Overview of Aquarium

Aquarium is a framework that implements Aspect-Oriented Programming (AOP) for Ruby. The premise of AOP is that some concerns in an application will cut across the natural object boundaries of the problem domain. Rather than scatter duplicated code in each object to handle the cross-cutting concern, AOP modularizes the specification of which execution points are affected (called join points) and the actions that should be invoked at those points.

For example, persistence of model objects is a cross-cutting concern, in the sense that the desired persistence approach (database, flat files, replication, etc.) is independent of the domain logic represented by the model. So, why should the model code have any persistence logic? Instead, capture the details of mapping the domain to the persistence approach in separate components and programmatically or declaratively modify the model objects to synchronize state changes with the persistent memory of the state.

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How to Use Aquarium

Aquarium provides a Domain Specific Language (DSL) with which you can express "aspectual" system behaviour in a modular way, i.e. using a succinct language and without repeating yourself all over your code base!

Imagine you want to trace all invocations of the public, instance methods in all classes whose names end with "Service". Here's how you can implement that behavior in Aquarium:

 1class ServiceTracer
 2  include Aquarium::DSL
 3  # jp => the current "join point"
 4  before :calls_to => :all_methods, :in_types => /Service$/ do |jp, object, *args|
 5    names = "#{jp.target_type.name}##{jp.method_name}"
 6    log "Entering: #{names}: object = #{object}, args = #{args}" 
 7  end
 8  after :calls_to => :all_methods, :in_types => /Service$/ do |jp, object, *args|
 9    names = "#{jp.target_type.name}##{jp.method_name}"
10    log "Leaving: #{names}: object = #{object}, args = #{args}" 
11  end
12end

The #before advice adds behavior that is invoked before each method is invoked, in this case, it logs a message with the name of the executing class and method, followed by the list of arguments passed to the method.

The #after advice adds similar behavior the is invoked after each method is invoked.

A more succinct implementation of this behavior uses #around advice:

 1class ServiceTracer
 2  include Aquarium::DSL
 3  around :calls_to => :all_methods, :in_types => /Service$/ do |jp, object, *args|
 4    names = "#{jp.target_type.name}##{jp.method_name}"
 5    log "Entering: #{names}: object = #{object}, args = #{args}" 
 6    result = join_point.proceed
 7    log "Leaving: #{names}: object = #{object}, args = #{args}" 
 8    result  # block needs to return the result of the "proceed"!
 9  end
10end

The special method #proceed invokes the advised method, passing it the args list (by default). For #around advice, you must call #proceed unless you specifically don't want the original method called!

See the Examples and the API section for more details.

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Install (or upgrade) with RubyGems:

$ gem install aquarium

See the [download] page for different options or go directly to the RubyForge download page.

V0.6.0
  • gem install aquarium
  • gem update aquarium

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