The driver should add the ability to distinguish between an explicit write concern of

and an acknowledged write concern that means to use the server's default. MongoClient instances by default should use the latter, as per specification.

This can be done with a small backwards breaking change. We can add a no-argument constructor to WriteConcern which means "this is acknowledged but it should use the server's default configured write concern. And then we can change the declaration of WriteConcern.ACKNOWLEDGED to:

public final static WriteConcern ACKNOWLEDGED = new WriteConcern();

The only breakage is that with this change, the following code

    WriteConcern.ACKNOWLEDGED.getW()    // getW() returns an integer

will throw an exception, since w will be null rather that an integer.

In addition, the driver should add additional constants to WriteConcern, e.g.

public final static WriteConcern W1 = new WriteConcern(1);
public final static WriteConcern W2 = new WriteConcern(2);
public final static WriteConcern W3 = new WriteConcern(3);