3 Catalyst::Manual::Tutorial::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::Intro>
20 L<Catalyst Basics|Catalyst::Manual::Tutorial::CatalystBasics>
24 L<More Catalyst Basics|Catalyst::Manual::Tutorial::MoreCatalystBasics>
28 L<Basic CRUD|Catalyst::Manual::Tutorial::BasicCRUD>
32 L<Authentication|Catalyst::Manual::Tutorial::Authentication>
36 L<Authorization|Catalyst::Manual::Tutorial::Authorization>
44 L<Testing|Catalyst::Manual::Tutorial::Testing>
48 L<Advanced CRUD|Catalyst::Manual::Tutorial::AdvancedCRUD>
52 L<Appendices|Catalyst::Manual::Tutorial::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|Catalyst::Log> for more detail.) For example, if
84 you add the 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, you a method is defined for
97 each of the following "logging levels" (in increasing order of
106 You can also use L<Data::Dumper|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::Books')->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 command is
139 encountered (it has no effect when Perl is run without the C<-d> flag).
141 B<NOTE:> The C<DB> here is the Perl Debugger, not the DB model.
143 To now run the Catalyst development server under the Perl debugger, simply
144 prepend C<perl -d> to the front of C<script/myapp_server.pl>:
146 $ perl -d script/myapp_server.pl
148 This will start the interactive debugger and produce output similar to:
150 $ perl -d script/myapp_server.pl
152 Loading DB routines from perl5db.pl version 1.3
153 Editor support available.
155 Enter h or `h h' for help, or `man perldebug' for more help.
157 main::(script/myapp_server.pl:16): my $debug = 0;
161 Press the C<c> key and hit C<Enter> to continue executing the Catalyst
162 development server under the debugger. Although execution speed will be
163 slightly slower than normal, you should soon see the usual Catalyst
164 startup debug information.
166 Now point your browser to L<http://localhost:3000/books/list> and log
167 in. Once the breakpoint is encountered in the
168 C<MyApp::Controller::list> method, the console session running the
169 development server will drop to the Perl debugger prompt:
171 MyApp::Controller::Books::list(/home/me/MyApp/script/../lib/MyApp/Controller/Books.pm:48):
172 48: $c->stash->{books} = [$c->model('DB::Books')->all];
176 You now have the full Perl debugger at your disposal. First use the
177 C<next> feature by typing C<n> to execute the C<all> method on the Book
178 model (C<n> jumps over method/subroutine calls; you can also use C<s> to
179 C<single-step> into methods/subroutines):
182 SELECT me.id, me.authors, me.title, me.rating FROM books me:
183 MyApp::Controller::Books::list(/home/me/MyApp/script/../lib/MyApp/Controller/Books.pm:53):
184 53: $c->stash->{template} = 'books/list.tt2';
188 This takes you to the next line of code where the template name is set.
189 Notice that because we enabled C<DBIC_TRACE=1> earlier, SQL debug
190 output also shows up in the development server debug information.
192 Next, list the methods available on our C<Book> model:
194 DB<1> m $c->model('DB::Books')
198 __source_handle_accessor
203 <lines removed for brevity>
207 We can also play with the model directly:
209 DB<2> x ($c->model('DB::Books')->all)[1]->title
210 SELECT me.id, me.title, me.rating FROM books me:
211 0 'TCP/IP Illustrated, Volume 1'
213 This uses the Perl debugger C<x> command to display the title of a book.
215 Next we inspect the C<books> element of the Catalyst C<stash> (the C<4>
216 argument to the C<x> command limits the depth of the dump to 4 levels):
218 DB<3> x 4 $c->stash->{books}
220 0 MyApp::Model::DB::Book=HASH(0xb8e702c)
221 '_column_data' => HASH(0xb8e5e2c)
224 'title' => 'CCSP SNRS Exam Certification Guide'
226 <lines removed for brevity>
228 Then enter the C<c> command to continue processing until the next
229 breakpoint is hit (or the application exits):
232 SELECT author.id, author.first_name, author.last_name FROM ...
234 Finally, press C<Ctrl+C> to break out of the development server.
235 Because we are running inside the Perl debugger, you will drop to the
236 debugger prompt. Press C<q> to exit the debugger and return to your OS
242 For more information on using the Perl debugger, please see C<perldebug>
243 and C<perldebtut>. You can also type C<h> or C<h h> at the debugger
244 prompt to view the built-in help screens.
247 =head1 DEBUGGING MODULES FROM CPAN
249 Although the techniques discussed above work well for code you are
250 writing, what if you want to use print/log/warn messages or set
251 breakpoints in code that you have installed from CPAN (or in module that
252 ship with Perl)? One helpful approach is to place a copy of the module
253 inside the C<lib> directory of your Catalyst project. When Catalyst
254 loads, it will load from inside your C<lib> directory first, only
255 turning to the global modules if a local copy cannot be found. You can
256 then make modifications such as adding a C<$DB::single=1> to the local
257 copy of the module without risking the copy in the original location.
258 This can also be a great way to "locally override" bugs in modules while
259 you wait for a fix on CPAN.
262 Matt Trout has suggested the following shortcut to create a local
263 copy of an installed module:
265 mkdir -p lib/Module; cp `perldoc -l Module::Name` lib/Module/
267 Note: If you are following along in Debian 5, you will need to install
268 the C<perl-doc> package to use the C<perldoc> command. Use
269 C<sudo aptitude install perl-doc> to do that.
271 For example, you could make a copy of
272 L<Catalyst::Plugin::Authentication|Catalyst::Plugin::Authentication>
273 with the following command:
275 mkdir -p lib/Catalyst/Plugin; cp \
276 `perldoc -l Catalyst::Plugin::Authentication` lib/Catalyst/Plugin
278 You can then use the local copy inside your project to place logging
279 messages and/or breakpoints for further study of that module.
281 B<Note:> Matt has also suggested the following tips for Perl
288 Check the version of an installed module:
290 perl -ME<lt>mod_nameE<gt> -e '"print $E<lt>mod_nameE<gt>::VERSION\n"'
294 $ perl -MCatalyst::Plugin::Authentication -e \
295 'print $Catalyst::Plugin::Authentication::VERSION;'
300 Check if a modules contains a given method:
302 perl -MModule::Name -e 'print Module::Name->can("method");'
306 $ perl -MCatalyst::Plugin::Authentication -e \
307 'print Catalyst::Plugin::Authentication->can("user");'
310 If the method exists, the Perl C<can> method returns a coderef.
311 Otherwise, it returns undef and nothing will be printed.
318 If you run into issues during the rendering of your template, it might
319 be helpful to enable TT C<DEBUG> options. You can do this in a Catalyst
320 environment by adding a C<DEBUG> line to the C<__PACKAGE__->config>
321 declaration in C<lib/MyApp/View/TT.pm>:
323 __PACKAGE__->config({
324 TEMPLATE_EXTENSION => '.tt2',
328 There are a variety of options you can use, such as 'undef', 'all',
329 'service', 'context', 'parser' and 'provider'. See
330 L<Template::Constants|Template::Constants> for more information
331 (remove the C<DEBUG_> portion of the name shown in the TT docs and
332 convert to lower case for use inside Catalyst).
334 B<NOTE:> B<Please be sure to disable TT debug options before continuing
335 with the tutorial> (especially the 'undef' option -- leaving this
336 enabled will conflict with several of the conventions used by this
337 tutorial to leave some variables undefined on purpose).
342 Kennedy Clark, C<hkclark@gmail.com>
344 Please report any errors, issues or suggestions to the author. The
345 most recent version of the Catalyst Tutorial can be found at
346 L<http://dev.catalyst.perl.org/repos/Catalyst/Catalyst-Manual/5.70/trunk/lib/Catalyst/Manual/Tutorial/>.
348 Copyright 2006-2008, Kennedy Clark, under Creative Commons License
349 (L<http://creativecommons.org/licenses/by-sa/3.0/us/>).