"Real" Object References in Distributed Cache? - distributed-cache

Question

I'm personally committed to .net distributed caching solutions, but I think this question is interesting across all platforms.
Is there a distributed caching solution (or generic strategy) that allows to both store objects in the cache while maintaining the integrity of the references between them?
To exemplify - Suppose I have an object Foo foo that references an object Bar bar and also and object Foo foo2 that references that same Bar bar. If I load foo to the cache, a copy of bar is stored along with it. If I also load foo2 to the cache, a separate copy of bar is stored along with that. If I change foo.bar in the cache, the change does not impact foo2.bar :(
Is there an existing distributed cache solution that will enable me to load foo, foo2 and bar into the cache while maintaining the foo.bar foo2.bar references?

Answer 1

First and foremost
I do not know of any distributed system, and I do not pretend to build one. This post explains how you can simulate this behavior with .NET and C# using the IObjectReference interface with serializable objects.
Now, lets go on with the show
I do not know of such a distributed system, but you can somewhat easily achive this with .NET using the IObjectReference interface. Your implementation of ISerializable.GetObjectData would need to call SerializationInfo.SetType to point out a proxy class that implements IObjectReference, and would be able (with help from data provided by your GetObjectData method) to get a reference to the real object that should be used.
Example code:
[Serializable]
internal sealed class SerializationProxy<TOwner, TKey> : ISerializable, IObjectReference {
private const string KeyName = "Key";
private const string InstantiatorName = "Instantiator";
private static readonly Type thisType = typeof(SerializationProxy<TOwner, TKey>);
private static readonly Type keyType = typeof(TKey);
private static readonly Type instantiatorType = typeof(Func<TKey, TOwner>);
private readonly Func<TKey, TOwner> _instantiator;
private readonly TKey _key;
private SerializationProxy() {
}
private SerializationProxy(SerializationInfo info, StreamingContext context) {
if (info == null) throw new ArgumentNullException("info");
_key = (TKey)info.GetValue(KeyName, keyType);
_instantiator = (Func<TKey, TOwner>)info.GetValue(InstantiatorName, instantiatorType);
}
void ISerializable.GetObjectData(SerializationInfo info, StreamingContext context) {
throw new NotSupportedException("This type should never be serialized.");
}
object IObjectReference.GetRealObject(StreamingContext context) {
return _instantiator(_key);
}
internal static void PrepareSerialization(SerializationInfo info, TKey key, Func<TKey, TOwner> instantiator) {
if (info == null) throw new ArgumentNullException("info");
if (instantiator == null) throw new ArgumentNullException("instantiator");
info.SetType(thisType);
info.AddValue(KeyName, key, keyType);
info.AddValue(InstantiatorName, instantiator, instantiatorType);
}
}
This code would be called with SerializationProxy.PrepareSerialization(info, myKey, myKey => LoadedInstances.GetById(myKey)) from your GetObjectData method, and your LoadedInstances.GetById should return the instance from a Dictionary<TKey, WeakReference> or load it from cache/database if it isnt already loaded.
EDIT:
I've wrote some example code to show what I mean.
public static class Program {
public static void Main() {
// Create an item and serialize it.
// Pretend that the bytes are stored in some magical
// domain where everyone lives happily ever after.
var item = new Item { Name = "Bleh" };
var bytes = Serialize(item);
{
// Deserialize those bytes back into the cruel world.
var loadedItem1 = Deserialize<Item>(bytes);
var loadedItem2 = Deserialize<Item>(bytes);
// This should work since we've deserialized identical
// data twice.
Debug.Assert(loadedItem1.Id == loadedItem2.Id);
Debug.Assert(loadedItem1.Name == loadedItem2.Name);
// Notice that both variables refer to the same object.
Debug.Assert(ReferenceEquals(loadedItem1, loadedItem2));
loadedItem1.Name = "Bluh";
Debug.Assert(loadedItem1.Name == loadedItem2.Name);
}
{
// Deserialize those bytes back into the cruel world. (Once again.)
var loadedItem1 = Deserialize<Item>(bytes);
// Notice that we got the same item that we messed
// around with earlier.
Debug.Assert(loadedItem1.Name == "Bluh");
// Once again, force the peaceful object to hide its
// identity, and take on a fake name.
loadedItem1.Name = "Blargh";
var loadedItem2 = Deserialize<Item>(bytes);
Debug.Assert(loadedItem1.Name == loadedItem2.Name);
}
}
#region Serialization helpers
private static readonly IFormatter _formatter
= new BinaryFormatter();
public static byte[] Serialize(ISerializable item) {
using (var stream = new MemoryStream()) {
_formatter.Serialize(stream, item);
return stream.ToArray();
}
}
public static T Deserialize<T>(Byte[] bytes) {
using (var stream = new MemoryStream(bytes)) {
return (T)_formatter.Deserialize(stream);
}
}
#endregion
}
// Supercalifragilisticexpialidocious interface.
public interface IDomainObject {
Guid Id { get; }
}
// Holds all loaded instances using weak references, allowing
// the almighty garbage collector to grab our stuff at any time.
// I have no real data to lend on here, but I _presume_ that this
// wont be to overly evil since we use weak references.
public static class LoadedInstances<T>
where T : class, IDomainObject {
private static readonly Dictionary<Guid, WeakReference> _items
= new Dictionary<Guid, WeakReference>();
public static void Set(T item) {
var itemId = item.Id;
if (_items.ContainsKey(itemId))
_items.Remove(itemId);
_items.Add(itemId, new WeakReference(item));
}
public static T Get(Guid id) {
if (_items.ContainsKey(id)) {
var itemRef = _items[id];
return (T)itemRef.Target;
}
return null;
}
}
[DebuggerDisplay("{Id} {Name}")]
[Serializable]
public class Item : IDomainObject, ISerializable {
public Guid Id { get; private set; }
public String Name { get; set; }
// This constructor can be avoided if you have a
// static Create method that creates and saves new items.
public Item() {
Id = Guid.NewGuid();
LoadedInstances<Item>.Set(this);
}
#region ISerializable Members
public void GetObjectData(SerializationInfo info, StreamingContext context) {
// We're calling SerializationProxy to call GetById(this.Id)
// when we should be deserialized. Notice that we have no
// deserialization constructor. Fxcop will hate us for that.
SerializationProxy<Item, Guid>.PrepareSerialization(info, Id, GetById);
}
#endregion
public static Item GetById(Guid id) {
var alreadyLoaded = LoadedInstances<Item>.Get(id);
if (alreadyLoaded != null)
return alreadyLoaded;
// TODO: Load from storage container (database, cache).
// TODO: The item we load should be passed to LoadedInstances<Item>.Set
return null;
}
}