Wednesday, 18 November 2009

The host in charge or How to design software the good way.

Software design is something that can be really challenging. This becomes more clear when you are working on a project now and than, or have some very complicated software.
In the last topic I talked about the challenge of defining the scheduler and what it should schedule. I thought it was a good and easy point to start with, but it is not. I think by starting with the scheduler I want to make it much to generic and I don't want to be to generic, I just want it to do what it should do.
To identify what it should do I have to start somewhere else.

The Host.

Within NetworkMonitor hosts play an important role. A host is a network device with a networkcard and an network address.
The state of the host can be checked and set from the outside. The host itself is responsible to notify others that a new check should be scheduled, but the user should be able to schedule a check manually as well.

This above text will leas us to the following design:

The SetState and ScheduleCheck methods will have parameters, but I don't know them yet.
The idea is that when the SetState method is called, the ScheduleCheck is called automaticly.
The ScheduleCheck will fire an event (ScheduleCheckEvent) to schedule the check. The event will contain all the necesary information for others to get the right host, the right check and the right time to perform the check.
Since the host has a public ScheduleCheck methode, the user can also schedule the check whenever he likes.

The Tests

Since this is a learning project I want to start using a Test first approach. This means we start with writing tests and then write the code to let the tests pass.
I am going to use specification style testing which means I write specifications/stories about what I need to test. Here are my specifications for the Host:
Given: an Host
When : ScheduleCheck is called
Then : a ScheduleCheckEvent is raised
Given: an Host
When : the status is set via SetStatus
Then : then a ScheduleCheckEvent is raised
I tried writing specifications before, but I found it always difficult to do. These specifications are as simple as can be. The reason for this (I think) is that I want to test less than before, or made things to complex.

ok, but that is not code.. I am going to use MSpec which is a .NET project for Behaviour Driven style testing.

public class When_scheduling_a_check
{
   static Host hots;
   Establish context = () => { host = new Host("10.1.1.2"); };
   Because of = () => { host.ScheduleCheck(DateTime.Now); };
   It should_have_raised_a_ScheduleCheckEvent;
}

public class When_setting_the_status
{
   static Host hots;
   Establish context = () => { host = new Host("10.1.1.2"); };
   Because of = () => { host.SetState(State.Ok); };
   It should_have_raise_a_ScheduleCheckEvent;
}

All that I have done here are writing the When (Establish context) and Then (Because of). The actual test is still pending.
The funny code (() => {};) is part of the .NET syntax and are called lambda expressions. This makes it possible to put functions/methods inside a property to be run at a later moment.

Well thats all for today. I will have to implement this first before I continue :)

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