概述
IIS线程池中的线程数量是有限制的。当有多个长时间请求时，可能会耗尽IIS可用线程。出现503错误。在MVC中。当遇到非CPU操作的长时间请求时，MVC提供了异步方法来解决这个问题。
例：利用async和await实现异步方法
// GET: Async
[AsyncTimeout(1000)]
public async Task<ActionResult> Index()
{
var data = await GetPageTaskAsync("http://www.baidu.com");
return data;
}
回到Orchard,在Orchard启动时,需要一定时间加载模块插件，这时候如果出现大量请求，则有可能出现上面提到的错误。来看一下Orchard中是如何解决这个问题。
项目结构Orchard.WarmupStarter ，是一个单独的项目。方便复用
具体实现
一句话概括实现步骤: 启动时初始化一个异步请求列表，初始化期间有请求到来时，把该请求添加到请求列表中。当系统初始化完成时回调起步请求列表并且清空。
Starter.cs
/// <summary>
/// Run the initialization delegate asynchronously in a queued work item
/// </summary>
public void LaunchStartupThread(HttpApplication application) {
// Make sure incoming requests are queued
WarmupHttpModule.SignalWarmupStart();
ThreadPool.QueueUserWorkItem(
state => {
try {
var result = _initialization(application);
_initializationResult = result;
}
catch (Exception e) {
lock (_synLock) {
_error = e;
_previousError = null;
}
}
finally {
// Execute pending requests as the initialization is over
WarmupHttpModule.SignalWarmupDone();
}
});
}
WarmupHttpModule.SignalWarmupStart(); 系统开始加载，初始化请求列表
public static void SignalWarmupStart() {
lock (_synLock) {
if (_awaiting == null) {
_awaiting = new List<Action>();
}
}
}
WarmupHttpModule.SignalWarmupDone(); 系统加载完成，回调请求列表并且清空
public static void SignalWarmupDone() {
IList<Action> temp;
lock (_synLock) {
temp = _awaiting;
_awaiting = null;
}
if (temp != null) {
foreach (var action in temp) {
action();
}
}
}
WebConfig中注册WarmupHttpModule
请求到来时，执行WarmupHttpModule.BeginBeginRequest回调, 如果加载中，则请求添加到异步列表，否则继续执行回调
private IAsyncResult BeginBeginRequest(object sender, EventArgs e, AsyncCallback cb, object extradata) {
// host is available, process every requests, or file is processed
if (!InWarmup() || WarmupUtility.DoBeginRequest(_context)) {
var asyncResult = new DoneAsyncResult(extradata);
cb(asyncResult);
return asyncResult;
}
else {
// this is the "on hold" execution path
var asyncResult = new WarmupAsyncResult(cb, extradata);
Await(asyncResult.Completed);
return asyncResult;
}
}
异步编程模型 DoneAsyncResult 和 WarmupAsyncResult 实现IAsyncResult
WarmupAsyncResult
/// <summary>
/// AsyncResult for "on hold" request (resumes when "Completed()" is called)
/// </summary>
private class WarmupAsyncResult : IAsyncResult {
private readonly EventWaitHandle _eventWaitHandle = new AutoResetEvent(false/*initialState*/);
private readonly AsyncCallback _cb;
private readonly object _asyncState;
private bool _isCompleted;
public WarmupAsyncResult(AsyncCallback cb, object asyncState) {
_cb = cb;
_asyncState = asyncState;
_isCompleted = false;
}
public void Completed() {
_isCompleted = true;
_eventWaitHandle.Set();
_cb(this);
}
bool IAsyncResult.CompletedSynchronously {
get { return false; }
}
bool IAsyncResult.IsCompleted {
get { return _isCompleted; }
}
object IAsyncResult.AsyncState {
get { return _asyncState; }
}
WaitHandle IAsyncResult.AsyncWaitHandle {
get { return _eventWaitHandle; }
}
}
DoneAsyncResult(不阻塞)
/// <summary>
/// Async result for "ok to process now" requests
/// </summary>
private class DoneAsyncResult : IAsyncResult {
private readonly object _asyncState;
private static readonly WaitHandle _waitHandle = new ManualResetEvent(true/*initialState*/);
public DoneAsyncResult(object asyncState) {
_asyncState = asyncState;
}
bool IAsyncResult.CompletedSynchronously {
get { return true; }
}
bool IAsyncResult.IsCompleted {
get { return true; }
}
WaitHandle IAsyncResult.AsyncWaitHandle {
get { return _waitHandle; }
}
object IAsyncResult.AsyncState {
get { return _asyncState; }
}
}
总结
Orchard.WarmupStarter 已封装好相关热启动代码， 实际项目中如果初始化时间比较长，稍改造Orchard.WarmupStarter就可复用到自己的项目中。
参考
http://www.cnblogs.com/alby/archive/2012/10/18/orchard-WarmupStarter.html