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Working Effectively With Legacy Perl Code


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  • 1. Working Effectively with Legacy Perl Code Erik Rantapaa Frozen Perl 2010
  • 2. What is Legacy Code?
  • 3. What is Legacy Code? Uses old/obsolete perl and modules Not well factored / organically grown Original developers gone No one fully understands it Uses outdated practices Hard to modify without breaking it Long new developer ramp-up time No documentation (or wrong documentation) No tests On the other hand... Doing useful work - generating revenue
  • 4. What is Legacy Code? Michael Feathers - "Code without tests" Ward Cunningham - "Accumulated technical debt" Stuart Halloway - "Legacy is the degree to which code: - fails to capture intent - fails to communicate intent - captures irrelevant detail (ceremony)"
  • 5. Example Legacy App e-commerce application over 10 years old "rewritten" at least a couple of times worked on by lots of different developers > 1000 .pm files > 150 database tables, >2000 columns 3 templating systems, 100's of template files used perl 5.8.0 until recently lots of old versions of CPAN modules (some customized) didn't compile with -cw until recently rm -rf not an option
  • 6. Overview Unit Testing from a Perl perspective Working with the Code Base Instrumentation Future Directions
  • 7. The Case for Testing "... With tests we can change our code quickly and verifiably. Without them we really don't know if our code is getting better or worse. ..." - Michael Feathers
  • 8. Cost of Fixing Defects
  • 9. The Feedback Cycle
  • 10. Testing vs. Debugging Debugging: manual set-up (set breakpoints, step code, etc.) temporarily modifies code (printf debugging) manual verification pay for it every time Testing: no source code modification automated set-up and verification pay once when you create the test reap the rewards every time you run it
  • 11. Your Application
  • 12. A Unit Test
  • 13. Unit Testing isolates a small piece of functionality eliminates dependencies on other components through mocking / substitution ideally runs quickly provides automated verification Benefits: safety net during refactoring after refactoring becomes a regression test speeds up the Edit-Compile-Run-Debug cycle
  • 14. Impediments to Unit Testing Main impediment: the way the application is glued together. use LWP; ... sub notify_user { my ($user, $message) = @_; ... my $ua = LWP::UserAgent->new; ... $ua->request(...); }
  • 15. Strongly Coupled Concerns sub emit_html { }
  • 16. Dependency Breaking Techniques Adapt Parameter Parameterize Constructor Break Out Method Object Parameterize Method Definition Completion Primitive Parameter Encapsulate Global Pull Up Feature References Push Down Dependency Extract and Override Call Replace Function with Extract and Override Factory Function Pointer Method Replace Global Reference Extract and Override Getter with Getter Extract Implementer Subclass and Override Extract Interface Method Introduce Instance Delegator Supersede Instance Variable Introduce Static Setter Template Redefinition Link Substitution Text Redefinition ...
  • 17. Sprouting replace a block of code with a subroutine/method call (even for new code) Benefits: simple and safe code transformation code block can be run independently permits the code to be redefined
  • 18. Sprouting Example - DBI calls Any DBI call is a good candidate for sprouting. DBI call = an action on a business concept Sprouting permits: testing of SQL syntax testing of query operation removal of interaction with the database
  • 19. Dependency Injection # use LWP; ... sub notify_user { my ($user, $message, $ua ) = @_; ... # my $ua = LWP::UserAgent->new; ... $ua->request(...); } "Ask for - Don't create"
  • 20. Preserving Signatures use LWP; ... sub notify_user { my ($user, $message, $ua ) = @_; ... $ua ||= LWP::UserAgent->new; ... $ua->request(...); }
  • 21. Perl-Specific Mocking/Isolation Techniques
  • 22. Replacing a Package alter @INC to load alternate code alter %INC to omit unneeded code
  • 23. Replacing Subroutines Monkey patch the symbol table: no warnings 'redefine'; *Product::saveToDatabase = sub { $_[0]->{saved} = 1 }; *CORE::GLOBAL::time = sub { ... } Create accessors for package lexicals
  • 24. Modifying Object Instances Re-bless to a subclass: package MockTemplateEngine; our @ISA = qw(RealTemplateEngine); sub process { ... new behavior ... } ... $template = RealTemplateEngine->new(...); bless $template, 'MockTemplateEngine';
  • 25. Use the Stack Define behavior based on caller(). Use in conjunction with monkey patching subs.
  • 26. Exploiting Perl's Dynamicism Use the dynamic capabilities of Perl to help you isolate code and create mock objects. Not the cleanest techniques, but they can greatly simplify getting dependency-laden code under test.
  • 27. Manual Testing Not always bad Some times it's the best option: automated verification is difficult / impossible automated test to costly to create
  • 28. Manual Testing Example
  • 29. Using an Wrapper Layer
  • 30. Real-World Experience Writing the first test is very difficult It gets easier! Biggest win: automated verification Next biggest win: efficient tests Manual testing is ok (save what you did!)
  • 31. More Real-World Experience Isolating code will help you understand the dependency structure of your application Let your tests guide refactoring Testable code ~ clean code ~ modular code
  • 32. Working with the Code Base
  • 33. Source Control Get everthing under source control: perl itself all CPAN modules shared libraries Apache, mod_perl, etc.
  • 34. Reliable Builds Automate building and deployment Break dependencies on absolute paths, port numbers Enable multiple deployments to co-exist on the same machine
  • 35. Logging make logs easy to access import logs into a database automate a daily synopsis SELECT count(*), substr(message, 0, 50) as "key" FROM log_message WHERE log_time BETWEEN ... AND ... GROUP BY key ORDER BY count(*) DESC;
  • 36. Wrappers Create deployment-specific wrappers for perl and perldoc: #!/bin/sh PATH=... PERL5LIB=... LD_LIBRARY_PATH=... ORACLE_HOME=... ... /path/to/app/perl "$@"
  • 37. Create Tools $ compile-check Runs app-perl -cw -MApp::PaymentService::Email::Formatter Full module name determined from current working directory.
  • 38. Automated Testing Continuous Integration Smoke Tests Regression Tests
  • 39. Instrumentation
  • 40. Devel::NYTProf Pros: full instrumentation of your code timing statistics on a per-line basis call-graphs, heat map useful for Cons: big performance hit (not suitable for production) you need to drive it Also see Aspect::Library::NYTProf for targeted profiling
  • 41. Devel::Leak::Object Original purpose: find leakage of objects due to cyclic references Modifications: Record how (the stack) objects get created Count the number of each type of object created Determine where objects of a certain class are created Robust facility to instrument bless and DESTROY.
  • 42. dtrace Pros: instrument events across the entire system runs at nearly full speed - usable in production Cons: only available for Mac OS and Solaris perl probes still under development
  • 43. Future Directions More static analysis manually added soft type assertions more help in editors/IDEs more refactoring tools better compilation diagnostics catch mistyped subroutine names catch errors in calling methods/subs catch other type errors
  • 44. Future Directions More dynamic instrumention usable in production low performance hit mainly interested in: the call chain (stack) types of variables / subroutine parameters / return values dtrace? modified perl interpreter?
  • 45. Future Directions Automated Instrumentation build a database of the runtime behavior of your program collect design details not expressed in the source mine the database to infer internal rules: call graphs signature of subroutines use rules to aid static analysis
  • 46. A Refactoring Problem Want to change $self->{PRICE} to $self->getPrice: package Product; sub foo { my ($self, ...) = @_; ... ... $self->{PRICE} ... ... }
  • 47. A Refactoring Problem Within package Product → easy! package Product; sub getPrice { ... } sub foo { my ($self, ...) = @_; ... ... $self->getPrice ... # know type of $self ... }
  • 48. A Refactoring Problem Can we change this? package SomeOtherClass; sub bar { ... ... $product->{PRICE} ... } Need to know if $product is from package Product (or a subclass)
  • 49. A Refactoring Problem Look at context for help. package SomeOtherClass; sub bar { ... my $product = Product->new(...); ... ... $product->{PRICE} ... }
  • 50. A Refactoring Problem Now what? package SomeOtherClass; sub bar { my ($self, $product, ...) = @_; ... ... $product->{PRICE} ... } Need to figure out where SomeOtherClass::bar() gets called. Instrumentation DB → just look up the answer
  • 51. References "Working Effectively with Legacy Code" - Michael Feathers "Clean Code Talks" - googletechtalks channel on YouTube Writing Testable Code - Misko Hevery http://misko.hevery. com/code_reviewers_guide "Clean Code: A Handbook of Agile Software Craftsmanship," Robert C. Martin, ed.
  • 52. Thanks! Feedback, suggestions, experiences?