Firebase is a set of tools with which to build scalable applications in the cloud. Among those tools, you'll find authentication, storage, databases, notifications, and hosting.

You can actually choose between two databases: the Firebase Realtime Database and Cloud Firestore. In this chapter, we'll be using the Cloud Firestore database, which is a NoSQL document database that simplifies storing, querying, and updating data in the cloud. More importantly in the context of this book, you can use it as the backend of your iOS and Android apps, with no need to write the code for a web service and, in many cases, without writing any code at all for your server-side service.

Relational databases use tables to store data, with a fixed schema that all records must follow. For example, if you store user data, you can create a users table with three fields...