Cluster: Document Conflicts in Client-side

What are conflicts?

When two or more changes of a single document are done concurrently in two separate nodes, RavenDB cannot know which one of the changes is the correct one. This is called document conflict.
For more information about conflicts and their resolution, see article about conflicts.

When is a conflict exception thrown?

DocumentConflictException will be thrown for any access of a conflicted document. Fetching attachments of a conflicted document will throw InvalidOperationException on the server.

How to resolve a conflict from the client side?

• PUT of a document with ID that belongs to conflicted document will resolve the conflict.

using (var session = store.OpenSession())
{
    session.Store(new User {Name = "John Doe"}, "users/123"); // users/123 is a conflicted document
    session.SaveChanges(); //when this request is finished, the conflict for users/132 is resolved.
}

• DELETE of a conflicted document will resolve its conflict.

using (var session = store.OpenSession())
{
    session.Delete("users/123"); // users/123 is a conflicted document
    session.SaveChanges(); //when this request is finished, the conflict for users/132 is resolved.
}                

• Incoming replication will resolve conflict if the incoming document has a larger change vector.

Modifying conflict resolution from the client-side

In RavenDB we can resolve conflicts either by resolving to the latest or by using a conflict resolution script to decide which one of the conflicted document variants are the ones that need to be kept. The following is an example of how we can set a conflict resolution script from the client-side.

using (var documentStore = new DocumentStore
{
    Urls = new []{ "http://<url of a database>" },
    Database = "<database name>"
})
{
    var resolveByCollection = new Dictionary<string, ScriptResolver>
    {
        {
            "ShoppingCarts", new ScriptResolver //specify conflict resolution for collection
            {
                // conflict resolution script is written in javascript
                Script = @"
                var final = docs[0];
                for(var i = 1; i < docs.length; i++)
                {
                    var currentCart = docs[i];
                    for(var j = 0; j < currentCart.Items.length; j++)
                    {
                        var item = currentCart.Items[j];
                        var match = final.Items
                                         .find( i => i.ProductId == item.ProductId);
                        if(!match)
                        {
                            // not in cart, add
                            final.Items.push(item);
                        }
                        else
                        {
                            match.Quantity = Math.max(
                                        item.Quantity ,
                                        match.Quantity);
                        }
                    }
                }
                return final; // the conflict will be resolved to this variant
                "
            }
        }
    };

    var op = new ModifyConflictSolverOperation(
        store.Database,
        resolveByCollection,    //we specify conflict resolution scripts by document collection 
        resolveToLatest: true); // if true, RavenDB will resolve conflict to the latest
                                // if there is no resolver defined for a given collection or
                                // the script returns null

    await store.Maintenance.Server.SendAsync(op);
}