3 Catalyst::Manual::Tutorial::07_Debugging - Catalyst Tutorial - Chapter 7: Debugging
8 This is B<Chapter 7 of 10> for the Catalyst tutorial.
10 L<Tutorial Overview|Catalyst::Manual::Tutorial>
16 L<Introduction|Catalyst::Manual::Tutorial::01_Intro>
20 L<Catalyst Basics|Catalyst::Manual::Tutorial::02_CatalystBasics>
24 L<More Catalyst Basics|Catalyst::Manual::Tutorial::03_MoreCatalystBasics>
28 L<Basic CRUD|Catalyst::Manual::Tutorial::04_BasicCRUD>
32 L<Authentication|Catalyst::Manual::Tutorial::05_Authentication>
36 L<Authorization|Catalyst::Manual::Tutorial::06_Authorization>
44 L<Testing|Catalyst::Manual::Tutorial::08_Testing>
48 L<Advanced CRUD|Catalyst::Manual::Tutorial::09_AdvancedCRUD>
52 L<Appendices|Catalyst::Manual::Tutorial::10_Appendices>
59 This chapter of the tutorial takes a brief look at the primary options
60 available for troubleshooting Catalyst applications.
62 Note that when it comes to debugging and troubleshooting, there are two
69 Fans of C<log> and C<print> statements embedded in the code.
73 Fans of interactive debuggers.
77 Catalyst is able to easily accommodate both styles of debugging.
82 Folks in the former group can use Catalyst's C<$c-E<gt>log> facility.
83 (See L<Catalyst::Log> for more detail.) For example, if you add the
84 following code to a controller action method:
86 $c->log->info("Starting the foreach loop here");
88 $c->log->debug("Value of \$id is: ".$id);
90 Then the Catalyst development server will display your message along
91 with the other debug output. To accomplish the same thing in a TT
94 [% c.log.debug("This is a test log message") %]
96 As with many other logging facilities, a method is defined for each of
97 the following "logging levels" (in increasing order of
106 You can also use L<Data::Dumper> in both Catalyst code
107 (C<use Data::Dumper; $c-E<gt>log-E<gt>debug("\$var is: ".Dumper($var));)>)
108 and TT templates (C<[% Dumper.dump(book) %]>.
111 =head1 RUNNING CATALYST UNDER THE PERL DEBUGGER
113 Members of the interactive-debugger fan club will also be at home with
114 Catalyst applications. One approach to this style of Perl debugging is
115 to embed breakpoints in your code. For example, open
116 C<lib/MyApp/Controller/Books.pm> in your editor and add the
117 C<DB::single=1> line as follows inside the C<list> method (I like to
118 "left-justify" my debug statements so I don't forget to remove them, but
119 you can obviously indent them if you prefer):
122 # Retrieve the usual Perl OO '$self' for this object. $c is the Catalyst
123 # 'Context' that's used to 'glue together' the various components
124 # that make up the application
129 # Retrieve all of the book records as book model objects and store in the
130 # stash where they can be accessed by the TT template
131 $c->stash->{books} = [$c->model('DB::Book')->all];
133 # Set the TT template to use. You will almost always want to do this
134 # in your action methods.
135 $c->stash->{template} = 'books/list.tt2';
138 This causes the Perl Debugger to enter "single step mode" when this
139 command is encountered (it has no effect when Perl is run without the
142 B<NOTE:> The C<DB> here is the Perl Debugger, not the DB model.
144 If you haven't done it already, enable SQL logging as before:
146 $ export DBIC_TRACE=1
148 To now run the Catalyst development server under the Perl debugger,
149 simply prepend C<perl -d> to the front of C<script/myapp_server.pl>:
151 $ perl -d script/myapp_server.pl
153 This will start the interactive debugger and produce output similar to:
155 $ perl -d script/myapp_server.pl
157 Loading DB routines from perl5db.pl version 1.3
158 Editor support available.
160 Enter h or `h h' for help, or `man perldebug' for more help.
162 main::(script/myapp_server.pl:16): my $debug = 0;
166 Press the C<c> key and hit C<Enter> to continue executing the Catalyst
167 development server under the debugger. Although execution speed will be
168 slightly slower than normal, you should soon see the usual Catalyst
169 startup debug information.
171 Now point your browser to L<http://localhost:3000/books/list> and log
172 in. Once the breakpoint is encountered in the
173 C<MyApp::Controller::list> method, the console session running the
174 development server will drop to the Perl debugger prompt:
176 MyApp::Controller::Books::list(/home/me/MyApp/script/../lib/MyApp/Controller/Books.pm:48):
177 48: $c->stash->{books} = [$c->model('DB::Book')->all];
181 You now have the full Perl debugger at your disposal. First use the
182 C<next> feature by typing C<n> to execute the C<all> method on the Book
183 model (C<n> jumps over method/subroutine calls; you can also use C<s> to
184 C<single-step> into methods/subroutines):
187 SELECT me.id, me.title, me.rating, me.created, me.updated FROM book me:
188 MyApp::Controller::Books::list(/home/me/MyApp/script/../lib/MyApp/Controller/Books.pm:53):
189 53: $c->stash->{template} = 'books/list.tt2';
193 This takes you to the next line of code where the template name is set.
194 Notice that because we enabled C<DBIC_TRACE=1> earlier, SQL debug output
195 also shows up in the development server debug information.
197 Next, list the methods available on our C<Book> model:
199 DB<1> m $c->model('DB::Book')
203 __result_class_accessor
204 __source_handle_accessor
210 <lines removed for brevity>
214 We can also play with the model directly:
216 DB<2> x ($c->model('DB::Book')->all)[1]->title
217 SELECT me.id, me.title, me.rating, me.created, me.updated FROM book me:
218 0 'TCP/IP Illustrated, Volume 1'
220 This uses the Perl debugger C<x> command to display the title of a book.
222 Next we inspect the C<books> element of the Catalyst C<stash> (the C<4>
223 argument to the C<x> command limits the depth of the dump to 4 levels):
225 DB<3> x 4 $c->stash->{books}
227 0 MyApp::Model::DB::Book=HASH(0xb8e702c)
228 '_column_data' => HASH(0xb8e5e2c)
229 'created' => '2009-05-08 10:19:46'
232 'title' => 'CCSP SNRS Exam Certification Guide'
233 'updated' => '2009-05-08 10:19:46'
235 <lines removed for brevity>
237 Then enter the C<c> command to continue processing until the next
238 breakpoint is hit (or the application exits):
241 SELECT author.id, author.first_name, author.last_name FROM ...
243 Finally, press C<Ctrl+C> to break out of the development server.
244 Because we are running inside the Perl debugger, you will drop to the
247 ^CCatalyst::Engine::HTTP::run(/usr/local/share/perl/5.10.0/Catalyst/Engine/HTTP.pm:260):
248 260: while ( accept( Remote, $daemon ) ) {
252 Finally, press C<q> to exit the debugger and return to your OS shell
258 For more information on using the Perl debugger, please see C<perldebug>
259 and C<perldebtut>. For those daring souls out there, you can dive down
260 even deeper into the magical depths of this fine debugger by checking
263 You can also type C<h> or C<h h> at the debugger prompt to view the
264 built-in help screens.
266 For an excellent book covering all aspects of the Perl debugger, we
267 highly recommend reading 'Pro Perl Debugging' by Richard Foley.
269 Oh yeah, before you forget, be sure to remove the C<DB::single=1> line
270 you added above in C<lib/MyApp/Controller/Books.pm>.
272 =head1 DEBUGGING MODULES FROM CPAN
274 Although the techniques discussed above work well for code you are
275 writing, what if you want to use print/log/warn messages or set
276 breakpoints in code that you have installed from CPAN (or in module that
277 ship with Perl)? One helpful approach is to place a copy of the module
278 inside the C<lib> directory of your Catalyst project. When Catalyst
279 loads, it will load from inside your C<lib> directory first, only
280 turning to the global modules if a local copy cannot be found. You can
281 then make modifications such as adding a C<$DB::single=1> to the local
282 copy of the module without risking the copy in the original location.
283 This can also be a great way to "locally override" bugs in modules while
284 you wait for a fix on CPAN.
286 Matt Trout has suggested the following shortcut to create a local copy
287 of an installed module:
289 mkdir -p lib/Module; cp `perldoc -l Module::Name` lib/Module/
291 Note: If you are following along in Debian 6 or Ubuntu, you will need to
292 install the C<perl-doc> package to use the C<perldoc> command. Use
293 C<sudo aptitude install perl-doc> to do that.
295 For example, you could make a copy of
296 L<Catalyst::Plugin::Authentication> with the following command:
298 mkdir -p lib/Catalyst/Plugin; cp \
299 `perldoc -l Catalyst::Plugin::Authentication` lib/Catalyst/Plugin
301 You can then use the local copy inside your project to place logging
302 messages and/or breakpoints for further study of that module.
304 B<Note:> Matt has also suggested the following tips for Perl debugging:
310 Check the version of an installed module:
312 perl -M<mod_name> -e 'print "$<mod_name>::VERSION\n"'
316 $ perl -MCatalyst::Plugin::Authentication -e \
317 'print $Catalyst::Plugin::Authentication::VERSION;'
320 and if you are using bash aliases:
322 alias pmver="perl -le '\$m = shift; eval qq(require \$m) \
323 or die qq(module \"\$m\" is not installed\\n); \
328 Check if a modules contains a given method:
330 perl -MModule::Name -e 'print Module::Name->can("method");'
334 $ perl -MCatalyst::Plugin::Authentication -e \
335 'print Catalyst::Plugin::Authentication->can("user");'
338 If the method exists, the Perl C<can> method returns a coderef.
339 Otherwise, it returns undef and nothing will be printed.
346 If you run into issues during the rendering of your template, it might
347 be helpful to enable TT C<DEBUG> options. You can do this in a Catalyst
348 environment by adding a C<DEBUG> line to the C<__PACKAGE__->config>
349 declaration in C<lib/MyApp/View/HTML.pm>:
351 __PACKAGE__->config({
352 TEMPLATE_EXTENSION => '.tt2',
356 There are a variety of options you can use, such as 'undef', 'all',
357 'service', 'context', 'parser' and 'provider'. See
358 L<Template::Constants> for more information (remove the C<DEBUG_>
359 portion of the name shown in the TT docs and convert to lower case for
360 use inside Catalyst).
362 B<NOTE:> B<Please be sure to disable TT debug options before continuing
363 with the tutorial> (especially the 'undef' option -- leaving this
364 enabled will conflict with several of the conventions used by this
365 tutorial to leave some variables undefined on purpose).
371 Kennedy Clark, C<hkclark@gmail.com>
373 Feel free to contact the author for any errors or suggestions, but the
374 best way to report issues is via the CPAN RT Bug system at
375 <https://rt.cpan.org/Public/Dist/Display.html?Name=Catalyst-Manual>.
377 The most recent version of the Catalyst Tutorial can be found at
378 L<http://dev.catalyst.perl.org/repos/Catalyst/Catalyst-Manual/5.80/trunk/lib/Catalyst/Manual/Tutorial/>.
380 Copyright 2006-2010, Kennedy Clark, under the
381 Creative Commons Attribution Share-Alike License Version 3.0
382 (L<http://creativecommons.org/licenses/by-sa/3.0/us/>).