[C#]簡易的Backoff window實現類別

2012-04-18

6912
0
C#

在無線網路的領域中，若是節點間要進行傳輸，會試圖嚐試發送RTS訊號，當接收端收到且允許傳送時，接收端會發送CTS訊號，傳送端就會知道可以進行傳送的動作。但若是傳送端發送了RTS後過段時間沒收到CTS訊號，代表訊號被碰撞掉了，這時會挑選個backoff值，決定要多久後再重試傳輸。這邊的backoff值會隨著碰撞而變大backoff值的區間，也就是無線網路領域所謂的Backoff window機制。

這樣的機制很簡單，但卻能分散碰撞，並將重試的時間調在適當的區間。這樣的機制一樣可以拿來套用在我們一般程式的開發，就像有時後訂Delay time時不知道該設多少，訂多訂少都會有不好的影響，那倒不如導入這樣的一個機制讓它自我調適。

這邊筆者憑著印象刻了一個簡單的BackOff類別。

程式碼如下：

using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections.ObjectModel;

namespace ConsoleApplication12
{
	public class BackOff
	{
		#region Var
		private int _level;
		private Random _random;
		#endregion

		#region Private Property
		private ReadOnlyCollection<int> m_BackOffWindows { get; set; }

		private Random m_Random
		{
			get
			{
				if (_random == null)
					_random = new Random(Guid.NewGuid().GetHashCode());
				return _random;
			}
		}
		#endregion

		#region Public Property
		/// <summary>
		/// Gets or sets the level.
		/// </summary>
		/// <value>The level.</value>
		public int Level 
		{
			get
			{
				return _level;
			}
			set
			{
				if (value <= 0)
					throw new Exception("Level value must be bigger than zero!");

				if (value > m_BackOffWindows.Count)
					throw new Exception("Level value incorrect!");

				_level = value;
			}
		}
		#endregion
		
		#region Constructor
		/// <summary>
		/// Initializes a new instance of the <see cref="BackOff"/> class.
		/// </summary>
		/// <param name="backOffWindows">The back off windows.</param>
		public BackOff(params int[] backOffWindows)
		{
			CheckBackOffWindows(backOffWindows);		

			this.m_BackOffWindows = new ReadOnlyCollection<int>(backOffWindows.ToList());
			ResetLevel();
		}
		#endregion


		#region Private Method
		/// <summary>
		/// Checks the back off windows.
		/// </summary>
		/// <param name="backOffWindows">The back off windows.</param>
		private void CheckBackOffWindows(params int[] backOffWindows)
		{
			if (backOffWindows == null)
				throw new ArgumentNullException("backOffWindows");

			if (backOffWindows.Length == 0)
				throw new ArgumentException("backOffWindows must contains elements!");

			if (backOffWindows.First() <= 0)
				throw new ArgumentOutOfRangeException("First backOffWindow must bigger than zero!");

			var minValue = -1;
			foreach (var backOffWindow in backOffWindows)
			{
				if (backOffWindow < 0)
					throw new ArgumentOutOfRangeException("backOffWindows must bigger than zero!");

				if (minValue > backOffWindow)
					throw new ArgumentOutOfRangeException("backOffWindows must order by increase!");

				if (minValue == backOffWindow)
					throw new ArgumentOutOfRangeException("Can't contains duplicated elements!");

				minValue = backOffWindow;
			}
		}
		#endregion


		#region Public Method
		/// <summary>
		/// Increases the level.
		/// </summary>
		/// <returns></returns>
		public Boolean IncreaseLevel()
		{
			if (Level >= m_BackOffWindows.Count)
				return false;
			
			Level += 1;
			return true;
		}


		/// <summary>
		/// Decreases the level.
		/// </summary>
		/// <returns></returns>
		public Boolean DecreaseLevel()
		{
			if (Level <= 1)
				return false;
			
			Level -= 1;
			return true;
		}

		public void ResetLevel()
		{
			Level = 1;
		}

		/// <summary>
		/// Gets the next value.
		/// </summary>
		/// <returns></returns>
		public int NextValue()
		{
			var minValue = (Level == 1) ? 0 : m_BackOffWindows[Level - 2];
			var maxValue = m_BackOffWindows[Level - 1];

			return m_Random.Next(minValue, maxValue);
		}
		#endregion
	}
}

實際使用時只要建立物件實體，並帶入backoff window的區間值，透過NextValue取得backoff值就可以了，若是碰撞則呼叫IncreaseLevel去將區間上調。

using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication12
{
	class Program
	{
		static void Main(string[] args)
		{
			BackOff backoff = new BackOff(16,32,64);

			Console.WriteLine("Level1...");
			Console.WriteLine(backoff.NextValue().ToString());
			Console.WriteLine(backoff.NextValue().ToString());
			Console.WriteLine(backoff.NextValue().ToString());
			Console.WriteLine(new string('=', 50));

			backoff.IncreaseLevel();
			Console.WriteLine("Level2...");
			Console.WriteLine(backoff.NextValue().ToString());
			Console.WriteLine(backoff.NextValue().ToString());
			Console.WriteLine(backoff.NextValue().ToString());
			Console.WriteLine(new string('=', 50));

			backoff.IncreaseLevel();
			Console.WriteLine("Level3...");
			Console.WriteLine(backoff.NextValue().ToString());
			Console.WriteLine(backoff.NextValue().ToString());
			Console.WriteLine(backoff.NextValue().ToString());
			Console.WriteLine(new string('=', 50));
		}
	}
}

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