ConsistentHash implementation in C# 4.0

For those not familiar with ConsistentHash, start here: Consistent Hashing

The following is an attempt to implement the functionality using features of C# 3/4. I needed this functionality for the distributed cache project that I was working on (Available at HoC )

namespace HoC.Common
{
    public class ConsistentHash  : ICloneable
    {
        SortedList<string, string> itemCircle = new SortedList<string, string>();

        public string GetNearestItem(string key)
        {
            if (itemCircle.Count == 0)
                throw new ConsistentHashCircleEmpty();

            string keyHash = Hasher.GetHash(key);

            //find the last item that is just after the passed in key (clockwise)
            Func<KeyValuePair<string, string>, bool> stringCompare = x => (x.Key.CompareTo(keyHash) > 0);
            KeyValuePair<string, string> item = itemCircle.FirstOrDefault(stringCompare);

            if (string.IsNullOrEmpty(item.Key)) 
            {
                //if no item, fallback to the first item => traverse circle clockwise 
                return itemCircle.First().Value; 
            }

            return item.Value;
        }

        public void StoreItem(string item)
        {
            string hash = Hasher.GetHash(item);
            itemCircle[hash] = item;
        }

        public void RemoveItem(string item)
        {
            string hash = Hasher.GetHash(item);
            if (itemCircle.ContainsKey(hash))
                itemCircle.Remove(hash);
        }

        public string GetNextItemInCircle(string key)
        {
            return GetNearestItem(key);
        }

        //this function traverses anticlockwise, whereas GetNearestItem() traverses clockwise
        public string GetPreviousItemInCircle(string key)
        {
            if (itemCircle.Count == 0)
                throw new ConsistentHashCircleEmpty();

            string keyHash = Hasher.GetHash(key);

            //traverse, anticlock wise , find first item
            Func<KeyValuePair<string, string>, bool> stringCompare = x => (x.Key.CompareTo(keyHash) < 0);
            KeyValuePair<string, string> item = itemCircle.FirstOrDefault(stringCompare);

            if (string.IsNullOrEmpty(item.Key))
            {
                //if no item, fallback to the last item => traverse circle anti clockwise 
                return itemCircle.Last().Value;
            }

            return item.Value;
        }



        public object Clone()
        {
            return (ConsistentHash)MemberwiseClone();
        }
    }

    class ConsistentHashCircleEmpty : ApplicationException
    {

    }


}

Couple of drill downs:

1.) Basically each objects gets assigned a hash (Hasher class internally uses RIPEMD160Managed) so that it can be arranged in the circle and then gets stored to the internal circle. RIPEMD160Managed though a bit slower, supposedly has the lowest collision.

2.) The circle as seen is implemented using a SortedList list class - itemCircle

3.) The core functionality is implemented using the two methods:

3.1) GetNearestItem : This method traverses clockwise - basically find the object that comes up next in the sorted list after the given key.

3.2) GetPreviousItemInCircle : Opposite of GetNearestItem. Traverses anti-clockwise.

In the project that I use this internally, the objects are all serializable, hence the objects could be easily represented as a string.

Object Pool - Quick and Short in .NET 4.0

A simple object pool that could be used to maintain a set of objects readily available in memory, especially if you see that the object creation time is heavy. Eg:- creation of a MemoryStream on a need basis is typically time consuming in a server based app. In this case, it could best to have a set of MemoryStream objects readily available in memory. But, we definitely have to make sure to couple of things first :

1.) The way the object pool is filled happens is async.
2.) Access to a pool instance from the client app is thread safe.

To solve 1, we could exploit the AsyncMethodCaller together with its BeginInvoke(), while to make sure that the access to the pool is thread safe, we could check out the ConcurrentBag in .NET 4.0. So, where is the code?!

namespace HoC.Client
{
    /// <summary>
    /// maintains a pool/list of objects, provides it on request
    /// usually used for classes whose construction call is heavy
    /// </summary>
    /// <typeparam name="W"></typeparam>
    public class ObjectInstancePool<W>
    {
        private const int objectCount = 20; //can be updated to receive through the constructor
        private ConcurrentBag<W> objectList = new ConcurrentBag<W>();

        public ObjectInstancePool()
        {
            //refresh the bag first
            new AsyncMethodCaller(Filler).BeginInvoke(null, null);
        }

        public W GetInstance()
        {
            W result;

            if (!(objectList.TryTake(out result)))
            {
                result = Activator.CreateInstance<W>();
            }
            else
                new AsyncMethodCaller(Filler).BeginInvoke(null, null); //refresh the bag
            
            return result;
        }

        public delegate void AsyncMethodCaller();

        private void Filler()
        {
            while (objectList.Count < objectCount)
            {
                objectList.Add(Activator.CreateInstance<W>());
            }
        }
    }
}

I totally like it short and sweet :)