Event Up-Conversion With Keeping Event-Class Name - neventstore

Question

NEventStore 3.2.0.0
As far as I found out it is required by NEventStore that old event-types must kept around for event up-conversion.
To keep them deserializing correctly in the future they must have an unique name. It is suggested to call it like EventEVENT_VERSION.
Is there any way to avoid EventV1, EventV2,..., EventVN cluttering up your domain model and simply keep using Event?
What are your strategies?

Answer 1

In a question long, long time ago, an answer was missing...
In the discussion referred in the comments, I came up with an - I would say - elegant solution:
Don't save the type-name but an (versioned) identifier
The identifier is set by an attribute on class-level, i.e.
namespace CurrentEvents
{
[Versioned("EventSomethingHappened", 0)] // still version 0
public class EventSomethingHappened
{
...
}
}
This identifier should get serialized in/beside the payload. In serialized form
"Some.Name.Space.EventSomethingHappened" -> "EventSomethingHappened|0"
When another version of this event is required, the current version is copied in an "legacy" assembly or just in another Namespace and renamed (type-name) to "EventSomethingHappenedV0" - but the Versioned-attribute remains untouched (in this copy)
namespace LegacyEvents
{
[Versioned("EventSomethingHappened", 0)] // still version 0
public class EventSomethingHappenedV0
{
...
}
}
In the new version (at the same place, under the same name) just the version-part of the attribute gets incremented. And that's it!
namespace CurrentEvents
{
[Versioned("EventSomethingHappened", 1)] // new version 1
public class EventSomethingHappened
{
...
}
}
Json.NET supports binders which maps type-identifiers to types and back. Here is a production-ready binder:
public class VersionedSerializationBinder : DefaultSerializationBinder
{
private Dictionary<string, Type> _getImplementationLookup = new Dictionary<string, Type>();
private static Type[] _versionedEvents = null;
protected static Type[] VersionedEvents
{
get
{
if (_versionedEvents == null)
_versionedEvents = AppDomain.CurrentDomain.GetAssemblies()
.Where(x => x.IsDynamic == false)
.SelectMany(x => x.GetExportedTypes()
.Where(y => y.IsAbstract == false &&
y.IsInterface == false))
.Where(x => x.GetCustomAttributes(typeof(VersionedAttribute), false).Any())
.ToArray();
return _versionedEvents;
}
}
public VersionedSerializationBinder()
{
}
private VersionedAttribute GetVersionInformation(Type type)
{
var attr = type.GetCustomAttributes(typeof(VersionedAttribute), false).Cast<VersionedAttribute>().FirstOrDefault();
return attr;
}
public override void BindToName(Type serializedType, out string assemblyName, out string typeName)
{
var versionInfo = GetVersionInformation(serializedType);
if (versionInfo != null)
{
var impl = GetImplementation(versionInfo);
typeName = versionInfo.Identifier + "|" + versionInfo.Revision;
}
else
{
base.BindToName(serializedType, out assemblyName, out typeName);
}
assemblyName = null;
}
private VersionedAttribute GetVersionInformation(string serializedInfo)
{
var strs = serializedInfo.Split(new[] { '|' }, StringSplitOptions.RemoveEmptyEntries);
if (strs.Length != 2)
return null;
return new VersionedAttribute(strs[0], strs[1]);
}
public override Type BindToType(string assemblyName, string typeName)
{
if (typeName.Contains('|'))
{
var type = GetImplementation(GetVersionInformation(typeName));
if (type == null)
throw new InvalidOperationException(string.Format("VersionedEventSerializationBinder: No implementation found for type identifier '{0}'", typeName));
return type;
}
else
{
var versionInfo = GetVersionInformation(typeName + "|0");
if (versionInfo != null)
{
var type = GetImplementation(versionInfo);
if (type != null)
return type;
// else: continue as it is a normal serialized object...
}
}
// resolve assembly name if not in serialized info
if (string.IsNullOrEmpty(assemblyName))
{
Type type;
if (typeName.TryFindType(out type))
{
assemblyName = type.Assembly.GetName().Name;
}
}
return base.BindToType(assemblyName, typeName);
}
private Type GetImplementation(VersionedAttribute attribute)
{
Type eventType = null;
if (_getImplementationLookup.TryGetValue(attribute.Identifier + "|" + attribute.Revision, out eventType) == false)
{
var events = VersionedEvents
.Where(x =>
{
return x.GetCustomAttributes(typeof(VersionedAttribute), false)
.Cast<VersionedAttribute>()
.Where(y =>
y.Revision == attribute.Revision &&
y.Identifier == attribute.Identifier)
.Any();
})
.ToArray();
if (events.Length == 0)
{
eventType = null;
}
else if (events.Length == 1)
{
eventType = events[0];
}
else
{
throw new InvalidOperationException(
string.Format("VersionedEventSerializationBinder: Multiple types have the same VersionedEvent attribute '{0}|{1}':\n{2}",
attribute.Identifier,
attribute.Revision,
string.Join(", ", events.Select(x => x.FullName))));
}
_getImplementationLookup[attribute.Identifier + "|" + attribute.Revision] = eventType;
}
return eventType;
}
}
...and the Versioned-attribute
[AttributeUsage(AttributeTargets.Class)]
public class VersionedAttribute : Attribute
{
public string Revision { get; set; }
public string Identifier { get; set; }
public VersionedAttribute(string identifier, string revision = "0")
{
this.Identifier = identifier;
this.Revision = revision;
}
public VersionedAttribute(string identifier, long revision)
{
this.Identifier = identifier;
this.Revision = revision.ToString();
}
}
At last use the versioned binder like this
JsonSerializer.Create(new JsonSerializerSettings
{
TypeNameHandling = TypeNameHandling.All,
TypeNameAssemblyFormat = FormatterAssemblyStyle.Simple,
Binder = new VersionedSerializationBinder()
});
For a full Json.NET ISerialize-implementation see (an little outdated) gist here:
https://gist.github.com/warappa/6388270