Optimistic Locking with cas(): Each document has a unique identifier associated with it (the CAS value), which changes when the document itself is mutated. You can fetch the CAS value for a given key and then use the cas() operation to update it. The update will only succeed, when the CAS value is still the same. This is why it's called optimistic locking: someone else can still read and try to update the document, but it will fail once the CAS value has changed. Here is a example on how to do it with the Java SDK:

// Reads the document with the CAS value.
String key = "eagle_brewing-golden";
CASValue<Object> beerWithCAS = client.gets(key);

// Updates the document and tries to store it back.
Beer beer = gson.fromJson((String)beerWithCAS.getValue(), Beer.class);
beer.name = "Some other Name";
CASResponse cas = client.cas(key, beerWithCAS.getCas(), gson.toJson(beer));

The CASResponse is a ENUM which identifies the status of the cas() operation. If it succeeds, CASResponse.OK is returned. If CASResponse.EXISTS is returned, the CAS has changed in the meantime. A common approach is to run the script shown above (the whole fetch-and-update process) in a loop until the operation did succeed. Depending on your application, you may need to do conflict management as well.

Note that this also means that all your application need to follow the same code path (cooperative locking). If you use set() somewhere else in the code on the same document, it will work even if the CAS itself is out of date (that's because the normal set() method doesn't care about those values at all). Of course, the CAS itself changes then and the cas() operation would fail afterwards.

Pessimistic Locking with getAndLock(): If you want to lock a document completely (or an object graph), you can use the getAndLock() method. Other threads can still run get() queries against the document, but set() operations without a CAS will fail.

// Get with Lock
String key = "eagle_brewing-golden";
CASValue<Object> beer = client.getAndLock(key, 20);
String beerJson = (String) beer.getValue();

// Try to set without the lock
OperationFuture<Boolean> result = client.set(key, 0, beerJson);
System.out.println(result.get()); // Prints false

// Try to set with the CAS aquired
CASResponse cas = client.cas(key, beer.getCas(), beerJson);
System.out.println(cas); // Prints OK

Once you update the document, the lock will be released. There is also the unlock() method available through which you can unlock the document. You are required to define how long the lock should be active (the maximum is 30 seconds).