Event Driven Applications Wrangle Cross-Cutting Concerns 

Chris Saylor Lead Engineer @cjsaylor 150+ countries 14 million weekly class participants 15 million monthly pageviews 47 million monthly service requests Zumba Fitness 

What to expect What are cross-cutting concerns? Origins from Observer pattern Asynchronous vs Synchronous Logic abstraction Demo Events in the wild Advantages and drawbacks Futher considerations 

What are cross-cutting concerns? 

Cross-cutting concerns are parts of a program that rely on or must affect many other parts of the system. Source: Wikipedia.org 

Examples of cross-cutting concerns Logging Caching Product feature interaction Monitoring Record audit trail State Management 

What are the risks of cross cutting concerns? Coupling systems too tightly (aka "Tangling") Lots of code duplications (aka "Scattering") 

Single Responsibility Principle If a class and its services should be responsible for one thing, how do we deal with unrelated business rules that must be addressed by that class? 

Origins from Observer pattern 

Observer Pattern The root of Event Driven Applications began with the observer pattern which tracks the state of a subject by attaching observers to the subject so that when it changes, all observers are notified. 

SplObserver The interface is a standard lib to implement this pattern. SplObserver An SplSubject object attaches the observer. The SplSubject then calls an update on the observer when notifying the registered observers. 

Asynchronous vs Synchronous 

App event in javascript Syncronous in order of execution Asyncronous in order of completion $ ( ' p ' ) . o n ( ' c l i c k ' , f u n c t i o n ( ) { ( f u n c t i o n ( ) { c o n s o l e . l o g ( ' f i r s t ? ' ) ; } ( ) ) ; } ) . o n ( ' c l i c k ' , f u n c t i o n ( ) { ( f u n c t i o n ( ) { c o n s o l e . l o g ( ' s e c o n d ? ' ) ; } ( ) ) ; } ) ; 

App event in PHP Syncronous Predictable order of execution and completion < ? p h p $ e v e n t - > o n ( ' e v e n t 1 ' , f u n c t i o n ( ) { e c h o " F i r s t ! " ; } ) - > o n ( ' e v e n t 1 ' , f u n c t i o n ( ) { e c h o " S e c o n d . " } ) ; 

Logic abstraction 

Separation of Concerns Use synchronous events to separate chunks of business logic away from core functionality Provides for modularity Allow for unit testing listeners Multiple messages - multiple listeners 

Example Before < ? p h p f u n c t i o n l o g i n ( $ u s e r n a m e , $ p a s s w o r d ) { t r y { / / g e t u s e r f r o m d b $ u s e r = R e p o s i t o r y : : g e t ( $ u s e r n a m e , $ p a s s w o r d ) ; / / C h e c k i f a n a d m i n i f ( $ u s e r - > r o l e - > a d m i n ) { $ _ S E S S I O N [ ' a d m i n ' ] = t r u e ; } / / L o g u s e r ' s l o g i n $ u s e r - > u p d a t e L a s t L o g i n ( t i m e ( ) ) ; R e p o s i t o r y : : s a v e U s e r ( $ u s e r ) ; } c a t c h ( \ E x c e p t i o n $ e ) { r e t u r n f a l s e ; } r e t u r n t r u e ; } 

Example After < ? p h p f u n c t i o n l o g i n ( $ u s e r n a m e , $ p a s s w o r d ) { t r y { / / g e t u s e r f r o m d b $ u s e r = R e p o s i t o r y : : g e t ( $ u s e r n a m e , $ p a s s w o r d ) ; / / F i r e p o s t l o g i n e v e n t $ e v e n t : : t r i g g e r ( ' a f t e r L o g i n ' , c o m p a c t ( ' u s e r ' ) ) ; } c a t c h ( \ E x c e p t i o n $ e ) { r e t u r n f a l s e ; } r e t u r n t r u e ; < ? p h p $ e v e n t : : l i s t e n ( ' a f t e r L o g i n ' , f u n c t i o n ( $ e v e n t ) { i f ( $ e v e n t - > d a t a [ ' u s e r ' ] - > r o l e - > a d m i n ) { $ _ S E S S I O N [ ' a d m i n ' ] = t r u e } } ) ; $ e v e n t : : l i s t e n ( ' a f t e r L o g i n ' , f u n c t i o n ( $ e v e n t ) { $ e v e n t - > d a t a [ ' u s e r ' ] - > u p d a t e L a s t L o g i n ( t i m e ( ) ) ; R e p o s i t o r y : : s a v e U s e r ( $ e v e n t - > d a t a [ ' u s e r ' ] ) ; 

Demo https://github.com/cjsaylor/event-driven-apps-demo Branch: cart-demo (cart-demo-event) 

Events in the wild 

CakePHP Controllers have beforeFilter, beforeRender, and components with similar callbacks Models have behaviors with before find/save Exposed event library 

Symfony 2 can be used as a standalone event library. Can be easily incorporated via Composer EventDispatcher 

Zend can be used to create events and listeners of said events Can be extracted from Zend, but not as easily as Symfony 2. EventManagers 

Many others... Guzzle Magento Lithium 

Advantages and Drawbacks 

Decouple Code Advantage: Modular design can reduce core functionality bugs when modifying modules. Allows for open frameworks to allow third parties to implement custom solutions without modifying core files. Disadvantage: Code is harder to follow and needs good organizational management. Documentation of what events will be called when is almost a must. 

Plugin Architecture Advantage: Enable or disable plugins on the fly. Essential for open source targeting developers to implement. Disadvantage: No dependencies should exist be between plugins. 

Testing Advantage: Test the listeners separate from the core functionality. Execute listeners from core test cases via external event triggers. Mock event callbacks to test event trigger placement. Disadvantage: Multiple event bindings in test suites results in undesired event triggers. Unbinding specific event callbacks can be difficult to do. 

Further considerations 

Synchronous or Asynchronous? Make a judgment on whether an event listener should process it now or post-request process with a call to a message queue. 

Events and Variable Reference Using an object for passage into the event so all event handlers have access to the same data. 

Specialization Is the logic I'm trying to abstract into an event worth doing so? Would this logic best be housed in the core logic? 

Further Reading Using application events to hook in plugins Decoupling applications with domain events Domain Events 

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