The check-and-set methods provide a mechanism for updating information only if the client knows the check (CAS) value. This can be used to prevent clients from updating values in the database that may have changed since the client obtained the value. Methods for storing and updating information support a CAS method that allows you to ensure that the client is updating the version of the data that the client retrieved.

The check value is in the form of a 64-bit integer which is updated every time the value is modified, even if the update of the value does not modify the binary data. Attempting to set or update a key/value pair where the CAS value does not match the value stored on the server will fail.

The cas() methods are used to explicitly set the value only if the CAS supplied by the client matches the CAS on the server, analogous to the Section 6.2, “Set Operations” method.

With all CAS operations, the CASResponse value returned indicates whether the operation succeeded or not, and if not why. The CASResponse is an Enum with three possible values:

The first form of the cas() method allows for an item to be set in the database only if the CAS value supplied matches that stored on the server.

For example:

CASResponse casr = client.cas("caskey", casvalue, "new string value");

if (casr.equals(CASResponse.OK)) {
    System.out.println("Value was updated");
}
else if (casr.equals(CASResponse.NOT_FOUND)) {
    System.out.println("Value is not found");
}
else if (casr.equals(CASResponse.EXISTS)) {
    System.out.println("Value exists, but CAS didn't match");
}

The second form of the method supports using a custom transcoder for storing a value.

This form of the cas() method updates both the key value and the expiry time for the value. For information on expiry values, see Section 4.4, “Expiry Values”.

For example the following attempts to set the key caskey with an updated value, setting the expiry times to 3600 seconds (one hour).

Transcoder<Integer> tc = new IntegerTranscoder();
CASResponse casr = client.cas("caskey", casvalue, 3600, 1200, tc);

Performs an asynchronous CAS operation on the given key/value. You can use this method to set a value using CAS without waiting for the response. The following example requests an update of a key, timing out after 5 seconds if the operation was not successful.

Future<CASResponse> future = client.asyncCAS("someKey", casvalue, "updatedvalue");

CASResponse casr;

try {
    casr = future.get(5, TimeUnit.SECONDS);
} catch(TimeoutException e) {
    future.cancel(false);
}

Performs an asynchronous CAS operation on the given key/value using a custom transcoder. The example below shows the update of an existing value using a custom Integer transcoder.

Transcoder<Integer> tc = new IntegerTranscoder();
Future<CASResponse> future = client.asyncCAS("someKey", casvalue, 1200, tc);

CASResponse casr;

try {
    casr = future.get(5, TimeUnit.SECONDS);
} catch(TimeoutException e) {
    future.cancel(false);
}

The final form of the asyncCAS() method supports a custom transcoder and setting the associated expiry value. For example, to update a value and set the expiry to 60 seconds:

Transcoder<Integer> tc = new IntegerTranscoder();
Future<CASResponse> future = client.asyncCAS("someKey", casvalue, 60, 1200, tc);

CASResponse casr;

try {
    casr = future.get(5, TimeUnit.SECONDS);
} catch(TimeoutException e) {
    future.cancel(false);
}