I have this structure on my Firestore database:

I have a collection named groups.
Each document inside represents a group of users.
Each group has a map field, called users.

Group1: {    // Group name is the document ID

  ...        // other fields
  users: {                 
    userID: { 
       nick: "myuser1"
    },
    ...
  }
}

Each user belongs to another collection, users, each user document has
an array field with a list of groups he belongs to.

userID: {              // document ID is the user ID
   nick: "myuser1",
   ...,
   ...,    // other fields
   ...,
   groups: [
     "Group1",
     ...
   ]

}    

When adding a user to a group, I must ensure both group document and user document are updated in a single transaction, otherwise the information will be inconsistent.

The code lives inside a callable cloud function.
In order to ensure atomicity, I used a transaction, see below.

The thing is, it works…. most of the time. I’d say a 20% of times it does NOT work, it leaves the user with the group name inside its groups array, but the group document is NOT updated. So, no atomicity.

Here’s the function, I am importing like this:

const functions = require('firebase-functions')

so I guess I’m using v1? Not sure about that.

exports.joinGroup = functions
  .region(firebaseRegion)
  .https.onCall(async (data, context) => {
    const groupCollection = data.groupCollection
    const userCollection = data.userCollection
    const groupName = data.groupName
    const token = data.token

    try {
      const res = await db.runTransaction(async t => {
        const groupRef = db.collection(groupCollection).doc(groupName)
        let docgr = await t.get(groupRef)
        if (!docgr.exists) return false
        const uid = context.auth.uid
        const userRef = db.collection(userCollection).doc(uid)
        const docUser = await t.get(userRef)
        if (!docUser.exists) return false
        const user = docUser.data()
        const nick = user.nick
        const him = {
          nick
        }

        t.update(groupRef, {
          [`users.${uid}`]: him
        })
        t.update(groupRef, {
          invites: FieldValue.arrayRemove(token)
        })
        t.update(userRef, {
          groups: FieldValue.arrayUnion(groupName)
        })
        return true
      })
      if (res) {
        console.log('JOINGROUP JOINED', groupName)
        return { ok: true }
      } else {
        const err = { code: 403, error: 'joinGroup failed' }
        throw err
      }
    } catch (etrans) {
      console.log('FAILED', etrans)
    }
  })