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 Source code for the tutorial in included in the F</home/catalyst/Final>
63 directory of the Tutorial Virtual machine (one subdirectory per
64 chapter). There are also instructions for downloading the code in
65 L<Catalyst::Manual::Tutorial::01_Intro>.
68 Note that when it comes to debugging and troubleshooting, there are two
75 Fans of C<log> and C<print> statements embedded in the code.
79 Fans of interactive debuggers.
83 Catalyst is able to easily accommodate both styles of debugging.
88 Folks in the former group can use Catalyst's C<$c-E<gt>log> facility.
89 (See L<Catalyst::Log> for more detail.) For example, if you add the
90 following code to a controller action method:
92 $c->log->info("Starting the foreach loop here");
94 $c->log->debug("Value of \$id is: ".$id);
96 Then the Catalyst development server will display your message along
97 with the other debug output. To accomplish the same thing in a TT
100 [% c.log.debug("This is a test log message") %]
102 As with many other logging facilities, a method is defined for each of
103 the following "logging levels" (in increasing order of
104 severity/importance):
112 You can also use L<Data::Dumper> in both Catalyst code
113 (C<use Data::Dumper; $c-E<gt>log-E<gt>debug("\$var is: ".Dumper($var));)>)
114 and TT templates (C<[% Dumper.dump(book) %]>.
116 B<NOTE:> Whether you are a logging fanatic or not, we strongly recommend
117 that you take advantage of L<Log::Log4perl> or L<Log::Dispatch>. It's
118 easy to use L<Catalyst::Log> with either of these and they will provide
119 a huge amount of extra functionality that you will want in virtually
120 every production application you run or support.
123 =head1 RUNNING CATALYST UNDER THE PERL DEBUGGER
125 Members of the interactive-debugger fan club will also be at home with
126 Catalyst applications. One approach to this style of Perl debugging is
127 to embed breakpoints in your code. For example, open
128 C<lib/MyApp/Controller/Books.pm> in your editor and add the
129 C<DB::single=1> line as follows inside the C<list> method (I like to
130 "left-justify" my debug statements so I don't forget to remove them, but
131 you can obviously indent them if you prefer):
134 # Retrieve the usual Perl OO '$self' for this object. $c is the Catalyst
135 # 'Context' that's used to 'glue together' the various components
136 # that make up the application
141 # Retrieve all of the book records as book model objects and store in the
142 # stash where they can be accessed by the TT template
143 $c->stash->{books} = [$c->model('DB::Book')->all];
145 # Set the TT template to use. You will almost always want to do this
146 # in your action methods.
147 $c->stash->{template} = 'books/list.tt2';
150 This causes the Perl Debugger to enter "single step mode" when this
151 command is encountered (it has no effect when Perl is run without the
154 B<NOTE:> The C<DB> here is the Perl Debugger, not the DB model.
156 If you haven't done it already, enable SQL logging as before:
158 $ export DBIC_TRACE=1
160 To now run the Catalyst development server under the Perl debugger,
161 simply prepend C<perl -d> to the front of C<script/myapp_server.pl>:
163 $ perl -d script/myapp_server.pl
165 This will start the interactive debugger and produce output similar to:
167 $ perl -d script/myapp_server.pl
169 Loading DB routines from perl5db.pl version 1.3
170 Editor support available.
172 Enter h or `h h' for help, or `man perldebug' for more help.
174 main::(script/myapp_server.pl:16): my $debug = 0;
178 Press the C<c> key and hit C<Enter> to continue executing the Catalyst
179 development server under the debugger. Although execution speed will be
180 slightly slower than normal, you should soon see the usual Catalyst
181 startup debug information.
183 Now point your browser to L<http://localhost:3000/books/list> and log
184 in. Once the breakpoint is encountered in the
185 C<MyApp::Controller::list> method, the console session running the
186 development server will drop to the Perl debugger prompt:
188 MyApp::Controller::Books::list(/home/catalyst/MyApp/script/../lib/MyApp/Controller/Books.pm:48):
189 48: $c->stash->{books} = [$c->model('DB::Book')->all];
193 You now have the full Perl debugger at your disposal. First use the
194 C<next> feature by typing C<n> to execute the C<all> method on the Book
195 model (C<n> jumps over method/subroutine calls; you can also use C<s> to
196 C<single-step> into methods/subroutines):
199 SELECT me.id, me.title, me.rating, me.created, me.updated FROM book me:
200 MyApp::Controller::Books::list(/home/catalyst/MyApp/script/../lib/MyApp/Controller/Books.pm:53):
201 53: $c->stash->{template} = 'books/list.tt2';
205 This takes you to the next line of code where the template name is set.
206 Notice that because we enabled C<DBIC_TRACE=1> earlier, SQL debug output
207 also shows up in the development server debug information.
209 Next, list the methods available on our C<Book> model:
211 DB<1> m $c->model('DB::Book')
215 __result_class_accessor
216 __source_handle_accessor
222 <lines removed for brevity>
226 We can also play with the model directly:
228 DB<2> x ($c->model('DB::Book')->all)[1]->title
229 SELECT me.id, me.title, me.rating, me.created, me.updated FROM book me:
230 0 'TCP/IP Illustrated, Volume 1'
232 This uses the Perl debugger C<x> command to display the title of a book.
234 Next we inspect the C<books> element of the Catalyst C<stash> (the C<4>
235 argument to the C<x> command limits the depth of the dump to 4 levels):
237 DB<3> x 4 $c->stash->{books}
239 0 MyApp::Model::DB::Book=HASH(0xb8e702c)
240 '_column_data' => HASH(0xb8e5e2c)
241 'created' => '2009-05-08 10:19:46'
244 'title' => 'CCSP SNRS Exam Certification Guide'
245 'updated' => '2009-05-08 10:19:46'
247 <lines removed for brevity>
249 Then enter the C<c> command to continue processing until the next
250 breakpoint is hit (or the application exits):
253 SELECT author.id, author.first_name, author.last_name FROM ...
255 Finally, press C<Ctrl+C> to break out of the development server.
256 Because we are running inside the Perl debugger, you will drop to the
259 ^CCatalyst::Engine::HTTP::run(/usr/local/share/perl/5.10.0/Catalyst/Engine/HTTP.pm:260):
260 260: while ( accept( Remote, $daemon ) ) {
264 Finally, press C<q> to exit the debugger and return to your OS shell
270 For more information on using the Perl debugger, please see C<perldebug>
271 and C<perldebtut>. For those daring souls out there, you can dive down
272 even deeper into the magical depths of this fine debugger by checking
275 You can also type C<h> or C<h h> at the debugger prompt to view the
276 built-in help screens.
278 For an excellent book covering all aspects of the Perl debugger, we
279 highly recommend reading 'Pro Perl Debugging' by Richard Foley.
281 Oh yeah, before you forget, be sure to remove the C<DB::single=1> line
282 you added above in C<lib/MyApp/Controller/Books.pm>.
284 =head1 DEBUGGING MODULES FROM CPAN
286 Although the techniques discussed above work well for code you are
287 writing, what if you want to use print/log/warn messages or set
288 breakpoints in code that you have installed from CPAN (or in module that
289 ship with Perl)? One helpful approach is to place a copy of the module
290 inside the C<lib> directory of your Catalyst project. When Catalyst
291 loads, it will load from inside your C<lib> directory first, only
292 turning to the global modules if a local copy cannot be found. You can
293 then make modifications such as adding a C<$DB::single=1> to the local
294 copy of the module without risking the copy in the original location.
295 This can also be a great way to "locally override" bugs in modules while
296 you wait for a fix on CPAN.
298 Matt Trout has suggested the following shortcut to create a local copy
299 of an installed module:
301 mkdir -p lib/Module; cp `perldoc -l Module::Name` lib/Module/
303 Note: If you are following along in Debian 6 or Ubuntu, you will need to
304 install the C<perl-doc> package to use the C<perldoc> command. Use
305 C<sudo aptitude install perl-doc> to do that.
307 For example, you could make a copy of
308 L<Catalyst::Plugin::Authentication> with the following command:
310 mkdir -p lib/Catalyst/Plugin; cp \
311 `perldoc -l Catalyst::Plugin::Authentication` lib/Catalyst/Plugin
313 You can then use the local copy inside your project to place logging
314 messages and/or breakpoints for further study of that module.
316 B<Note:> Matt has also suggested the following tips for Perl debugging:
322 Check the version of an installed module:
324 perl -M<mod_name> -e 'print "$<mod_name>::VERSION\n"'
328 $ perl -MCatalyst::Plugin::Authentication -e \
329 'print $Catalyst::Plugin::Authentication::VERSION;'
332 and if you are using bash aliases:
334 alias pmver="perl -le '\$m = shift; eval qq(require \$m) \
335 or die qq(module \"\$m\" is not installed\\n); \
340 Check if a modules contains a given method:
342 perl -MModule::Name -e 'print Module::Name->can("method");'
346 $ perl -MCatalyst::Plugin::Authentication -e \
347 'print Catalyst::Plugin::Authentication->can("user");'
350 If the method exists, the Perl C<can> method returns a coderef.
351 Otherwise, it returns undef and nothing will be printed.
358 If you run into issues during the rendering of your template, it might
359 be helpful to enable TT C<DEBUG> options. You can do this in a Catalyst
360 environment by adding a C<DEBUG> line to the C<__PACKAGE__->config>
361 declaration in C<lib/MyApp/View/HTML.pm>:
363 __PACKAGE__->config({
364 TEMPLATE_EXTENSION => '.tt2',
368 There are a variety of options you can use, such as 'undef', 'all',
369 'service', 'context', 'parser' and 'provider'. See
370 L<Template::Constants> for more information (remove the C<DEBUG_>
371 portion of the name shown in the TT docs and convert to lower case for
372 use inside Catalyst).
374 B<NOTE:> B<Please be sure to disable TT debug options before continuing
375 with the tutorial> (especially the 'undef' option -- leaving this
376 enabled will conflict with several of the conventions used by this
377 tutorial to leave some variables undefined on purpose).
382 You can jump to the next chapter of the tutorial here:
383 L<Testing|Catalyst::Manual::Tutorial::08_Testing>
388 Kennedy Clark, C<hkclark@gmail.com>
390 Feel free to contact the author for any errors or suggestions, but the
391 best way to report issues is via the CPAN RT Bug system at
392 L<https://rt.cpan.org/Public/Dist/Display.html?Name=Catalyst-Manual>.
394 Copyright 2006-2011, Kennedy Clark, under the
395 Creative Commons Attribution Share-Alike License Version 3.0
396 (L<http://creativecommons.org/licenses/by-sa/3.0/us/>).