see on GitHub

Session : How to enable optimistic concurrency?

By default, optimistic concurrency checks are turned off, which means that changes made outside our session object will be overwritten. Checks can be turned on by setting UseOptimisticConcurrency property from Advanced session operations to true and may cause ConcurrencyExceptions to be thrown.

Example I

using (IDocumentSession session = store.OpenSession())
{
	session.Advanced.UseOptimisticConcurrency = true;

	Product product = new Product { Name = "Some Name" };

	session.Store(product, "products/999");
	session.SaveChanges();

	using (IDocumentSession otherSession = store.OpenSession())
	{
		Product otherProduct = otherSession.Load<Product>("products/999");
		otherProduct.Name = "Other Name";

		otherSession.SaveChanges();
	}

	product.Name = "Better Name";
	session.SaveChanges(); // will throw ConcurrencyException
}

The above example shows how to enable optimistic concurrency for a particular session. However that can be also turned on globally, that is for all opened sessions by using the convention DefaultUseOptimisticConcurrency.

Example II

store.Conventions.DefaultUseOptimisticConcurrency = true;

using (IDocumentSession session = store.OpenSession())
{
	bool isSessionUsingOptimisticConcurrency = session.Advanced.UseOptimisticConcurrency; // will return true
}

Remarks

Optimistic Concurrency and Replication

When Optimistic Concurrency is used with Replication then we advise to set ForceReadFromMaster when only read operations are load-balanced (FailoverBehavior.ReadFromAllServers) and you want to perform document update.

store.Conventions.DefaultUseOptimisticConcurrency = true;

using (DocumentSession session = (DocumentSession)store.OpenSession())
using (session.DatabaseCommands.ForceReadFromMaster())
{
    // In replicated setup where ONLY reads are load balanced (FailoverBehavior.ReadFromAllServers)
    // and optimistic concurrency checks are turned on
    // you must set 'ForceReadFromMaster' to get the appropriate ETag value for the document
    // when you want to perform document updates (writes)
    // because writes will go to the master server and ETag values between servers are not synchronized

    Product product = session.Load<Product>("products/999");
    product.Name = "New Name";

    session.SaveChanges();
}