What is a backend?

The vanishing saves

When Kai and Remy first built Garden Defense, saves lived in local storage: a small store of data inside one browser on one device. Clear the browser or get a new phone, and the garden is gone.

To fix that, our games need a partner on the internet called a backend: the part of a game that runs on servers instead of on the player's device.

Client and server

The client is the game running on your device: your phone, tablet or laptop. It draws the ship, plays the sounds and reads your buttons.

The server is a computer in a data centre, a building packed with thousands of servers, cooling fans and backup power. It's always on and always online.

They talk by sending messages. The client sends a request ("save my garden, please") and the server sends back a response ("saved").

What lives on the backend?

• SavesEvery player's progress, safe even if their phone falls in a pond.
• ScoresHigh scores from players all over the world, in one place.
• EventsSettings that switch on a holiday event for everyone at once (Stage 9).
• AccountsWho each player is, so the right save goes to the right person.

Bolt says: A simple rule: if many players or many devices need it, it belongs on the backend.

Running servers is a huge job

Running your own servers is serious work. They must stay on day and night, because somewhere in the world a player is always logging in. They need security updates as soon as a weakness is found, and backups in case a disk breaks.

And they must scale: cope with 1 player on a quiet Tuesday or 100,000 players on launch day. Big studios have whole teams, often called DevOps or live ops engineers, just for this.

Backend-as-a-Service: Firebase

• AuthenticationLogs players in and gives each one an ID.
• FirestoreA database that stores saves and scores.
• Remote ConfigSwitches we can flip without updating the game.

Backend-as-a-Service means another company runs the servers, and we use just the parts we need. Our pick is Firebase, made by Google. Its free plan, Spark, has a daily allowance that's plenty for a small studio. A pay-as-you-go plan, Blaze, is there when we grow.

When we create the database we pick a location, like a region in Europe or the US, close to most players. It can't be changed later, so we choose with care.

Setting up as a team

A Firebase project lives inside a Google account, and creating it means agreeing to Google's terms of service. Terms of service are a contract: promises with legal weight on both sides.

The law requires an adult to sign contracts, and Google accounts for children under 13 (older in some countries) are managed by a parent. So you and a parent create the project together. The parent owns the account, and the two of you build in it side by side.

Next: how the messages work

Client asks, server answers. Every save, score and login in this stage is a request and a response.

Next we open one of those messages and look inside: URLs, status codes and JSON. Then we plan our data, and then come logins, so the server knows who is asking.

Bolt says: Client, server, backend. Three words, and you can already explain how online games keep your progress.