T21:56:45.681731+02:00 cgi[2631]: req(126139968): GET http://t.example.net/~sveniu/debug/test.cgi HTTP/1.1, script_filename=/home/sveniu/public_html/debug/test.cgi
2012-06-04T21:56:45.681737+02:00 cgi[2631]: req(126139968): Connecting to db, dbname=cgi, dbuser=cgi
2012-06-04T21:56:45.685075+02:00 cgi[2631]: req(126139968): SQL query: select * from hits
2012-06-04T21:56:45.686670+02:00 cgi[2631]: req(126139968): SQL analyze: Seq Scan on hits  (cost=0.00..29.40 rows=1940 width=12) (actual time=0.008..0.009 rows=2 loops=1)
2012-06-04T21:56:45.686689+02:00 cgi[2631]: req(126139968): SQL analyze: Total runtime: 0.061 ms
2012-06-04T21:56:45.687194+02:00 cgi[2631]: req(126139968): Wall clock elapsed: 0.005704
2012-06-04T21:56:45.687306+02:00 cgi[2631]: req(126139968): resource.struct_rusage(ru_utime=0.044002, ru_stime=0.004, ru_maxrss=8028, ru_ixrss=0, ru_idrss=0, ru_isrss=0, ru_minflt=2347, ru_majflt=0, ru_nswap=0, ru_inblock=0, ru_oublock=0, ru_msgsnd=0, ru_msgrcv=0, ru_nsignals=0, ru_nvcsw=6, ru_nivcsw=8)

# Varnish receives the backend response.
2012-06-04T21:56:45.694026+02:00 varnishd[2542]: vcl_fetch(126139968) Backend response complete: default (127.0.0.1:80): 200 OK, ttl=120.000, grace=0.000

# Apache logs the request using a custom format. While this should in
# theory happen before Varnish receives the response, it could be due
# to syslog() call timing or Varnish logging the response before
# Apache has finished the delivery completely.
2012-06-04T21:56:45.694051+02:00 apache2log[2603]: response(126139968) http://t.example.net/~sveniu/debug/test.cgi 200 (200), remote=127.0.0.1:42791, local=127.0.0.1:80 usecs=63383; bytes=20; pid=2630; keepalives=0; xff=127.0.0.1

# Varnish delivers the object to the client.
2012-06-04T21:56:45.694057+02:00 varnishd[2542]: vcl_deliver(126139968) Delivering object.