Concurrent Requests for Writeable Session Variables

2017-02-13

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RM的開發日記
2017-11-30

在之前有討論到不同網頁使用到 Writeable Session 時會卡來卡去的狀況 Lock or Blocking(使用Session要小心網頁會被卡住哦!)，

或是 Web Service 如果開啟 Session 存取的話，預設是 Writeable Session 模式(設定 WebService 使用 ReadOnly Session)。

解法除了設定 SessionStateBehavior.ReadOnly 外，

When a Single ASP.NET Client makes Concurrent Requests for Writeable Session Variables 這篇還提供2個方式，

1.降低 Session Lock Check 的時間

2.實作 Lockless SessionStateStoreProvider

維護舊系統的朋友，如果改了 ReadOnly 又怕會影響到什麼功能，可以參考看看哦!

在寫入 Session 時會造成 500ms 的 Delay 時間(Storing Anything in ASP.NET Session Causes 500ms Delays)，

如果一定會寫到 Writeable Session 時，有以下2個方式，

1.降低 Session Lock Check 的時間

可以在 Global.asax.cs 的 Application_Start Method 去設定預設值，如下，

protected void Application_Start(object sender, EventArgs e)
{
	var sessionStateModuleType = typeof(SessionStateModule);
	var pollingIntervalFieldInfo = sessionStateModuleType.GetField("LOCKED_ITEM_POLLING_INTERVAL", BindingFlags.NonPublic | BindingFlags.Static);
	var orgpollingInterval = pollingIntervalFieldInfo.GetValue(null);
	pollingIntervalFieldInfo.SetValue(null, 30); // default 500ms
	var pollingDeltaFieldInfo = sessionStateModuleType.GetField("LOCKED_ITEM_POLLING_DELTA", BindingFlags.NonPublic | BindingFlags.Static);
	var orgPollingDelta = pollingDeltaFieldInfo.GetValue(null);
	pollingDeltaFieldInfo.SetValue(null, TimeSpan.FromMilliseconds(15.0)); // default 250ms
}

所以每次 request 時，可以發現 Polling Interval 從 500ms 改成 30ms, Polling Delta 從 250ms 改成 15ms，如下，

2.實作 Lockless SessionStateStoreProvider

public class LocklessInProcSessionStateStore : SessionStateStoreProviderBase
{
	private SessionStateStoreProviderBase _store;

	public override void Initialize(string name, NameValueCollection config)
	{
		base.Initialize(name, config);

		var storeType = typeof(SessionStateStoreProviderBase).Assembly.GetType("System.Web.SessionState.InProcSessionStateStore");
		_store = (SessionStateStoreProviderBase)Activator.CreateInstance(storeType);
		_store.Initialize(name, config);
	}

	public override void Dispose()
	{
		_store.Dispose();
	}

	public override bool SetItemExpireCallback(SessionStateItemExpireCallback expireCallback)
	{
		return _store.SetItemExpireCallback(expireCallback);
	}

	public override void InitializeRequest(HttpContext context)
	{
		_store.InitializeRequest(context);
	}

	public override SessionStateStoreData GetItem(HttpContext context, string id, out bool locked, out TimeSpan lockAge, out object lockId,
		out SessionStateActions actions)
	{
		var returnValue = _store.GetItem(context, id, out locked, out lockAge, out lockId, out actions);
		if (returnValue == null && lockId != null)
		{
			_store.ReleaseItemExclusive(context, id, lockId);
			returnValue = _store.GetItem(context, id, out locked, out lockAge, out lockId, out actions);
		}

		return returnValue;
	}

	public override SessionStateStoreData GetItemExclusive(HttpContext context, string id, out bool locked, out TimeSpan lockAge,
		out object lockId, out SessionStateActions actions)
	{
		var returnValue = _store.GetItemExclusive(context, id, out locked, out lockAge, out lockId, out actions);
		if (returnValue == null && lockId != null)
		{
			_store.ReleaseItemExclusive(context, id, lockId);
			returnValue = _store.GetItemExclusive(context, id, out locked, out lockAge, out lockId, out actions);
		}

		return returnValue;
	}

	public override void ReleaseItemExclusive(HttpContext context, string id, object lockId)
	{
		_store.ReleaseItemExclusive(context, id, lockId);
	}

	public override void SetAndReleaseItemExclusive(HttpContext context, string id, SessionStateStoreData item, object lockId, bool newItem)
	{
		_store.SetAndReleaseItemExclusive(context, id, item, lockId, newItem);
	}

	public override void RemoveItem(HttpContext context, string id, object lockId, SessionStateStoreData item)
	{
		_store.RemoveItem(context, id, lockId, item);
	}

	public override void ResetItemTimeout(HttpContext context, string id)
	{
		_store.ResetItemTimeout(context, id);
	}

	public override SessionStateStoreData CreateNewStoreData(HttpContext context, int timeout)
	{
		return _store.CreateNewStoreData(context, timeout);
	}

	public override void CreateUninitializedItem(HttpContext context, string id, int timeout)
	{
		_store.CreateUninitializedItem(context, id, timeout);
	}

	public override void EndRequest(HttpContext context)
	{
		_store.EndRequest(context);
	}

}

在 web.config 中加入設定自定的 SessionState，如下，

<sessionState mode="Custom" customProvider="LocklessInProcSessionStateStore" cookieless="false" timeout="1" >
  <providers>
	<add name="LocklessInProcSessionStateStore" 
		 type="你專案的namespace.LocklessInProcSessionStateStore"/>
  </providers>
</sessionState>

有需要的朋友可以試看看哦! 特別是那種維護舊系統的朋友，如果改了 ReadOnly 又怕會影響到什麼功能，就可以用方法1 哦!