I found that although the primaryPreferred is specified, a small number of request will still be sent to the secondary node, even though the primary node is available at the time. 

mongo-go-driver: v1.10.2  

mongodb: 4.0 replicaset

```
var url = flag.String("url", "127.0.0.1:27031", "mongodb url")
 
func main() {
        flag.Parse()
 
        f := func() error {
                clientOptions := options.Client().ApplyURI(*url).SetReadPreference(readpref.PrimaryPreferred())
 
                client, err := mongo.NewClient(clientOptions)
                if err != nil

{                         return fmt.Errorf("new client failed:%v", err)                 }

 
                if err = client.Connect(context.Background()); err != nil

{                         return fmt.Errorf("connect failed: %v", err)                 }

                defer client.Disconnect(context.Background())
               
               /*
                if err = client.Ping(context.Background(), readpref.Primary()); err != nil

{                         fmt.Println("ping failed:", err)                 }

else

{                         fmt.Println("ping...")                 }

                */
 
                coll := client.Database("mydb").Collection("coll")
 
                filter := bson.D{{Key: "_id", Value: "test_val_readpref"}}
                res := coll.FindOne(context.Background(), filter)
                if res.Err() != nil

{                         fmt.Println("err:", res.Err())                         return res.Err()                 }

 
                return nil
        }
 
        for i := 0; i < 1000; i++ {
                if err := f(); err != nil

{                         fmt.Println("err ", err)                 }

else

{                         fmt.Println("success")                 }

        }
}
```

I have to ping the primary node before sending the request to make sure I got data from the primary. 

From the documentation here: https://www.mongodb.com/docs/v5.0/core/read-preference-use-cases/ 

The description of primaryPreferred is that

```

operations read from the primary but if it is unavailable, operations read from secondary members in most situations.

```

When the primary node is available and should all operations read from primary members?