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Active Object Design Pattern
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- 2. Active Object • Decouplesmethod execution from method invocation • Simplifies synchronized access to objects that reside in their own threads of control • Commonly used in distributed systems requiring multi-threaded servers • Often implemented using message passing • Similar to the Actor pattern. In the Actor pattern each Actor is an Active Object.
- 3. Example Class Diagram •A proxy provides an interface that allows clients to access methods of an object • A concrete method request is created for every method invoked on the proxy • A scheduler receives the method requests & dispatches them on the servant when they become runnable • An activation list maintains pending method requests • A servant implements the methods • A future allows clients to access the results of a method call on the proxy Future Scheduler insert dispatch MethodRequest can_call call * Proxy method_1 method_n Activation List insert remove Servant method_1 method_n creates creates maintains Concrete MethodRequest1 Concrete MethodRequest2
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- 6. Method Scheduling (cont) • Aclient calls a function on a proxy object • The proxy creates a future which will be returned to the client (If the method returns a result) • The proxy creates a method request which is told about the future • The method request is then sent to the scheduler which places it in the activation list • If the method is to return a result, the future is returned to the client
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- 8. Method Dispatch (cont) • Thescheduler checks if the method request can be run • If the method request can be run it is removed from the activation list and called • The method request calls the method on the servant. • If the method has a result, the result is written to the associated future
- 9. Method Dispatch and Guards •Methods have guards which can delay them from being called. • Before the scheduler can have the servant run a method, it checks if the method can be called • If a method cannot be called, the scheduler goes on to the next method to be run • Example: Queue implemented as an Active Object • Enqueue cannot be called if the queue is full • Dequeue cannot be called if the queue is empty
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- 11. Retrieving Results (cont) • Afterthe result has been written to the future the client can then retrieve it • Reading the result is asynchronous of calling the method
- 12. Example Implementations / References •Actor implementation in Python: http://jodal.github.com/pykka/ • Actors for Scala/Java: http://akka.io/ • http://www.erlang.org/ • “Prefer Using Active Objects Instead of Naked Threads” by Herb Sutter http://drdobbs.com/high-performance-computing/225700095 • “Pattern-Oriented Software Architecture: Patterns for Concurrent and Networked Objects” by Douglas C. Schmidt, et al. www1.cse.wustl.edu/~schmidt/POSA/POSA2/ www.cs.wustl.edu/~schmidt/posa2.ppt • http://en.wikipedia.org/wiki/Actor_model