private static void registerKeyForClient(DistributedMember clientId, Object key) {
    try {
      ClientSession cs = server.getClientSession(clientId);
      if (cs.isPrimary()) {
        cs.registerInterest(SEPARATOR + REGION_NAME, key, InterestResultPolicy.KEYS_VALUES,
            false);
      }
    } catch (Exception ex) {
      Assert.fail("failed while registering key(" + key + ")", ex);
    }
  }
exercises the ClientSession interface to register interest in a server on behalf of a client

@param clientId the DM of the client
@param key the key that the client is interested in
geode-core/src/distributedTest/java/org/apache/geode/internal/cache/tier/sockets/InterestListDUnitTest.java