Replication Conflicts

What is a Conflict?

A conflict occurs when the same document is updated concurrently on two different nodes.
This can happen because of a network split or because of several client updates that each talked to different nodes faster than we could replicate the information between the nodes.

In a distributed system, we can either choose to run a consensus (which requires consulting a majority on every decision) or accept the potential for conflicts.

For document writes, RavenDB chooses to accept conflicts as a trade-off of always being able to accept writes on any node.

Conflict Detection

Each document in a RavenDB cluster has a Change Vector which is used for conflict detection.

When the server receives an incoming replication batch, it compares the Change Vector of the incoming document with the Change Vector of the local document.

Let's assume remote_cv to be the change vector of a remote document, and local_cv to be a change vector of a local document.
The Comparison of the Change Vectors may yield three possible results:

• remote_cv <= local_cv -> Nothing to do, the local document is more up-to-date.
• remote_cv > local_cv -> Remote document is more recent than local, replace local document with remote
• remote_cv conflicts with local_cv -> Try to resolve conflict.

When there is a conflict between two or more document versions, RavenDB will try multiple steps to resolve it. If any of the steps succeeds, the conflict resolution will end and resolved document will be written to the storage.

1. If ResolveToLatest flag is set (by default it is true), resolve the conflict to the document variant where the Latest Modified property is the latest.
2. Check whether the document contents are identical, if so, then there is no conflict and the change vector of the two documents is merged.

   Note

   The identity check applies to Tombstone as well, and it will always resolve to the local one, since the tombstones are always considered equal.

3. Try to resolve the conflict by using a script, which is set up by configuring a Conflict Resolver.
4. If all else fails, record conflicting document variants as "Conflicted Documents" which will have to be resolved Manually.

var final = docs[0];

for(var i = 1; i < docs.length; i++)
{
	var currentOrder = docs[i];
	for(var j = 0; j < currentOrder.Lines.length; j++)
	{
		var item = currentOrder.Lines[j];
		var match = final.Lines
			             .find(i => i.Product == item.Product);
		if(!match)
		{
			// not in Order, add
			final.Lines.push(item);
		}
		else
		{
			match.Quantity = Math.max(
				item.Quantity,
				match.Quantity);
		}
	}
}

return final;