title: How constant is the Firebase Anonymous ID
body: <p>I'm building an app with Firebase Real-Time Database. The app will be mainly offline at first, but we are planning an online update later. I'm planning to use the anonymous sign in, to get an ID for the user and store all his data under this ID:</p>
  
  <pre><code>Auth.auth().signInAnonymously(completion: { (user, error) -&gt; Void })
  </code></pre>
  
  <p>After signing in I get an ID that looks like something like this: <code>pCfgFOvEYEYvfWHaaaaavKgs8h33</code></p>
  
  <p>Is it guaranteed that this ID will <strong>ALWAYS</strong> stay unchanged on a given device? I couldn't find any documentation about this.</p>