应用缓存和慢进程

Question

我想使用应用程序缓存在我的 ASP.net 3.5 网站上创建一个应用程序范围的提要。我用来填充缓存的数据获取速度很慢，可能长达 10 秒（来自远程服务器的数据馈送）。我的问题/困惑是，构建缓存管理的最佳方式是什么。

private const string CacheKey = "MyCachedString";
private static string lockString = "";

public string GetCachedString()
{
    string data = (string)Cache[CacheKey];
    string newData = "";

    if (data == null)
    {
        // A - Should this method call go here?
        newData = SlowResourceMethod();

        lock (lockString)
        {
            data = (string)Cache[CacheKey];

            if (data != null)
            {
               return data;
            }

            // B - Or here, within the lock?
            newData = SlowResourceMethod();
            Cache[CacheKey] = data = newData;
        }
    }

    return data;
}

实际的方法将由 HttpHandler (.ashx) 呈现。

如果我在“A”点收集数据，我会缩短锁定时间，但最终可能会多次调用外部资源（来自所有试图引用提要的网页）。如果我把它放在'B'点，锁定时间会很长，我认为这是一件坏事。

什么是最好的方法，或者我可以使用更好的模式吗？

任何建议将不胜感激。

Answer 1

我在代码上添加了 cmets。

private const string CacheKey = "MyCachedString";
private static readonly object syncLock = new object();

public string GetCachedString()
{
    string data = (string)Cache[CacheKey];
    string newData = "";

    // start to check if you have it on cache
    if (data == null)
    {
        // A - Should this method call go here?
        // absolut not here
        // newData = SlowResourceMethod();

        // we are now here and wait for someone else to make it or not
        lock (syncLock)
        {
            // now lets see if some one else make it...
            data = (string)Cache[CacheKey];

            // we have it, send it
            if (data != null)
            {
               return data;
            }

            // not have it, now is the time to look for it.
            // B - Or here, within the lock?
            newData = SlowResourceMethod();
            // set it on cache
            Cache[CacheKey] = data = newData;
        }
    }

    return data;
}

对我来说更好的是使用mutex 并且锁定取决于名称CacheKey 而不是锁定所有资源和非相关资源。使用互斥锁，一个基本的简单示例是：

private const string CacheKey = "MyCachedString";
public string GetCachedString()
{
    string data = (string)Cache[CacheKey];
    string newData = "";

    // start to check if you have it on cache
    if (data == null)
    {
        // lock it base on resource key 
        //  (note that not all chars are valid for name)
        var mut = new Mutex(true, CacheKey);

        try
        {   
            // Wait until it is safe to enter.
            // but also add 30 seconds max
            mut.WaitOne(30000);

            // now lets see if some one else make it...
            data = (string)Cache[CacheKey];

            // we have it, send it
            if (data != null)
            {
               return data;
            }

            // not have it, now is the time to look for it.
            // B - Or here, within the lock?
            newData = SlowResourceMethod();
            // set it on cache
            Cache[CacheKey] = data = newData;

        }
        finally
        {
            // Release the Mutex.
            mut.ReleaseMutex();
        }    
    }

    return data;
}

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