The document discusses the design principles of writing extensible applications, emphasizing when and why extensibility is crucial. It outlines common solutions such as modular design, dependency injection, and event-driven architecture, and underscores what Symfony offers, including bundles and service containers. The conclusion highlights that extensions facilitate the addition and modification of applications while minimizing impact on existing functionality.

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Writing Extensible Applications

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About meabout me…
Andrey Yatsenco
● PHP Developer at Oro Inc.
● 3 years with Symfony
● 6 years with PHP
https://www.facebook.com/yatsenco
https://github.com/anyt

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Writing Extensible Applications
Agenda
● Definition
● Common solutions
● What Symfony offers

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When applications should be extensible?
When you write:
● a lot of similar apps
● requirements change a lot
● a lot of features in one app
● open-source

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provide for change (enhancements)
● while minimizing impact to existing system
functions.
● is a design principle where the
implementation takes future growth into
consideration.
● extensions can be through the addition of
new functionality or through modification
of existing functionality
Extensibility

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Extensible design is to accept that
not everything can be designed in advance

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Why code must
be extensible?

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Typical problems of not extensible apps
● Difficult to replace part of an app
● Difficult to scale an application

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Common solutions
● Abstractions and DI
● Modular design
● Extensions provider
● Event-driven architecture

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Common solutions
● Abstractions and DI
● Modular design
● Extensions provider
● Event-driven architecture

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Abstractions and DI
Abstractions: Dependency Injection:

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Common solutions
● Abstractions and DI
● Modular design
● Extensions provider
● Event-driven architecture

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Modular design
Examples:
● Symfony
● ZF
● Yii
● Drupal
● Joomla
● Wordpress
● etc.
Naming:
● modules
● bundles
● packages
● plugins
● service providers
● etc.

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Modular design
Make your framework modular:
● Symfony HttpKernel component (bundles)

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Modular design
Separating the functionality of a program into
independent, interchangeable modules,
such that each contains everything
necessary to execute only one aspect of the
desired functionality.

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Modular design
Bad practices:
● High Cohesion
● Circular references

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Modular design
Bad practices:
How to avoid this?

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Modular design
Bad practices:
How to avoid this?
● Event-driven architecture

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Event-driven architecture
● Observer
● Mediator

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Event-driven architecture
Observer:

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Event-driven architecture
Mediator

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Modular design
Bad practices:
How to avoid this?
● Event-driven architecture
● Providing extensions

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Providing extensions

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What Symfony offers?
● Bundles
● Service container
● Event dispatcher

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What Symfony offers?
Service container
● Dependency Injection

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What Symfony offers?
Service container
● Overriding classes of services with the
same dependencies with parameters

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What Symfony offers?
Service container
● Overriding services in compiler passes with
the same interface or by extending the
original one in Compiler Passes

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What Symfony offers?
Service container
● Decorating services (new in Symfony 2.5)

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What Symfony offers?
Service container
● Providing bundle configuration

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What Symfony offers?
Event dispatcher (mediator pattern)

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What Symfony offers?
You still can provide extension.
There is no out of the box solution, because
you don’t need it.
You can check examples:
● Twig
● Form

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Conclusion
● Extensions helps you to make addition and
modification of your application while
minimizing impact to existing functionality.

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Conclusion
Common solutions
● Abstractions and DI
● Modular design
● Extensions provider
● Event-driven architecture

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Conclusion
Symfony offers
● Bundles
● Service container
● Event dispatcher

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P.S.
Ideal OO code not the answer, because of
performance.
You can scale your infrastructure, because it's
cheaper than development in some situations
but usually not for a long time.

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References
● The Philosophy of Extensible Software
● Github - PHP Design Patterns
● Wikipedia - Extensibility
● Wikipedia - List of software development philosophies
● Wikipedia - SOLID
● SO - Designing extensible software
● Java Docs - Creating Extensible Applications
● Symfony Books

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