Firebase Database, Cloud Functions, Rules

Question

How to restrict write access to /lastmodified table in database named /functions-project-12345 only to cloud function, read & write access to table /chat to everyone

/functions-project-12345
    /lastmodified: 1234567890
    /chat
        /key-123456
            username: "Mat"
            text: "Hey Bob!"
        /key-123457
            username: "Bob"
            text: "Hey Mat"

Answer 1

Cloud Functions run with administrative privileges, which means they bypass the security rules of your database. Knowing that, it is quite simple to secure the database to your requirements:

{
  "rules": {
    ".write": false,
    "chat": {
      ".write": true
    }
  }
}