Mongo writes query.UpsertId multiple documents before I can validate with query.Count()

Question

Description: I`m using mongoDB on my project. This is short logic for handler when user tries to put his item for sale. Before putting offer to mongo I validate the offer, so there would be no active offers with save assetId

Using:

• mgo.v2

• mongo 3.6

• golang 1.10

Problem: If user clicks really fast sends several requests to my handler (lets say if he double click the mouse fast), validation doesn`t work as it seems like the first offer is not in the collection yet, so as a result I get 2-3 offers with same assetId.

I tried

• Set mongoUrl?replicaSet=rs0, so our master and slaves would now about each other
• Set time.Sleep(200 * time.Millisecond) after validation

Question:

Is there any way I can handle this with mongo instruments, or someone would suggest some other workaround?

Thank you in advance!

       count, _ := r.DB.C(sellOfferCollectionName).Find(
         bson.M{
            "state":     someState,
            "asset_id":  assetId,
            "seller_id": seller,
         },
       ).Count()

       if count > 0 {
           return 
       }       

       id := uuid.New().String()
            OfferModel := Offer{
                Id:           id,
                AssetId:      assetId,
                State:        someState,
                SellerId:     sellerId,
                CreatingDate: time.Now(),
            }
        if _, err := r.DB.C(sellOfferCollectionName).UpsertId(offer.Id, offer); err != nil {
            return err
        }

UPDATE

I tried to recreate the problem even more. So I wrote this little test code, so in result managed to write 60 documents before validation (count > 0) worked. This example fully recreates my problem.

type User struct {
  Id        string `bson:"_id"`
  FirstName string `bson:"first_name"`
  LastName  string `bson:"last_name"`
  State     bool   `bson:"state"`
}

func main() {
  mongoSession, mgErr := mgo.Dial("127.0.0.1:27018")

  if mgErr != nil {
    panic(mgErr)
  }

  var mongoDbSession = mongoSession.DB("test_db")

  for i := 0; i < 1000; i++ {
    go func() {
      count, _ := mongoDbSession.C("users").Find(
        bson.M{
          "state":      true,
          "first_name": "testUser",
        },
      ).Count()

      if count > 0 {
        return
      }

      user := User{
        Id:        uuid.New().String(),
        FirstName: "testUser",
        LastName:  "testLastName",
        State:     true,
      }

      if _, err := mongoDbSession.C("users").UpsertId(user.Id, user); err != nil {
        panic(mgErr)
      }
    }()
  }

  count, _ := mongoDbSession.C("users").Find(
    bson.M{
      "state":      true,
      "first_name": "testUser",
    },
  ).Count()

  fmt.Println(count)
  fmt.Scanln()
}

Answer 1

Eventually, after close investigation the bug, we found out that the reason was when user sent request it was handled in goroutine. Means a lot requests = a lot of concurrent goroutines. So, it out validator (check if the offer is in the collection), couldn't find it as it was not in the mongo yet. So, in the end, we decided to use redis as our validator.

Here is short implementation:

incr, err := redisClient.Incr(offer.AssetId).Result()
    if err != nil {
        return err
    }

    if incr > 1 {
        return errors.New("ASSET_ALREADY_ON_SALE")
    }

    redisClient.Expire(offer.AssetId, time.Second*10)

Hope it will help someone facing same issue.

Link on implementation description: