Allowing write access only to Cloud Functions for Firebase

Question

How can we secure database via the rules that only allow Cloud Functions for Firebase to write data to certain locations, previously there was an option to add uid to admin client databaseAuthVariableOverride and use that uid in rules section, but now we initialise via admin.initializeApp(functions.config().firebase); so I’m not to sure about how to add additional params in.

EDIT is it a good idea to initiate with certificate for this instead? i.e

admin.initializeApp({
  credential: admin.credential.cert("/path-to-cert"),
  databaseURL: "database-url",
  databaseAuthVariableOverride: { uid: "some-id" }
});

What benefit does admin.initializeApp(functions.config().firebase) have over above and where is functions.config() actually getting data from, isn't this just a node module?

Answer 1

Normally, at the top of your Cloud Functions code, you have:

var functions = require('firebase-functions');

As part of the firebase-functions node module, you have access to a functions.config().firebase which is just an object which has everything you need to initialize the Admin SDKs, including your Database URL and a credential implementation (based off of Application Default Credentials). If you console.log(functions.config().firebase) in your code, you will see it just is an object with these properties and a few other ones you may want to use in your code.

You can add databaseAuthVariableOverride to this object to limit the Admin SDK's privileges. You can just overwrite the object itself:

var firebaseConfig = functions.config().firebase;
firebaseConfig.databaseAuthVariableOverride = {
  uid: 'some-uid',
  foo: true,
  bar: false
};
admin.initializeApp(firebaseConfig);

Or you can use something like Object.assign() to copy the relevant details to a new object:

var firebaseConfig = Object.assign({}, functions.config().firebase, {
  databaseAuthVariableOverride: {
    uid: 'some-uid',
    foo: true,
    bar: false
  }
});
admin.initializeApp(firebaseConfig);