Session: How to Enable Optimistic Concurrency

By default, optimistic concurrency checks are turned off. Changes made outside our session object will be overwritten. Concurrent changes to the same document will use the Last Write Wins strategy so a lost update anomaly is possible with the default configuration of the session.

You can enable the optimistic concurrency strategy either globally, at the document store level or a per session basis.
In either case, with optimistic concurrency enabled, RavenDB will generate a concurrency exception (and abort all modifications in the current transaction) when trying to save a document that has been modified on the server side after the client loaded and modified it.

The ConcurrencyException that might be thrown upon the saveChanges call, needs to be handled by the caller.
The operation can be retried (the document needs to be reloaded since it got changed meanwhile) or handle the error in a way that is suitable in a given scenario.

You can also control optimistic concurrency per specific document. To enable it, supply a Change Vector to Store.
If you don't supply a 'Change Vector' or if the 'Change Vector' is null, then optimistic concurrency will be disabled.
Setting the 'Change Vector' to an empty string will cause RavenDB to ensure that this document is a new one and doesn't already exists.

Setting optimistic concurrency per specific document overrides the use of the useOptimisticConcurrency field from the advanced session operations.

Enabling for a specific Session

try (IDocumentSession session = store.openSession()) {
    session.advanced().setUseOptimisticConcurrency(true);

    Product product = new Product();
    product.setName("Some Name");

    session.store(product, "products/999");
    session.saveChanges();

    try (IDocumentSession otherSession = store.openSession()) {
        Product otherProduct = otherSession.load(Product.class, "products/999");
        otherProduct.setName("Other Name");

        otherSession.saveChanges();
    }

    product.setName("Better Name");
    session.saveChanges(); //  will throw ConcurrencyException
}

Enabling Globally

The first example shows how to enable optimistic concurrency for a particular session. This can be also turned on globally, for all opened sessions by using the convention store.getConventions().setUseOptimisticConcurrency.

store.getConventions().setUseOptimisticConcurrency(true);

try (IDocumentSession session = store.openSession()) {
    boolean isSessionUsingOptimisticConcurrency
        = session.advanced().isUseOptimisticConcurrency(); // will return true
}

Turning Off Optimistic Concurrency for a Single Document when it is Enabled on Session

Optimistic concurrency can be turned off for a single document by passing null as a change vector value to store method even when it is turned on for an entire session (or globally).

try (IDocumentSession session = store.openSession()) {
    Product product = new Product();
    product.setName("Some Name");

    session.store(product, "products/999");
    session.saveChanges();
}

try (IDocumentSession session = store.openSession()) {
    session.advanced().setUseOptimisticConcurrency(true);

    Product product = new Product();
    product.setName("Some Other Name");

    session.store(product, null, "products/999");
    session.saveChanges(); // will NOT throw Concurrency exception
}

Turning On Optimistic Concurrency for a New Document when it is Disabled on Session

Optimistic concurrency can be turned on for a new document by passing "" as a change vector value to store method even when it is turned off for an entire session (or globally). It will cause to throw ConcurrencyException if the document already exists.

try (IDocumentSession session = store.openSession()) {
    Product product = new Product();
    product.setName("Some Name");
    session.store(product, "products/999");
    session.saveChanges();
}

try (IDocumentSession session = store.openSession()) {
    session.advanced().setUseOptimisticConcurrency(false); // default value

    Product product = new Product();
    product.setName("Some Other Name");

    session.store(product, "", "products/999");
    session.saveChanges(); // will throw Concurrency exception
}