FoundationDB Key-Value Store Unique Constraint

UniqueDemo.java

import com.foundationdb.*;
import com.foundationdb.async.*;
import com.foundationdb.directory.*;
import com.foundationdb.tuple.*;

public class UniqueDemo
{
    public static void main(String[] args) {
        FDB fdb = FDB.selectAPIVersion(200);
        UniqueDemo app = new UniqueDemo(fdb);
        test(app);
    }

    public static void test(UniqueDemo app) {
        System.out.println("Testing");
        app.insertUser(1, "foo@example.com");
        // same ID allowed
        app.insertUser(1, "foo@example.com");
        app.insertUser(2, "bar@example.com");
        try {
            app.insertUser(3, "foo@example.com");
            throw new IllegalStateException();
        } catch(IllegalArgumentException e) { /* expected */ }
        app.insertUser(3, "zap@example.com");
        System.out.println("Passed");
    }


    final Database db;
    final DirectorySubspace users;
    final DirectorySubspace usersByEmail;

    public UniqueDemo(FDB fdb) {
        this.db = fdb.open();
        this.users = DirectoryLayer.getDefault().createOrOpen(db, PathUtil.from("users")).get();
        this.usersByEmail = DirectoryLayer.getDefault().createOrOpen(db, PathUtil.from("users_by_email")).get();
    }

    public void insertUser(final int userID, final String email) {
        db.run(new Function<Transaction, Void>() {
            @Override
            public Void apply(Transaction tr) {
                 // Insert user into primary key index
                 tr.set(users.pack(userID), Tuple.from(userID, email).pack());
             
                 // UNIQUE constraint on email field.  As in an RDBMS, this requires an
                 // index on the field.  If you want, you can of course use this index
                 // for reads, too
                 byte[] curID = tr.get(usersByEmail.pack(email)).get();
                 if((curID != null) && (Tuple.fromBytes(curID).getLong(0) != userID)) {
                     throw new IllegalArgumentException("Duplicate e-mail address");
                 }
                 tr.set(usersByEmail.pack(email), Tuple.from(userID).pack());
                 return null;
            }
        });
    }
}