| Log::Trace::Manual - A guide to using Log::Trace |
Log::Trace::Manual - A guide to using Log::Trace
This is a brief guide to how you can use the Log::Trace module in your scripts
and modules. The Log::Trace documentation has a comprehensive list of
options.
You can enable tracing by specifying the tracing target via the 'use'
statement or at runtime via the import() method. In most cases, you'll want
to keep the code that enables tracing in a single point, usually the main
script of your application. In general, modules should avoid directly
setting tracing options.
Here's a slightly contrived example which demonstrates the TRACE, TRACEF,
DUMP and TRACE_HERE functions:
#!/usr/bin/perl -w
use strict;
use Another::Module;
use Log::Trace log => '/var/log/myapp.log';
TRACE("-------- Starting archiver ---------");
TRACEF("We are going to try to archive %d items", scalar @ARGV);
DUMP("List of things to archive", \@ARGV);
archive_em($_) foreach(@ARGV);
sub archive_em {
TRACE_HERE();
my $thing = shift;
unless (Another::Module::check_safe($thing)) {
warn "bad chars in: $thing";
return;
}
rename $thing, $thing.".archive" or warn "Couldn't archive $thing: $!";
TRACE("Tried to archive $thing");
}
Note the way Log::Trace is imported. The import list controls where the
output of the four tracing functions goes. Instead we could have done:
use Log::Trace qw(warn);
and the trace output would have gone to STDERR.
In the previous example, tracing was enabled only in the main script. Now
we'll see how to enable tracing in Another::Module at the same time.
First, Another::Module needs to define a TRACE subroutine. It may also
define TRACEF, TRACE_HERE and DUMP stubs. It can do that simply by
using Log::Trace. However, if Another::Module defines its own stub
tracing functions, we can remove the dependency on Log::Trace.
package Another::Module;
sub check_safe {my_routine {
my $filename = shift;
TRACE("Checking that '$filename' has safe characters");
return $filename =~ /^([\w.\-/]+)$/
}
sub my_other_routine {
TRACE_HERE();
}
# tracing stubs
sub TRACE {}
sub TRACE_HERE {}
Now, in the main script, we can change the 'use' statement so tracing will
be enabled in Another::Module:
use Log::Trace log => '/var/log/myapp.log', {Deep => 1};
By default, the Deep option will force Log::Trace to export tracing
functions to any modules that define a TRACE subroutine. That includes
modules that are not directly used by the main script. But this behaviour can
be relaxed or tightened with other options. See Deep import for examples.
Adding TRACE and other stub functions to your module is an Interface Contract
between your module and Log::Trace (in some software circles this might be given a name such as
ISupportsTracing). Of course you can write other code that takes advantage of this
interface completely independent of Log::Trace, e.g.
use Another::Module;
if($ENV{DEBUG}) {
*Another::Module::TRACE = sub {print "TRACE: ".join("\t",@_)."\n"};
}
Since Log::Trace is designed with debugging in mind, all tracing failures
are non-fatal, so allowing normal execution to continue. However, Log::Trace
will report to STDERR that a problem has occurred.
For example, this code:
use Log::Trace file => '/myapp.log';
TRACE('Running');
print "Hello World!\n";
Will produce this output:
Log::Trace: Cannot open /myapp.log : Permission denied at lib/Log/Trace.pm line <nnn>.
Hello World!
You can invoke tracing on the command line:
perl -MLog::Trace=print -e "TRACE('hello')"
perl -MLog::Trace=warn -e "TRACE('hello')"
perl -MLog::Trace=log,test.log -e "TRACE('hello')"
However you can't apply this approach to scripts that use Log::Trace or
define a TRACE stub as these will clobber *main::TRACE set up by -M when
they are compiled. Fortunately it is straightforward to write your
command-line scripts so you can, for example, get trace output with -t and deep
trace output with -T:
use Log::Trace;
use Getopt::Std;
use MyModule;
use vars qw($opt_t $opt_T);
getopts("tT");
# tracing
import Log::Trace 'print' if $opt_t;
import Log::Trace 'print' => {Deep => 1} if $opt_T;
do_something_involving_tracing();
Whilst tracing to a log file or STDERR is tolerable for CGIs, it's often far
more convenient to return the tracing information back to the browser of the
client-side developer.
use CGI;
use constant DEV_SERVER => 1;
my $trace_buffer;
if(DEV_SERVER && CGI::param('Tracing')) {
require Log::Trace;
import Log::Trace buffer => \$trace_buffer, {Deep => 1};
}
my $output = do_everything();
print CGI::header();
print $output;
if (DEV_SERVER && $trace_buffer)
{
print "\n\n", "<pre>", CGI::escapeHTML($trace_buffer), "</pre>";
}
You should remember to change the DEV_SERVER constant when releasing the CGI
to a production environment.
Log::Trace can filter the tracing output by referring to the logging level.
The logging level is defined when you enable tracing. Log::Trace doesn't
impose any conventions on the levels. The default levels implementation
requires that the levels be numeric, but that can be overriden.
In the simplest case, you can specify the level as a threshold value:
use Log::Trace print => {Level => 3};
In this example, all trace messages at level 3 or below will be output.
You can also specify a list of valid levels:
use Log::Trace print => {Level => [0 .. 3, 7]};
All the tracing functions accept a hash as an optional first parameter where you can specify the level for that trace message. E.g.:
TRACE({Level => 4}, "This is a warning");
TRACEF({Level => 6}, "%d items found", scalar @items);
TRACE_HERE({Level => 10});
DUMP({Level => 8}, 'Retrieved data', \%data);
DUMP is designed to accept a hash as its first parameter, but there may be
cases where you wish to dump a hash that contains a Level key. In those
cases, you can take advantage of the return value of DUMP():
my $dumped = DUMP({Level => 1, Health => '0.68'});
TRACE({Level => 8}, 'Game stats', $dumped);
If you specify a tracing level when you enable Log::Trace, then tracing
messages that do not specify a level will not be output, unless you include
undef in the trace levels:
use Log::Trace print => {Level => [3, undef]};
TRACE("This is level undef, and will be output");
TRACE({Level => 3}, "This will also be output");
TRACE({Level => 8}, "... but this won't");
Here are some sample tracing levels (borrowed from Log::Agent) which you can use as a guide:
0 emergency
1 alert
2 critical
3 error
4 warning
6 notice
8 info
10 debug
Occasionally you won't want to see the trace output from ALL your modules in
your application. For example your application may give a module a huge data
structure or call it in a long loop. The Exclude option allows you to mask
out one or more modules.
use Log::Trace warn => {'Deep' => 1, 'Exclude' => 'MyVerboseModule'};
or
use Log::Trace warn => {'Deep' => 1, 'Exclude' => ['MyVerboseModule', 'Another::Module']};
Conversely you can use an opt-in approach rather than opt-out. The Match
option allows a regular expression to be used to select which packages are
initialised by Log::Trace. For example:
use Log::Trace print => {'Deep' => 1, 'Match' => qr/^MySubSystem::/};
When the Deep or Everywhere options are used, Log::Trace is imported into all the packages which have been compiled so far.
use Package::Foo;
use Log::Trace ('print' => {Deep => 1});
use Package::Bar; #Compiled after Log::Trace is imported
In this example, the TRACE function in Package::Bar won't be overridden. It's trivial to swap the order in the example above so that Log::Trace is the last module used, but suppose you have a module (such as a factory) that loads others on demand:
package MyApp::Reader;
sub new {
my $package = shift;
my $type = shift;
die unless($type =~ /^MyApp::Reader::\w+$/);
eval "require $type";
die($@) if($@);
return $type->new(@_);
}
How do you ensure Log::Trace gets imported into the backend MyApp::Reader::* modules (without polluting
all your modules with Log::Trace::import calls)?
CORE::require so that from now on any modules
that are loaded will have the Log::Trace import run against them:
use Log::Trace('log' => '/var/log/myapp.log', {'Deep' => 1, 'AutoImport' => 1});
This only works with recent versions of perl (see the ENVIRONMENT NOTES in the Log::Trace manpage).
package MyApp::Reader;
sub new {
my $package = shift;
my $type = shift;
die unless($type =~ /^MyApp::Reader::\w+$/);
eval "require $type";
die($@) if($@);
# Wire the component we've created into whatever
# our TRACE etc function has been wired to
*{"$type\::TRACE"} = \&MyApp::Reader::TRACE;
*{"$type\::DUMP"} = \&MyApp::Reader::DUMP;
return $type->new(@_);
}
If STDOUT, STDERR, syslog, a file, a file handle, or a buffer is not
to your liking then the custom method is for you.
Suppose you want to send your Log::Trace output into a database:
our $sth;
$sth = setup_logging_statement();
use Log::Trace custom => \&log_to_database;
sub log_to_database {
#TRACE can get any number of arguments
my $message = join(",", @_);
$sth->execute($message);
}
By default, Data::Dumper is used with a fixed set of options for DUMP output.
You can choose a different serialiser using the Dumper option:
import Log::Trace('print' => {Dumper => "YAML"}});
Where the string refers to a Data::Serializer::* backend. You can also control the options passed to the Data::Serializer backend (and thus customise the DUMP output) by passing a hashref of Data::Serializer contructor options:
import Log::Trace('print' => {Dumper => {
serializer => 'XML::Dumper',
options => {
dtd => 'path/to/my.dtd'
}
}});
At the time of writing, not all the configuration options of the underlying serialisation modules are exposed via their Data::Serializer wrappers. If you find this a limitation, please contribute patches to extend these modules as this will benefit a number of other modules that make use of the Data::Serializer API.
You can use the AllSubs tracing option to trace the execution path through each
subroutine. By default Log::Trace only wraps each subroutine in packages
with TRACE defined. You can force it to do it to all modules using the
Everywhere option. The following:
use Data::Dumper;
use Log::Trace print => {AllSubs => 1, Verbose => 1, Everywhere => 1, Exclude => 'Config'};
Data::Dumper->Dumpperl([[4]]);
generates the output:
main::__ANON__ (3) :: Data::Dumper::Dumpperl( )
Data::Dumper::Dumpperl (3) :: Data::Dumper::new( )
Data::Dumper::Dumpperl (3) :: Data::Dumper::_dump( Data::Dumper, ... )
Data::Dumper::_dump (205) :: overload::StrVal( ARRAY, ... )
overload::StrVal (239) :: overload::OverloadedStringify( ARRAY, ... )
overload::OverloadedStringify (92) :: overload::mycan( )
overload::OverloadedStringify (92) :: overload::ov_method( )
overload::OverloadedStringify (92) :: overload::mycan( )
overload::OverloadedStringify (92) :: overload::ov_method( )
overload::OverloadedStringify (92) :: overload::mycan( )
overload::OverloadedStringify (92) :: overload::ov_method( )
overload::OverloadedStringify (92) :: overload::mycan( )
overload::OverloadedStringify (92) :: overload::ov_method( )
Data::Dumper::_dump (205) :: Data::Dumper::_dump( Data::Dumper, ... )
(eval) (0) :: Data::Dumper::DESTROY( Data::Dumper, ... )
You may wonder ``How do I trace what's going on in module Acme::Foo I downloaded from CPAN that isn't Log::Trace enabled?''.
Assuming the module doesn't have any other kind of tracing that you can hook into, all you can do is use the AllSubs approach.
Assuming that's OK, you can restrict this to just the offending module with:
use Log::Trace print => {AllSubs => 1, Everywhere => 1, Match => qr/^Acme:Foo$/};
Although the trace stubs don't do anything, they do incur a small function call
overhead. If this performance hit is unacceptable, you can use a constant to
enable/disable all the Log::Trace statements in your code. The test for the
constant value will be optimised out at compile time so no runtime overhead is
incurred if the constant has a false value:
package ThrashMe;
use constant TRACING_ENABLED => 1; #Set to zero to optimise
sub performance_critical {
TRACE("this may slow things down") if(TRACING_ENABLED);
do_stuff();
}
sub TRACE{}
1;
$Revision: 1.9 $
| Log::Trace::Manual - A guide to using Log::Trace |