Wednesday, August 7, 2013

Last in Win Replication for RavenDB

One of my favorite features of RavenDB is how easy it is customize and extend.

RavenDB offers an extremely easy to use built in replication bundle. To deal with replication conflicts, the RavenDB.Database NuGet Package includes an abstract base class (the AbstractDocumentReplicationConflictResolver) that you can implement with your own conflict resolution rules.

Last In Wins Replication Conflict Resolver

John Bennett wrote a LastInWinsReplicationConflictResolver for RavenDB 1.0, and I have updated it for RavenDB 2.0 and 2.5. As always you can get that code from GitHub!

Download RavenExtensions from GitHub

Once you have built your resolver, you need only drop the assembly into the Plugins folder at the root of your RavenDB server and it will automatically be detected and loaded the next time that your server starts.

public class LastInWinsReplicationConflictResolver
    : AbstractDocumentReplicationConflictResolver
{
    private readonly ILog _log = LogManager.GetCurrentClassLogger();
 
    public override bool TryResolve(
        string id,
        RavenJObject metadata,
        RavenJObject document,
        JsonDocument existingDoc,
        Func<string, JsonDocument> getDocument)
    {
        if (ExistingDocShouldWin(metadata, existingDoc))
        {
            ReplaceValues(metadata, existingDoc.Metadata);
            ReplaceValues(document, existingDoc.DataAsJson);
            _log.Debug(
                "Replication conflict for '{0}' resolved with existing doc",
                id);
        }
        else
        {
            _log.Debug(
                "Replication conflict for '{0}' resolved with inbound doc",
                id);
        }
 
        return true;
    }
 
    private static bool ExistingDocShouldWin(
        RavenJObject newMetadata, 
        JsonDocument existingDoc)
    {
        if (existingDoc == null ||
            ExistingDocHasConflict(existingDoc) ||
            ExistingDocIsOlder(newMetadata, existingDoc))
        {
            return false;
        }
 
        return true;
    }
 
    private static bool ExistingDocHasConflict(JsonDocument existingDoc)
    {
        return existingDoc.Metadata[Constants.RavenReplicationConflict] != null;
    }
 
    private static bool ExistingDocIsOlder(
        RavenJObject newMetadata,
        JsonDocument existingDoc)
    {
        var newLastModified = GetLastModified(newMetadata);
 
        if (!existingDoc.LastModified.HasValue ||
            newLastModified.HasValue &&
            existingDoc.LastModified <= newLastModified)
        {
            return true;
        }
 
        return false;
    }
 
    private static DateTime? GetLastModified(RavenJObject metadata)
    {
        var lastModified = metadata[Constants.LastModified];
 
        return (lastModified == null)
            ? new DateTime?()
            : lastModified.Value<DateTime?>();
    }
 
    private static void ReplaceValues(RavenJObject target, RavenJObject source)
    {
        var targetKeys = target.Keys.ToArray();
        foreach (var key in targetKeys)
        {
            target.Remove(key);
        }
 
        foreach (var key in source.Keys)
        {
            target.Add(key, source[key]);
        }
    }
}
Shout it

Enjoy,
Tom

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