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The driver should add the ability to distinguish between an explicit write concern of
{w : 1}
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();
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The only breakage is that with this change, the following code
WriteConcern.ACKNOWLEDGED.getW() // getW() returns an integer
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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);
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public final static WriteConcern W2 = new WriteConcern(2);
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public final static WriteConcern W3 = new WriteConcern(3);
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