Topical Paper Presentation #07 Computer Science 490.002 Topical Paper Presentation #07 Pattern Emissary Design Pattern – Page 1 John Jenkins The Emissary Design Pattern “The Emissary Design Pattern” by Ramiro González Maciel LIFIA – La Plata National University, Argentina

The Problem Pattern Emissary Design Pattern – Page 2 One of the major issues with Distributed Systems is code synchronicity. When the client application has certain error codes, the server must be updated accordingly. There must be duplicate error code in both the client and server code. Communication from the server back to the client can be complicated. Multiple solutions are nearly impossible with a massive amount of code and client communication with the server on how to handle an error makes the solution get very complex very quickly. This is a real problem in any Distributed System environment.

The Emissary Design Pattern Pattern Emissary Design Pattern – Page 3 The client makes calls to the server with an additional parameter, an Emissary object. The Server handles calls to it as normal, but, when an error arises, it calls the appropriate handle event within the parameterized Emissary object. All Helper objects may use this Emissary object as well. The ConcreteEmissary objects are the different types of ways that a Server may handle an error.

Real-World Application Pattern Emissary Design Pattern – Page 4 The client is the BigBangOriginCalculator, and it will make calls to the HubbleTelescope, SuperComputerArray, and TriangulationEquipment with at least one parameter of an Emissary object. Each Emissary object describes a different method for handling an error. Rather than having duplicate code for handling and signaling errors on both the client and server sides, one Emissary object can handle a specific method for handling an error, which is created by the client and passed to the server.

Pattern Analysis Pattern Emissary Design Pattern – Page 6 This pattern is extremely useful for code that is already very complex by preventing or removing error handling code. It is allows for a much simpler method of communicating with the client rather than having predefined solutions to errors. Easier implementation of multiple error handling techniques. Overall, his presentation of the material was very well organized, and I feel that this is a very useful pattern. Not only is this pattern applicable in the Distributed Systems world, I would be surprised if it wasn’t used rather regularly.