How to ensure uniqueness of a property in a NoSQL record ( Golang + tiedot ) - go

Question

I'm working on a simple application written in golang, using tiedot as NoSQL database engine.
I need to store some users in the database.
type User struct {
Login string
PasswordHash string
Salt string
}
Of course two users cannot have the same login, and - as this engine does not provide any transaction mechanism - I'm wondering how to ensure that there's no duplicated login in the database when writing.
I first thought that I could just search for user by login before inserting, but as the database will be
used concurently, it is not reliable.
Maybe I could wait for a random time and if there is another user with the same login in the collection, delete it, but that does not sound reliable either.
Is this even possible, or should I switch to a database engine that support transactions ?

Answer 1

Below is my solution. It is not Tiedot specific, but It uses CQRS and can be applied to various DBs.
You can also have other benefits using it, such as caching and bulk write (in case DB supports it) to prevent asking DB on every request.
package main
import (
"sync"
"log"
"errors"
)
type User struct {
Login string
PasswordHash string
Salt string
}
type MutexedUser struct {
sync.RWMutex
Map map[string]User
}
var u = &MutexedUser{}
func main() {
var user User
u.Sync()
// Get new user here
//...
if err := u.Insert(user); err != nil {
// Ask to provide new login
//...
log.Println(err)
}
}
func (u *MutexedUser) Insert(user User) (err error) {
u.Lock()
if _, ok := u.Map[user.Login]; !ok {
u.Map[user.Login] = user
// Add user to DB
//...
u.Unlock()
return err
}
u.Unlock()
return errors.New("duplicated login")
}
func (u *MutexedUser) Read(login string) User {
u.RLock()
value := u.Map[login]
u.RUnlock()
return value
}
func (u *MutexedUser) Sync() (err error) {
var users []User
u.Lock()
defer u.Unlock()
// Read users from DB
//...
u.Map = make(map[string]User)
for _, user := range users {
u.Map[user.Login] = user
}
return err
}

Answer 2

I first thought that I could just search for user by login before inserting, but as the database will be used concurently, it is not reliable.
Right, it creates a race condition. The only way to resolve this is:
Lock the table
Search for the login
Insert if the login is not found
Unlock the table
Table-locks are not a scalable solution, because it creates an expensive bottleneck in your application. It's why non-transactional storage engines like MySQL's MyISAM are being phased out. It's why MongoDB has to use clusters to scale up.
It can work if you have a small dataset size and a light amount of concurrency, so perhaps it's adequate for login creation on a lightly-used website. New logins probably aren't created so frequently that they need to scale up so much.
But users logging in, or password changes, or other changes to account attributes, do happen more frequently.
The solution for this is to make this operation atomic, to avoid race conditions. For example, attempt the insert and have the database engine verify uniqueness and reject the insert if it violates that constraint.
Unfortunately, I don't see any documentation in tiedot that shows that it supports a unique constraint or a uniqueness enforcement on indexes.
Tiedot is 98% written by a single developer, in a period of about 2 years (May 2013 - April 2015). Very little activity since then (see https://www.openhub.net/p/tiedot). I would consider tiedot to be an experimental project, unlikely to expand in feature set.