Patterns for Software Modernization (IPC14)

Patterns like MVC and dependency injection help the development teams of modern applications frameworks to deliver an extendable and maintainable product. Why shouldn't those patterns help you modernizing your legacy application? In this session, we will have a look at ways to apply modern patterns to legacy code by using small refactoring steps.

Transcript

1. [AMT] Alexander M. Turek Patterns for
 Software Modernization Alexander M. Turek
 International PHP Conference 2014 1
2. [AMT] Alexander M. Turek about:me freelance software developer. builds web applications with php. likes the Symfony framework. lives in Munich Berlin, Germany. 2
3. [AMT] Alexander M. Turek A Web Application 3 Data Layer Business Logic HTTP Layer HTTP-Request HTTP-Response SQL
 Database
4. [AMT] Alexander M. Turek The Application A Legacy Application 4 HTTP-Request HTTP-Response Magic SQL
 Database
5. [AMT] Alexander M. Turek A Legacy Web Project • The team maintains an old software system. • The system still fulfills current requirements. • The company earns money with the system. • The original developers have left the team. 5
6. [AMT] Alexander M. Turek Legacy Maintenance • poorly tested code base • poor code quality • requirements got lost over time • implementing new features takes much time • high bug rate 6
7. [AMT] Alexander M. Turek Unclear Dependencies 7 If I change one component, I might accidentally break another.
8. [AMT] Alexander M. Turek Legacy Dilemma 8 • Developers work differently on legacy code. • Tendency of coding as it has always been done. • No tests are written, as the codebase is considered to be untestable.
9. [AMT] Alexander M. Turek Continuous Under-Engineering 9 As you attempt to deliver future releases, you go slower and slower as the junky code multiplies, until people lose faith in the system, the programmers, and even the process that got everyone into this position. Joshua Kerievsky: Refactoring to Patterns
10. [AMT] Alexander M. Turek Let's do a rewrite? 10
11. [AMT] Alexander M. Turek Refactoring 11 Refactoring is the process of changing a software system in such a way that it does not alter the external behavior of the code yet improves its internal structure. Martin Fowler: Refactoring – Improving the Design of Existing Code
12. [AMT] Alexander M. Turek Reasons for Refactoring 12 • Make it easier to add new code. • Improve the design of existing code. • Gain a better understanding of code. • Make coding less annoying.
13. [AMT] Alexander M. Turek Refactor Everything? 13 ✓ improved code quality - time-consuming - error-prone - no direct business value
14. [AMT] Alexander M. Turek Fun Fact 14 Your application probably conTains much code that „just works“.
15. [AMT] Alexander M. Turek Refactoring for Changes 15 • If you need to change a legacy component, refactor it first. Afterwards, implement the new feature. • Gain understanding for the code. • Make the code testable. • Make the code changeable.
16. [AMT] Alexander M. Turek Autobahn 8 16 „A8palmbach“ (de.wikipedia.org) CC-BY-SA
17. [AMT] Alexander M. Turek Decomposition Pull out a component. Rewrite it. Reintegrate it. 17
18. [AMT] Alexander M. Turek Risk Reduction What if we're running out of
 time/money/developers? 18
19. [AMT] Alexander M. Turek Flexibility What if we need to
 deliver features again? 19
20. [AMT] Alexander M. Turek Workload I don't want to maintain
 the same functionality twice. 20
21. [AMT] Alexander M. Turek Application Split 21 Data Layer Business Logic HTTP Layer Data Layer Business Logic HTTP Layer Master Database Application 1 Application 2 Authentication Service …
22. [AMT] Alexander M. Turek Application Split A monolithic application
 is split into smaller parts. • standalone applications • common services 22 Master Database Application 1 Application 2 Authentication Service …
23. [AMT] Alexander M. Turek Split Applications When… • You can isolate an application that only reads from the database. • You can isolate an application that is small enough to be rewritten completely. • You can replace a part of your application by an
 off-the-shelf solution (open source or commercial). 23
24. [AMT] Alexander M. Turek Example: Web Shop • shop frontend
 (search, product pages) • checkout process • administration area • data import • data warehouse • CRM 24 read-only! use third-party software?
25. [AMT] Alexander M. Turek Legacy Strangler Application 25 Data Layer Business Logic HTTP Layer Request Routing
26. [AMT] Alexander M. Turek Strangler Application • A new application is built to server a small subset of routes of the legacy application. • A router directs matching requests to the new application. • Repeat. 26 Legacy Data Layer Business Logic HTTP Layer Request Routing Event Interception
27. [AMT] Alexander M. Turek Use Strangulation When… • You need to keep old URLs working (SEO, inbound traffic). • You expect future changes to a subset of your routes. • You expect all new routes being added. • You want to enable an application split. • You want to migrate to a new full-stack framework. • You migrate between platforms. 27
28. [AMT] Alexander M. Turek Routing:
 Reverse Proxy • Legacy and strangler are listening on different
 servers/ports/vhosts/… • The proxy decides which application should serve an incoming request based on configured rules. ✓easy rollback ✓isolated environments - ruleset complexity - infrastructure dependency 28
29. [AMT] Alexander M. Turek Routing:
 Web Server • Legacy and Strangler are deployed to different directories on the web server. • The HTTP server decides which application should server an incoming request (rewrite rules). ✓easy rollback - ruleset complexity - dependency on web server software 29
30. [AMT] Alexander M. Turek Routing:
 Front Controller Script • All requests are routed to a front controller script. • front controller boots either legacy or strangler ✓easier to debug than mod_rewrite - no isolated environments - rollback requires deployment 30
31. [AMT] Alexander M. Turek Routing:
 404 Event Handler • Try to serve the request with the strangler app. • Catch the 404 framework event and boot the legacy application. ✓no temporary request router - Booting both apps might cause side-effects. - Performance on legacy routes might suffer. 31
32. [AMT] Alexander M. Turek Façade Pattern Provides a simplified interface to a larger body of code. 32 Source: Wikipedia (CC-BY-SA)
 http://en.wikipedia.org/wiki/File:Example_of_Facade_design_pattern_in_UML.png
33. [AMT] Alexander M. Turek Façade Pattern Provide legacy functionality. Hide legacy complexity. 33 Legacy Business Logic Legacy API
34. [AMT] Alexander M. Turek Migration By Component 34 Component Legacy
35. [AMT] Alexander M. Turek Migration By Component • Rewrite a legacy component. • Connect the legacy component where ever the old component is used. • Drop the legacy component. • Repeat. 35 Component Legacy
36. [AMT] Alexander M. Turek Migrate By Component When… • You want to introduce 3rd party libraries.
 (Monolog, Symfony HTTP Foundation, Doctrine) • You want to migrate to a component framework.
 (Symfony Components) • You expect future changes to legacy components.
 (new requirements, high bug rate) 36
37. [AMT] Alexander M. Turek Adapter Pattern AKA Wrapper AKA Translator Provide the functionality of a class under a different interface. 37
38. [AMT] Alexander M. Turek Legacy Wrapper A wrapper simulates the interface of the old component. Only minimal changes to the legacy system required. 38 New Component Legacy Application Wrapper
39. [AMT] Alexander M. Turek Strangler
 XOR
 Components? Combine both techniques! 39
40. [AMT] Alexander M. Turek Dependencies 40 © joyosity on flickr
41. [AMT] Alexander M. Turek class Customer {
 public $id;
 public function getName() {
 $db = new mysqli(
 'db.example.com', 'root', 'p741n-t3xt'
 );
 $res = $db->query(
 "SELECT name FROM customer WHERE id = $id;"
 );
 list($name) = $res->fetch_row();
 return $name;
 }
 } Database Dependency 41
42. [AMT] Alexander M. Turek Hard-Wired Dependencies 42 • Dependencies... • ... are not changeable. • ... are not configurable. • ... will always be tested with your code. • ... are not explicit. new MyClass
43. [AMT] Alexander M. Turek Global State Dependency 43 • Code depends on... • ... global variables/functions. • ... static fields/methods. • ... constants.
44. [AMT] Alexander M. Turek Dependency Injection 44 class Newsletter {
 public function send() {
 $mailer = new Mailer();
 $mailer->send(…);
 }
 }
 class Newsletter {
 private $mailer;
 
 public function __construct(Mailer $mailer) {
 $this->mailer = $mailer;
 }
 
 public function send() {
 $this->mailer->send(…);
 }
 }

45. [AMT] Alexander M. Turek Dependency Injection • core pattern of modern frameworks • make dependencies explicit • allows changing dependencies • improves testability 45
46. [AMT] Alexander M. Turek Dependency Injection Containers • make the assembly of components configurable • service locator pattern • request a service • receive a readily configured object • Not a registry: objects are constructed on demand! 46
47. [AMT] Alexander M. Turek Dependency Injection Containers • Pimple • Symfony DependencyInjection Component • Zend\Di • TYPO3 Flow Object Framework • Aura DI 47
48. [AMT] Alexander M. Turek Avoiding DI Anti-Patterns • Avoid depending on your DI container! • Never make your container global/static. 48
49. [AMT] Alexander M. Turek DI and Legacy? 49
50. [AMT] Alexander M. Turek Prerequisites • Raise your project to PHP 5.3. • … or implement your own DIC. • Introduce class autoloading. 50 not recommended
51. [AMT] Alexander M. Turek Breaking the Rules • Configure the DIC in your bootstrap script. • Make your DIC global accessible – but use this access point in your legacy code only! • Your legacy code may pull your shiny new components from the DIC directly. 51
52. [AMT] Alexander M. Turek Extracting Dependencies 52 class Customer {
 public $id;
 public function getName() {
 $db = new mysqli('db.example.com', 'root', 'p741n-t3xt');
 $res = $db->query("SELECT name FROM customer WHERE id = $id;");
 if (!$res) {
 $file = fopen('/var/log/myapp/error.log', 'a');
 fwrite($file, sprintf('DB error: %s', $db->error));
 return null;
 }
 list($name) = $res->fetch_row();
 return $name;
 }
 }
53. [AMT] Alexander M. Turek Service Location via DIC 53 class Customer {
 public $id;
 public function getName() {
 global $container;
 
 $db = $container->get('db');
 $res = $db->query("SELECT name FROM customer WHERE id = $id;");
 if (!$res) {
 $container->get('logger')->log(sprintf('DB error: %s', $db->error));
 return null;
 }
 list($name) = $res->fetch_row();
 return $name;
 }
 }
54. [AMT] Alexander M. Turek Dependency Injection 54 class Customer {
 private $id;
 private $db;
 private $logger;
 
 public function __construct(mysqli $db, Logger $logger, $id) { … }
 
 public function getName() {
 $res = $this->db->query("SELECT name FROM customer WHERE id = $id;");
 if (!$res) {
 $this->logger->log(sprintf('DB error: %s', $db->error));
 return null;
 }
 list($name) = $res->fetch_row();
 return $name;
 }
 }
55. [AMT] Alexander M. Turek DI: Connecting the Worlds 55
56. [AMT] Alexander M. Turek That‘s all. 56
57. [AMT] Alexander M. Turek Thank You spam me: me@derrabus.de follow me: @derrabus hire me: https://derrabus.de 57