Firebase database runTransaction at node with lot of data (e.g. root node)-CodePudding

Short question: Is it a bad practice in Firebase Realtime Database to run a transaction operation on a root node? (or any parent node with a lot of data.)

Long-ish version: Let's do an example based on a database structure recommended in the Firebase doc:

{
  "users": {
    // users indexed by their unique id
    "alovelace": {
      "name": "Ada Lovelace",
      "groups": {
         "techpioneers": true,
         "womentechmakers": true
      }
    },
    ... // other users
  },
  "groups": {
    // groups indexed by their unique id
    "techpioneers": {
      "name": "Historical Tech Pioneers",
      "members": {
        "alovelace": true,
        "ghopper": true,
        "eclarke": true
      }
    },
    ... // other groups
  }
}

Let's say I want to add a user to a group with runTransaction(). The only common shared node is the root, so I'll need to do something like:


runTransaction(ref(database, '/'), (value) => {
// do something with the data
})

To do this I need to have read access to the root node, which is kinda fine. As in, it's probably not doable from the client anymore, but you can rely on a Cloud Function or custom BE.

However, it seems like this is far from recommendable, as this operation requires downloading the whole database (or even a large portion of it in case you are not running the transaction on the root node, but still on some parent node with a lot of data).

So, what am I missing here? It seems that either I'm misunderstanding how runTransaction() works and why it requires read access, or it's a very unpractical method in real-life scenarios (also considering the best practices -presented by Firebase as well- on how to structure NoSQL databases).

Can somebody help me understand? thanks!

CodePudding user response：

Your understanding is correct: Firebase reads the entire node where a transactions runs, so you'll want to run it low in your JSON tree.

In the example you shared, adding a group doesn't require a transaction though, but can be accomplished with a single multi-path write operation:

firebase.database().ref().update({
  "users/alovelace/groups/newGroup": true,
  "groups/newGroup/members/alovelace": true
})