Dynamically setting the Yaws log level

Debugging a web-based application is more often than not done by first analyzing log files. Even when you can attach a REPL to a running application. They are an integral part of the developer's tool set.

But production applications usually do not enable tracing by default, because this could degrade performance and waste essential resources (memory, CPU time, disk, etc.). Would you dump the content of every HTTP request received by a web server on disk? Of course, when problems occur, trace level is simply raised, with the hope that the logs will contain some useful debugging info.

With the Yaws Erlang-based web server, the log level can be set programmatically like this:

-module(my_yaws_utils).
-export([set_yaws_trace_level/1]).

set_yaws_trace_level(Level) ->
  {ok, GC, Groups} = yaws_api:getconf(),
  yaws_config:hard_setconf(GC#gconf{trace=Level}, Groups).

with Level being either none, {true, http}, or {true, traffic}. The first value disables tracing, the second enables tracing of the HTTP requests in file trace.http, while the last enables tracing of the whole traffic (requests and responses).

It is certainly possible to attach to the Yaws server from another Erlang node to call this function. But if your application is administered by someone not really used to Erlang, it may be more appropriate to provide a command-line script for that purpose.

Basically, the script would connect to the Yaws node (named yaws_daemon@lelouch) and invoke the function with the appropriate log level:

#!/bin/sh
node=yaws_daemon@lelouch
# we should check the number of arguments to the script here...
case $1 in
  none) level=none;;
  http) level="{true, http}";;
  traffic) level="{true, traffic}"
esac

erl -sname trace_script -noinput \
    -setcookie ErlangCookie \
    -eval "rpc:call($node, my_yaws_utils, set_yaws_trace_level, [$level])." \
    -s init stop

The call to rpc:call does the actual RPC call to set the log level on the Yaws server.

The cool thing is the fact that this technique can be applied to a whole bunch of other utility tools, like scripts to stop the server, to reload the application configuration, etc. And this applies to all Erlang applications as well, this is in no way specific to Yaws.

And there are probably other ways to do the same thing. Let me know if you have a better solution.