我知道在某些情况下，例如长时间运行的进程，锁定ASP.NET缓存非常重要，以避免其他用户对该资源的后续请求再次执行长进程而不是访问缓存。
在ASP.NET中实现缓存锁定的最佳方法是什么？
答案 0 :(得分：104)
这是基本模式：
在代码中，它看起来像这样：
private static object ThisLock = new object();
public string GetFoo()
{
// try to pull from cache here
lock (ThisLock)
{
// cache was empty before we got the lock, check again inside the lock
// cache is still empty, so retreive the value here
// store the value in the cache here
}
// return the cached value here
}
答案 1 :(得分：28)
为了完整性，完整的例子看起来像这样。
private static object ThisLock = new object();
...
object dataObject = Cache["globalData"];
if( dataObject == null )
{
lock( ThisLock )
{
dataObject = Cache["globalData"];
if( dataObject == null )
{
//Get Data from db
dataObject = GlobalObj.GetData();
Cache["globalData"] = dataObject;
}
}
}
return dataObject;
答案 2 :(得分：13)
为了回应帕维尔的说法，我相信这是最安全的写作方式
private T GetOrAddToCache<T>(string cacheKey, GenericObjectParamsDelegate<T> creator, params object[] creatorArgs) where T : class, new()
{
T returnValue = HttpContext.Current.Cache[cacheKey] as T;
if (returnValue == null)
{
lock (this)
{
returnValue = HttpContext.Current.Cache[cacheKey] as T;
if (returnValue == null)
{
returnValue = creator(creatorArgs);
if (returnValue == null)
{
throw new Exception("Attempt to cache a null reference");
}
HttpContext.Current.Cache.Add(
cacheKey,
returnValue,
null,
System.Web.Caching.Cache.NoAbsoluteExpiration,
System.Web.Caching.Cache.NoSlidingExpiration,
CacheItemPriority.Normal,
null);
}
}
}
return returnValue;
}
答案 3 :(得分：12)
不需要锁定整个缓存实例，而只需要锁定要插入的特定密钥。 即使用男厕时，无需阻止进入女厕所：）
下面的实现允许使用并发字典锁定特定的缓存密钥。这样，您可以同时为两个不同的键运行GetOrAdd（） - 但不能同时为同一个键运行。
using System;
using System.Collections.Concurrent;
using System.Web.Caching;
public static class CacheExtensions
{
private static ConcurrentDictionary<string, object> keyLocks = new ConcurrentDictionary<string, object>();
/// <summary>
/// Get or Add the item to the cache using the given key. Lazily executes the value factory only if/when needed
/// </summary>
public static T GetOrAdd<T>(this Cache cache, string key, int durationInSeconds, Func<T> factory)
where T : class
{
// Try and get value from the cache
var value = cache.Get(key);
if (value == null)
{
// If not yet cached, lock the key value and add to cache
lock (keyLocks.GetOrAdd(key, new object()))
{
// Try and get from cache again in case it has been added in the meantime
value = cache.Get(key);
if (value == null && (value = factory()) != null)
{
// TODO: Some of these parameters could be added to method signature later if required
cache.Insert(
key: key,
value: value,
dependencies: null,
absoluteExpiration: DateTime.Now.AddSeconds(durationInSeconds),
slidingExpiration: Cache.NoSlidingExpiration,
priority: CacheItemPriority.Default,
onRemoveCallback: null);
}
// Remove temporary key lock
keyLocks.TryRemove(key, out object locker);
}
}
return value as T;
}
}
答案 4 :(得分：2)
Craig Shoemaker在asp.net缓存方面做了很棒的表演： http://polymorphicpodcast.com/shows/webperformance/
答案 5 :(得分：2)
我提出了以下扩展方法：
private static readonly object _lock = new object();
public static TResult GetOrAdd<TResult>(this Cache cache, string key, Func<TResult> action, int duration = 300) {
TResult result;
var data = cache[key]; // Can't cast using as operator as TResult may be an int or bool
if (data == null) {
lock (_lock) {
data = cache[key];
if (data == null) {
result = action();
if (result == null)
return result;
if (duration > 0)
cache.Insert(key, result, null, DateTime.UtcNow.AddSeconds(duration), TimeSpan.Zero);
} else
result = (TResult)data;
}
} else
result = (TResult)data;
return result;
}
我使用了@John Owen和@ user378380的答案。我的解决方案允许您在缓存中存储int和bool值。
如果有任何错误或是否可以写得更好，请纠正我。
答案 6 :(得分：1)
我最近看到一种称为正确状态袋访问模式的模式，似乎触及了这一点。
我修改了一下它是线程安全的。
http://weblogs.asp.net/craigshoemaker/archive/2008/08/28/asp-net-caching-and-performance.aspx
private static object _listLock = new object();
public List List() {
string cacheKey = "customers";
List myList = Cache[cacheKey] as List;
if(myList == null) {
lock (_listLock) {
myList = Cache[cacheKey] as List;
if (myList == null) {
myList = DAL.ListCustomers();
Cache.Insert(cacheKey, mList, null, SiteConfig.CacheDuration, TimeSpan.Zero);
}
}
}
return myList;
}
答案 7 :(得分：0)
CodeGuru的这篇文章解释了各种缓存锁定方案以及ASP.NET缓存锁定的一些最佳实践：
答案 8 :(得分：0)
我已经写了一个解决该特定问题的库：Rocks.Caching
此外，我还详细介绍了这个问题，并解释了为什么它很重要here。
答案 9 :(得分：0)
我修改了@ user378380的代码以获得更大的灵活性。而不是返回TResult现在返回对象以按顺序接受不同类型。还添加了一些灵活性参数。所有的想法都属于 @ user378380。
private static readonly object _lock = new object();
//If getOnly is true, only get existing cache value, not updating it. If cache value is null then set it first as running action method. So could return old value or action result value.
//If getOnly is false, update the old value with action result. If cache value is null then set it first as running action method. So always return action result value.
//With oldValueReturned boolean we can cast returning object(if it is not null) appropriate type on main code.
public static object GetOrAdd<TResult>(this Cache cache, string key, Func<TResult> action,
DateTime absoluteExpireTime, TimeSpan slidingExpireTime, bool getOnly, out bool oldValueReturned)
{
object result;
var data = cache[key];
if (data == null)
{
lock (_lock)
{
data = cache[key];
if (data == null)
{
oldValueReturned = false;
result = action();
if (result == null)
{
return result;
}
cache.Insert(key, result, null, absoluteExpireTime, slidingExpireTime);
}
else
{
if (getOnly)
{
oldValueReturned = true;
result = data;
}
else
{
oldValueReturned = false;
result = action();
if (result == null)
{
return result;
}
cache.Insert(key, result, null, absoluteExpireTime, slidingExpireTime);
}
}
}
}
else
{
if(getOnly)
{
oldValueReturned = true;
result = data;
}
else
{
oldValueReturned = false;
result = action();
if (result == null)
{
return result;
}
cache.Insert(key, result, null, absoluteExpireTime, slidingExpireTime);
}
}
return result;
}