MemCache分布式缓存的一个bug

       Memcached分布式缓存策略不是由服务器端至支持的，多台服务器之间并不知道彼此的存在。分布式的实现是由客户端代码（Memcached.ClientLibrary）通过缓存key-server映射来实现的，基本原理就是对缓存key求hash值，用hash值对服务器数量进行模运算，该key值被分配到模运算结果为索引的那台server上。

       Memcached.ClientLibrary对缓存key计算hashcode的核心算法如下：

/// <summary>
/// Returns appropriate SockIO object given
/// string cache key and optional hashcode.
/// 
/// Trys to get SockIO from pool.  Fails over
/// to additional pools in event of server failure.
/// </summary>
/// <param name="key">hashcode for cache key</param>
/// <param name="hashCode">if not null, then the int hashcode to use</param>
/// <returns>SockIO obj connected to server</returns>
public SockIO GetSock(string key, object hashCode)
{
    string hashCodeString = "<null>";
    if(hashCode != null)
        hashCodeString = hashCode.ToString();

    if(Log.IsDebugEnabled)
    {
        Log.Debug(GetLocalizedString("cache socket pick").Replace("$$Key$$", key).Replace("$$HashCode$$", hashCodeString));
    }

    if (key == null || key.Length == 0)
    {
        if(Log.IsDebugEnabled)
        {
            Log.Debug(GetLocalizedString("null key"));
        }
        return null;
    }

    if(!_initialized)
    {
        if(Log.IsErrorEnabled)
        {
            Log.Error(GetLocalizedString("get socket from uninitialized pool"));
        }
        return null;
    }

    // if no servers return null
    if(_buckets.Count == 0)
        return null;

    // if only one server, return it
    if(_buckets.Count == 1)
        return GetConnection((string)_buckets[0]);

    int tries = 0;

    // generate hashcode
    int hv;
    if(hashCode != null)
    {
        hv = (int)hashCode;
    }
    else
    {

        // NATIVE_HASH = 0
        // OLD_COMPAT_HASH = 1
        // NEW_COMPAT_HASH = 2
        switch(_hashingAlgorithm)
        {
            case HashingAlgorithm.Native:
                hv = key.GetHashCode();
                break;

            case HashingAlgorithm.OldCompatibleHash:
                hv = OriginalHashingAlgorithm(key);
                break;

            case HashingAlgorithm.NewCompatibleHash:
                hv = NewHashingAlgorithm(key);
                break;

            default:
                // use the native hash as a default
                hv = key.GetHashCode();
                _hashingAlgorithm = HashingAlgorithm.Native;
                break;
        }
    }

    // keep trying different servers until we find one
    while(tries++ <= _buckets.Count)
    {
        // get bucket using hashcode 
        // get one from factory
        int bucket = hv % _buckets.Count;
        if(bucket < 0)
            bucket += _buckets.Count;

        SockIO sock = GetConnection((string)_buckets[bucket]);

        if(Log.IsDebugEnabled)
        {
            Log.Debug(GetLocalizedString("cache choose").Replace("$$Bucket$$", _buckets[bucket].ToString()).Replace("$$Key$$", key));
        }

        if(sock != null)
            return sock;

        // if we do not want to failover, then bail here
        if(!_failover)
            return null;

        // if we failed to get a socket from this server
        // then we try again by adding an incrementer to the
        // current key and then rehashing 
        switch(_hashingAlgorithm)
        {
            case HashingAlgorithm.Native:
                hv += ((string)("" + tries + key)).GetHashCode();
                break;

            case HashingAlgorithm.OldCompatibleHash:
                hv += OriginalHashingAlgorithm("" + tries + key);
                break;

            case HashingAlgorithm.NewCompatibleHash:
                hv += NewHashingAlgorithm("" + tries + key);
                break;

            default:
                // use the native hash as a default
                hv += ((string)("" + tries + key)).GetHashCode();
                _hashingAlgorithm = HashingAlgorithm.Native;
                break;
        }
    }

    return null;
}

       从源码中（62--82行代码）可以发现，计算hashcode的算法共三种：

      （1）HashingAlgorithm.Native: 即使用.NET本身的hash算法，速度快，但与其他client可能不兼容，例如需要和java、ruby的client共享缓存的情况；

      （2）HashingAlgorithm.OldCompatibleHash: 可以与其他客户端兼容，但速度慢；

      （3）HashingAlgorithm.NewCompatibleHash: 可以与其他客户端兼容，据称速度快。

       进一步分析发现，Memcached.ClientLibrary默认计算缓存key的hashcode的方式就是HashingAlgorithm.Native，而HashingAlgorithm.Native计算hashcode的算法为“hv = key.GetHashCode()”，即用了.net类库string类型自带的GetHashCode()方法。

       Bug就要浮现出来了，根据微软（http://msdn.microsoft.com/zh-cn/library/system.object.gethashcode.aspx）对GetHashCode的解释：the .NET Framework does not guarantee the default implementation of the GetHashCode method, and the value this method returns may differ between .NET Framework versions and platforms, such as 32-bit and 64-bit platforms。string类型的GetHashCode()函数并不能保证不同平台同一个字符串返回的hash值相同，这样问题就出来了，对于不同服务器的同一缓存key来说，产生的hashcode可能不同，同一key对应的数据可能缓存到了不同的MemCache服务器上，数据的一致性无法保证，清除缓存的代码也可能失效。

// 64位 4.0
[__DynamicallyInvokable, ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail), SecuritySafeCritical]
public unsafe override int GetHashCode()
{
    if (HashHelpers.s_UseRandomizedStringHashing)
    {
        return string.InternalMarvin32HashString(this, this.Length, 0L);
    }
    IntPtr arg_25_0;
    IntPtr expr_1C = arg_25_0 = this;
    if (expr_1C != 0)
    {
        arg_25_0 = (IntPtr)((int)expr_1C + RuntimeHelpers.OffsetToStringData);
    }
    char* ptr = arg_25_0;
    int num = 5381;
    int num2 = num;
    char* ptr2 = ptr;
    int num3;
    while ((num3 = (int)(*(ushort*)ptr2)) != 0)
    {
        num = ((num << 5) + num ^ num3);
        num3 = (int)(*(ushort*)(ptr2 + (IntPtr)2 / 2));
        if (num3 == 0)
        {
            break;
        }
        num2 = ((num2 << 5) + num2 ^ num3);
        ptr2 += (IntPtr)4 / 2;
    }
    return num + num2 * 1566083941;
}


// 64位 2.0
// string
[ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
public unsafe override int GetHashCode()
{
    IntPtr arg_0F_0;
    IntPtr expr_06 = arg_0F_0 = this;
    if (expr_06 != 0)
    {
        arg_0F_0 = (IntPtr)((int)expr_06 + RuntimeHelpers.OffsetToStringData);
    }
    char* ptr = arg_0F_0;
    int num = 5381;
    int num2 = num;
    char* ptr2 = ptr;
    int num3;
    while ((num3 = (int)(*(ushort*)ptr2)) != 0)
    {
        num = ((num << 5) + num ^ num3);
        num3 = (int)(*(ushort*)(ptr2 + (IntPtr)2 / 2));
        if (num3 == 0)
        {
            break;
        }
        num2 = ((num2 << 5) + num2 ^ num3);
        ptr2 += (IntPtr)4 / 2;
    }
    return num + num2 * 1566083941;
}

//32位 4.0
[__DynamicallyInvokable, ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail), SecuritySafeCritical]
public unsafe override int GetHashCode()
{
    if (HashHelpers.s_UseRandomizedStringHashing)
    {
        return string.InternalMarvin32HashString(this, this.Length, 0L);
    }
    IntPtr arg_25_0;
    IntPtr expr_1C = arg_25_0 = this;
    if (expr_1C != 0)
    {
        arg_25_0 = (IntPtr)((int)expr_1C + RuntimeHelpers.OffsetToStringData);
    }
    char* ptr = arg_25_0;
    int num = 352654597;
    int num2 = num;
    int* ptr2 = (int*)ptr;
    int i;
    for (i = this.Length; i > 2; i -= 4)
    {
        num = ((num << 5) + num + (num >> 27) ^ *ptr2);
        num2 = ((num2 << 5) + num2 + (num2 >> 27) ^ ptr2[(IntPtr)4 / 4]);
        ptr2 += (IntPtr)8 / 4;
    }
    if (i > 0)
    {
        num = ((num << 5) + num + (num >> 27) ^ *ptr2);
    }
    return num + num2 * 1566083941;
}

      解决问题的方法就是不要用MemCache默认的hash算法，实现方式有两种：

     （1）初始化MemCache服务器的时候，指定为MemCahce自带其它的hash算法，代码为“this.pool.HashingAlgorithm = HashingAlgorithm.OldCompatibleHash;”。

     （2）自定义hash算法，调用set()、get()、delete()等方式时传递hash值，这几个方法有参数传递hashcode的重载。

       参考资料：分析Memcached客户端如何把缓存数据分布到多个服务器上(转)、memcached client - memcacheddotnet (Memcached.ClientLibrary) 1.1.5、memcache分布式实现、Object.GetHashCode 方法、关于 HashCode做key的可能性。