如何实现定时推送？
详细内容大概分为4个部分，1.应用场景 2.遇到问题 3.设计 4.实现 5.运行效果
1.应用场景
需要定时推送数据，且轻量化的实现。
2.遇到问题
- 如果启动一个定时器去定时轮询
- （1）轮询效率比较低
- （2）每次扫库，已经被执行过记录，仍然会被扫描（只是不会出现在结果集中），会做重复工作
- （3）时效性不够好，如果每小时轮询一次，最差的情况下会有时间误差
- 如何利用“延时消息”，对于每个任务只触发一次，保证效率的同时保证实时性，是今天要讨论的问题。
3.设计
高效延时消息，包含两个重要的数据结构：
（1）环形队列，例如可以创建一个包含3600个slot的环形队列（本质是个数组）
（2）任务集合，环上每一个slot是一个Set
同时，启动一个timer，这个timer每隔1s，在上述环形队列中移动一格，有一个Current Index指针来标识正在检测的slot。
Task结构中有两个很重要的属性：
（1）Cycle-Num：当Current Index第几圈扫描到这个Slot时，执行任务
（2）Task-Function：需要执行的任务指针
假设当前Current Index指向第一格，当有延时消息到达之后，例如希望3610秒之后，触发一个延时消息任务，只需：
（1）计算这个Task应该放在哪一个slot，现在指向1，3610秒之后，应该是第11格，所以这个Task应该放在第11个slot的Set中
（2）计算这个Task的Cycle-Num，由于环形队列是3600格（每秒移动一格，正好1小时），这个任务是3610秒后执行，所以应该绕3610/3600=1圈之后再执行，于是Cycle-Num=1
Current Index不停的移动，每秒移动到一个新slot，这个slot中对应的Set，每个Task看Cycle-Num是不是0：
（1）如果不是0，说明还需要多移动几圈，将Cycle-Num减1
（2）如果是0，说明马上要执行这个Task了，取出Task-Funciton执行（可以用单独的线程来执行Task），并把这个Task从Set中删除
使用了“延时消息”方案之后，“订单48小时后关闭评价”的需求，只需将在订单关闭时，触发一个48小时之后的延时消息即可：
（1）无需再轮询全部订单，效率高
（2）一个订单，任务只执行一次
（3）时效性好，精确到秒（控制timer移动频率可以控制精度）
4.实现
首先写一个方案要理清楚自己的项目结构，我做了如下分层。
Interfaces ， 这层里主要约束延迟消息队列的队列和消息任务行。
public interface IRingQueue<T>
{
/// <summary>
/// Add tasks [add tasks will automatically generate: task Id, task slot location, number of task cycles]
/// </summary>
/// <param name="delayTime">The specified task is executed after N seconds.</param>
/// <param name="action">Definitions of callback</param>
void Add(long delayTime,Action<T> action);
/// <summary>
/// Add tasks [add tasks will automatically generate: task Id, task slot location, number of task cycles]
/// </summary>
/// <param name="delayTime">The specified task is executed after N seconds.</param>
/// <param name="action">Definitions of callback.</param>
/// <param name="data">Parameters used in the callback function.</param>
void Add(long delayTime, Action<T> action, T data);
/// <summary>
/// Add tasks [add tasks will automatically generate: task Id, task slot location, number of task cycles]
/// </summary>
/// <param name="delayTime"></param>
/// <param name="action">Definitions of callback</param>
/// <param name="data">Parameters used in the callback function.</param>
/// <param name="id">Task ID, used when deleting tasks.</param>
void Add(long delayTime, Action<T> action, T data, long id);
/// <summary>
/// Remove tasks [need to know: where the task is, which specific task].
/// </summary>
/// <param name="index">Task slot location</param>
/// <param name="id">Task ID, used when deleting tasks.</param>
void Remove(long id);
/// <summary>
/// Launch queue.
/// </summary>
void Start();
}
public interface ITask
{
}Achieves，这层里实现之前定义的接口，这里写成抽象类是为了后面方便扩展。
using System;
using System.Collections.Concurrent;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using DelayMessageApp.Interfaces;
namespace DelayMessageApp.Achieves.Base
{
public abstract class BaseQueue<T> : IRingQueue<T>
{
private long _pointer = 0L;
private ConcurrentBag<BaseTask<T>>[] _arraySlot;
private int ArrayMax;
/// <summary>
/// Ring queue.
/// </summary>
public ConcurrentBag<BaseTask<T>>[] ArraySlot
{
get { return _arraySlot ?? (_arraySlot = new ConcurrentBag<BaseTask<T>>[ArrayMax]); }
}
public BaseQueue(int arrayMax)
{
if (arrayMax < 60 && arrayMax % 60 == 0)
throw new Exception("Ring queue length cannot be less than 60 and is a multiple of 60 .");
ArrayMax = arrayMax;
}
public void Add(long delayTime, Action<T> action)
{
Add(delayTime, action, default(T));
}
public void Add(long delayTime,Action<T> action,T data)
{
Add(delayTime, action, data,0);
}
public void Add(long delayTime, Action<T> action, T data,long id)
{
NextSlot(delayTime, out long cycle, out long pointer);
ArraySlot[pointer] = ArraySlot[pointer] ?? (ArraySlot[pointer] = new ConcurrentBag<BaseTask<T>>());
var baseTask = new BaseTask<T>(cycle, action, data,id);
ArraySlot[pointer].Add(baseTask);
}
/// <summary>
/// Remove tasks based on ID.
/// </summary>
/// <param name="id"></param>
public void Remove(long id)
{
try
{
Parallel.ForEach(ArraySlot, (ConcurrentBag<BaseTask<T>> collection, ParallelLoopState state) =>
{
var resulTask = collection.FirstOrDefault(p => p.Id == id);
if (resulTask != null)
{
collection.TryTake(out resulTask);
state.Break();
}
});
}
catch (Exception e)
{
Console.WriteLine(e);
}
}
public void Start()
{
while (true)
{
RightMovePointer();
Thread.Sleep(1000);
Console.WriteLine(DateTime.Now.ToString());
}
}
/// <summary>
/// Calculate the information of the next slot.
/// </summary>
/// <param name="delayTime">Delayed execution time.</param>
/// <param name="cycle">Number of turns.</param>
/// <param name="index">Task location.</param>
private void NextSlot(long delayTime, out long cycle,out long index)
{
try
{
var circle = delayTime / ArrayMax;
var second = delayTime % ArrayMax;
var current_pointer = GetPointer();
var queue_index = 0L;
if (delayTime - ArrayMax > ArrayMax)
{
circle = 1;
}
else if (second > ArrayMax)
{
circle += 1;
}
if (delayTime - circle * ArrayMax < ArrayMax)
{
second = delayTime - circle * ArrayMax;
}
if (current_pointer + delayTime >= ArrayMax)
{
cycle = (int)((current_pointer + delayTime) / ArrayMax);
if (current_pointer + second - ArrayMax < 0)
{
queue_index = current_pointer + second;
}
else if (current_pointer + second - ArrayMax > 0)
{
queue_index = current_pointer + second - ArrayMax;
}
}
else
{
cycle = 0;
queue_index = current_pointer + second;
}
index = queue_index;
}
catch (Exception e)
{
Console.WriteLine(e);
throw;
}
}
/// <summary>
/// Get the current location of the pointer.
/// </summary>
/// <returns></returns>
private long GetPointer()
{
return Interlocked.Read(ref _pointer);
}
/// <summary>
/// Reset pointer position.
/// </summary>
private void ReSetPointer()
{
Interlocked.Exchange(ref _pointer, 0);
}
/// <summary>
/// Pointer moves clockwise.
/// </summary>
private void RightMovePointer()
{
try
{
if (GetPointer() >= ArrayMax - 1)
{
ReSetPointer();
}
else
{
Interlocked.Increment(ref _pointer);
}
var pointer = GetPointer();
var taskCollection = ArraySlot[pointer];
if (taskCollection == null || taskCollection.Count == 0) return;
Parallel.ForEach(taskCollection, (BaseTask<T> task) =>
{
if (task.Cycle > 0)
{
task.SubCycleNumber();
}
if (task.Cycle <= 0)
{
taskCollection.TryTake(out task);
task.TaskAction(task.Data);
}
});
}
catch (Exception e)
{
Console.WriteLine(e);
throw;
}
}
}
}
using System;
using System.Threading;
using DelayMessageApp.Interfaces;
namespace DelayMessageApp.Achieves.Base
{
public class BaseTask<T> : ITask
{
private long _cycle;
private long _id;
private T _data;
public Action<T> TaskAction { get; set; }
public long Cycle
{
get { return Interlocked.Read(ref _cycle); }
set { Interlocked.Exchange(ref _cycle, value); }
}
public long Id
{
get { return _id; }
set { _id = value; }
}
public T Data
{
get { return _data; }
set { _data = value; }
}
public BaseTask(long cycle, Action<T> action, T data,long id)
{
Cycle = cycle;
TaskAction = action;
Data = data;
Id = id;
}
public BaseTask(long cycle, Action<T> action,T data)
{
Cycle = cycle;
TaskAction = action;
Data = data;
}
public BaseTask(long cycle, Action<T> action)
{
Cycle = cycle;
TaskAction = action;
}
public void SubCycleNumber()
{
Interlocked.Decrement(ref _cycle);
}
}
}Logic，这层主要实现调用逻辑，调用者最终只需要关心把任务放进队列并指定什么时候执行就行了，根本不需要关心其它的任何信息。
public static void Start()
{
//1.Initialize queues of different granularity.
IRingQueue<NewsModel> minuteRingQueue = new MinuteQueue<NewsModel>();
//2.Open thread.
var lstTasks = new List<Task>
{
Task.Factory.StartNew(minuteRingQueue.Start)
};
//3.Add tasks performed in different periods.
minuteRingQueue.Add(5, new Action<NewsModel>((NewsModel newsObj) =>
{
Console.WriteLine(newsObj.News);
}), new NewsModel() { News = "Trump's visit to China！" });
minuteRingQueue.Add(10, new Action<NewsModel>((NewsModel newsObj) =>
{
Console.WriteLine(newsObj.News);
}), new NewsModel() { News = "Putin Pu's visit to China！" });
minuteRingQueue.Add(60, new Action<NewsModel>((NewsModel newsObj) =>
{
Console.WriteLine(newsObj.News);
}), new NewsModel() { News = "Eisenhower's visit to China！" });
minuteRingQueue.Add(120, new Action<NewsModel>((NewsModel newsObj) =>
{
Console.WriteLine(newsObj.News);
}), new NewsModel() { News = "Xi Jinping's visit to the US！" });
//3.Waiting for all tasks to complete is usually not completed. Because there is an infinite loop.
//F5 Run the program and see the effect.
Task.WaitAll(lstTasks.ToArray());
Console.Read();
}Models，这层就是用来在延迟任务中带入的数据模型类而已了。自己用的时候换成任意自定义类型都可以。
