3 Catalyst::Manual::Tutorial::Debugging - Catalyst Tutorial - Part 7: Debugging
8 This is B<Part 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 part 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.
81 Folks in the former group can use Catalyst's C<$c-E<gt>log> facility.
82 (See L<Catalyst::Log> for more detail.) For example, if you add the
83 following code to a controller action method:
85 $c->log->info("Starting the foreach loop here");
87 $c->log->debug("Value of \$id is: ".$id);
89 Then the Catalyst development server will display your message along
90 with the other debug output. To accomplish the same thing in a TTSite
93 [% Catalyst.log.debug("This is a test log message") %]
95 You can also use L<Data::Dumper|Data::Dumper> in both Catalyst code
96 (C<use Data::Dumper; $c-E<gt>log-E<gt>debug("\$var is: ".Dumper($var));)>)
97 and TT templates (C<[% Dumper.dump(book) %]>.
99 =head1 RUNNING CATALYST UNDER THE PERL DEBUGGER
101 Members of the interactive-debugger fan club will also be at home with
102 Catalyst applications. One approach to this style of Perl debugging is
103 to embed breakpoints in your code. For example, open
104 C<lib/MyApp/Controller/Books.pm> in your editor and add the
105 C<DB::single=1> line as follows inside the C<list> method (I like to
106 "left-justify" my debug statements so I don't forget to remove them, but
107 you can obviously indent them if you prefer):
110 # Retrieve the usual Perl OO '$self' for this object. $c is the Catalyst
111 # 'Context' that's used to 'glue together' the various components
112 # that make up the application
117 # Retrieve all of the book records as book model objects and store in the
118 # stash where they can be accessed by the TT template
119 $c->stash->{books} = [$c->model('MyAppDB::Books')->all];
121 # Set the TT template to use. You will almost always want to do this
122 # in your action methods.
123 $c->stash->{template} = 'books/list.tt2';
126 This causes the Perl Debugger to enter "single step mode" when this command is
127 encountered (it has no effect when Perl is run without the C<-d> flag).
129 To now run the Catalyst development server under the Perl debugger, simply
130 prepend C<perl -d> to the front of C<script/myapp_server.pl>:
132 $ perl -d script/myapp_server.pl
134 This will start the interactive debugger and produce output similar to:
136 $ perl -d script/myapp_server.pl
138 Loading DB routines from perl5db.pl version 1.27
139 Editor support available.
141 Enter h or `h h' for help, or `man perldebug' for more help.
143 main::(script/myapp_server.pl:14): my $debug = 0;
147 Press the C<c> key and hit C<Enter> to continue executing the Catalyst
148 development server under the debugger. Although execution speed will be
149 slightly slower than normal, you should soon see the usual Catalyst
150 startup debug information.
152 Now point your browser to L<http://localhost:3000/books/list> and log
153 in. Once the breakpoint is encountered in the
154 C<MyApp::Controller::list> method, the console session running the
155 development server will drop to the Perl debugger prompt:
157 MyApp::Controller::Books::list(/home/me/MyApp/script/../lib/MyApp/Controller/Books.pm:40):
158 40: $c->stash->{books} = [$c->model('MyAppDB::Books')->all];
162 You now have the full Perl debugger at your disposal. First use the
163 C<next> feature by typing C<n> to execute the C<all> method on the Book
164 model (C<n> jumps over method/subroutine calls; you can also use C<s> to
165 C<single-step> into methods/subroutines):
168 SELECT me.id, me.authors, me.title, me.rating FROM books me:
169 MyApp::Controller::Books::list(/home/me/MyApp/script/../lib/MyApp/Controller/Books.pm:44):
170 44: $c->stash->{template} = 'books/list.tt2';
174 This takes you to the next line of code where the template name is set.
175 Notice that because we enabled C<DBIC_TRACE=1> earlier, SQL debug
176 output also shows up in the development server debug information.
178 Next, list the methods available on our C<Book> model:
180 DB<1> m $c->model('MyAppDB::Books')
184 MODIFY_CODE_ATTRIBUTES
189 _result_class_accessor
190 _result_source_accessor
193 <lines removed for brevity>
197 We can also play with the model directly:
199 DB<2> x ($c->model('MyAppDB::Books')->all)[1]->title
200 SELECT me.id, me.title, me.rating FROM books me:
201 0 'TCP/IP Illustrated, Volume 1'
203 This uses the Perl debugger C<x> command to display the title of a book.
205 Next we inspect the C<books> element of the Catalyst C<stash> (the C<4>
206 argument to the C<x> command limits the depth of the dump to 4 levels):
208 DB<3> x 4 $c->stash->{books}
210 0 MyApp::Model::MyAppDB::Book=HASH(0xb8e702c)
211 '_column_data' => HASH(0xb8e5e2c)
214 'title' => 'CCSP SNRS Exam Certification Guide'
216 <lines removed for brevity>
218 Then enter the C<c> command to continue processing until the next
219 breakpoint is hit (or the application exits):
222 SELECT author.id, author.first_name, author.last_name FROM ...
224 Finally, press C<Ctrl+C> to break out of the development server.
225 Because we are running inside the Perl debugger, you will drop to the
226 debugger prompt. Press C<q> to exit the debugger and return to your OS
232 For more information on using the Perl debugger, please see C<perldebug>
233 and C<perldebtut>. You can also type C<h> or C<h h> at the debugger
234 prompt to view the built-in help screens.
237 =head1 DEBUGGING MODULES FROM CPAN
239 Although the techniques discussed above work well for code you are
240 writing, what if you want to use print/log/warn messages or set
241 breakpoints in code that you have installed from CPAN (or in module that
242 ship with Perl)? One helpful approach is to place a copy of the module
243 inside the C<lib> directory of your Catalyst project. When Catalyst
244 loads, it will load from inside your C<lib> directory first, only
245 turning to the global modules if a local copy cannot be found. You can
246 then make modifications such as adding a C<$DB::single=1> to the local
247 copy of the module without risking the copy in the original location.
248 This can also be a great way to "locally override" bugs in modules while
249 you wait for a fix on CPAN.
252 Matt Trout has suggested the following shortcut to create a local
253 copy of an installed module:
255 mkdir -p lib/Module; cp `perldoc -l Module::Name` lib/Module/
257 Note: If you are following along in Ubuntu, you will need to install
258 the C<perl-doc> package to use the C<perldoc> command. Use
259 C<sudo apt-get install perl-doc> to do that.
261 For example, you could make a copy of
262 L<Catalyst::Plugin::Authentication|Catalyst::Plugin::Authentication>
263 with the following command:
265 mkdir -p lib/Catalyst/Plugin; cp \
266 `perldoc -l Catalyst::Plugin::Authentication` lib/Catalyst/Plugin
268 You can then use the local copy inside your project to place logging
269 messages and/or breakpoints for further study of that module.
271 B<Note:> Matt has also suggested the following tips for Perl
278 Check the version of an installed module:
280 perl -MModule::Name -e 'print $Module::Name::VERSION;'
284 $ perl -MCatalyst::Plugin::Authentication -e \
285 'print $Catalyst::Plugin::Authentication::VERSION;'
290 Check if a modules contains a given method:
292 perl -MModule::Name -e 'print Module::Name->can("method");'
296 $ perl -MCatalyst::Plugin::Authentication -e \
297 'print Catalyst::Plugin::Authentication->can("user");'
300 If the method exists, the Perl C<can> method returns a coderef.
301 Otherwise, it returns undef and nothing will be printed.
308 Kennedy Clark, C<hkclark@gmail.com>
310 Please report any errors, issues or suggestions to the author. The
311 most recent version of the Catalyst Tutorial can be found at
312 L<http://dev.catalyst.perl.org/repos/Catalyst/trunk/Catalyst-Manual/lib/Catalyst/Manual/Tutorial/>.
314 Copyright 2006, Kennedy Clark, under Creative Commons License
315 (L<http://creativecommons.org/licenses/by-nc-sa/2.5/>).