@Override
    public Collection<String> allClientId() {
        // client id is unique in the application
        // use set to replace array list
        // it will improve the performance
        Collection<String> clientIds = new HashSet<>(clients.size());
        clientIds.addAll(clients.keySet());
        return clientIds;
    }
@Test
    void testAllClientId() {
        Collection<String> allClientIds = persistentIpPortClientManager.allClientId();
        assertEquals(1, allClientIds.size());
        assertTrue(allClientIds.contains(clientId));
    }