Revision as of 22:18, 30 September 2010

Background

My design study is related to my part time job, where I am developing an application that can retrieve and plot data from a device that monitors air quality. The application is connected to a back end server where certain data is stored.

User operations that require communication with the server roughly include:

• Requesting help (and that the user be contacted)
• Electronically registering a device
• Calibrating a device - data about the calibration process is uploaded

Additionally, server communication is needed when the application:

• Authenticates with the server
• Pings the server to ensure network connectivity

My goal here is to create a design for the application side server interface framework. HTTPS POST requests are used to transfer data to the server.

Requirements

As this is a just a part of the app, the requirements are largely self imposed & generic:

• something that works
• is as nicely designed as practical
• is as easy to understand and maintain as practical
• is flexible so that new server requests can be added without much hassle

And not to forget:

• apply & learn better OO techniques

Design Study

Deliverables

• Classes that model the different request types
• Classes that allow requests to be sent over the network

Initial Design

This is my initial attempt at the design.

Design Summary

Sending Requests

Server requests are sent, and responses received using concrete insteances of IRequestSender. HttpPOSTRequestSender is the most generic concrete implementation of this interface - the two methods it has implement the actual sending/receiving mechanisms. XRequestSender is a class with a specific implementation of the IRequestSender interface for the application's purpose. It uses a composition relationship, containing an HttpPOSTRequestSender instance. Functionally, after computing and attaching a checksum to a given IRequest, the class is used to delegate Send/Receive calls to the contained HttpPOSTRequestSender.

Representing Requests

The IRequest interface is the most generic representation of a server request. In the style of HTTP POST/GET requests, a request is defined simply as something which can return a list of name-value pairs, representing the request parameters and their names (in particular a .NET 2.0 NameValueCollection). In the context of the application, the children of XRequest define exactly which parameters are required/optional for a particular request and their types.

XRequest is the abstract implementation of IRequest, implementing the shared functionality (send/get response) for all child classes. Each concrete request is then defined by a particular implementation of GetPOSTParameters() method - returning a list with different name-value parameter contents.

Using the classes together

The application creates concrete instances of the request classes, and interacts with them using the Send/GetResponse methods. The individual request objects then interact with the server using supplied instances of IRequestSender (concretely, XRequestSender). The application does not need knowledge of the IRequestSender family of objects, except for creating them and supplying them to request objects. Perhaps a factory method/class could tidy this up slightly.

Possible Design Flaws

• Possibly Lazy class smell, for the numerous concrete implementations of XRequest. Many 1 method classes, which have a very similar implementation (populate a list, though with different parameters). This raises the issue of whether the requests can more appropriately be handled by representing them as classes, or as runtime object instances of a more general class.

• Something feels weird in the diagram about how requests are sent/ An IRequestSender is capable of sending any/many IRequest instances; an IRequest cannot send itself, while an XRequest can send itself and be sent by an IRequestSender. The whole interaction feels strange, there are cyclic dependencies between reuquest classes and IRequestSender.

• The request class hierarchy works. Polymorphism is only utilized in dealing with IRequest instances in IRequestSender implementations. The child classes of XRequest are always used directly; XRequest only exists to factor out the common functionality of child classes. Possibly breaks Class hierarchies should be deep and narrow, however as suggested by that maxim, does extract common behaviour into XRequest superclass. The shallow but wide hierarchy "smells".

Design Patterns

• Bridge pattern used to separate interface/implementation dependencies between IRequest and IRequestSender instances.

• Proxy for composition: XRequestSender is composed of, and forwards most method calls to an HttpPOSTRequestSender, additionally containing (private) methods for calculating & adding a checksum. Alternatively a Decorator could add this functionality. (Also probably better suited to have this implemented somewhere in the IRequest hierarchy. Behavioral completeness,Separation of concerns,Coupling and cohesion)

• Decorator/Strategy/Chain of Responsibility might be useful for reducing the number of IRequest/XRequest child classes by further factoring out common functionality.

Code & Installation

Since my study concerns server requests, testing it out depends on interaction with an actual server/server pages. I've provided a heavily stubbed set of pages that return sample data. A local web-server has to be installed in order to test my design. The code implementation is not 100% complete and I have taken some shortcuts for demo purposes, but most of the expected functionality is there and it largely resembles the UML design above.

Steps:

• Download and install Xampp - http://www.apachefriends.org/en/xampp-windows.html
• Run the Xampp control application (xampp-control.exe) from the install directory
• Ensure the Apache module is running
• Create a directory called "requests" inside xampp_install_dir\htdocs (e.g. C:\xampp\htdocs\requests)
• Copy the php pages from the following archive to the "requests" directory
• The source and VS2008 solution are available in this archive. Open the solution and run the program. The program tries to connect to pages in "http://localhost/requests/", so you must have completed the above steps successfully.