Connection Pool Issue about mongodb-erlang HOT 2 OPEN

start_pool(Config) ->
    ReplicaSet      = data:get(<<"rs">>, Config),
    Hosts           = data:get(<<"hosts">>, Config),
    Database        = data:get(<<"database">>, Config),
    Login           = data:get(<<"login">>, Config),
    Password        = data:get(<<"password">>, Config),
    PoolSizeMaster  = data:get(<<"pool_size_master">>, Config),
    Credentials     = case Login of undefined -> []; _ -> [{login, Login}, {password, Password}, {auth_source, Database}] end,
    {ok, _Pid} = mongoc:connect(
        {rs, ReplicaSet, Hosts},
        [
            {name, ?MONGO_MASTER}, 
            {register, ?MONGO_MASTER},
            {max_overflow, 0},
            {pool_size, PoolSizeMaster},
            {rp_mode, primary}
        ],
        [{database, Database} | Credentials]
    ),
    PoolSizeSlave  = data:get(<<"pool_size_slave">>, Config),
    {ok, _} = mongoc:connect(
        {rs, ReplicaSet, Hosts},
        [
            {name, ?MONGO_SLAVE}, 
            {register, ?MONGO_SLAVE}, 
            {pool_size, PoolSizeSlave}, 
            {max_overflow, 0},
            {rp_mode, secondaryPreferred}
        ],
        [{database, Database}, {r_mode, slave_ok} | Credentials]
    ),
    ok.