Firebase basics

Why Firebase?

Many apps need the same backend features: user login, a cloud database, file storage, push notifications and crash reports. Building and running a server for these takes time and skills. Firebase provides them as managed services with Android SDKs, so a small team or a solo developer can ship a full app quickly.

Adding Firebase to an Android app

1. Create a project in the Firebase console and add an Android app with your exact applicationId.
2. Download google-services.json and put it in the app/ folder.
3. Add the Google services Gradle plugin and the Firebase BoM, then the libraries you need.

// app/build.gradle.kts
plugins {
    id("com.google.gms.google-services")
}

dependencies {
    implementation(platform("com.google.firebase:firebase-bom:<version>"))
    implementation("com.google.firebase:firebase-auth")
    implementation("com.google.firebase:firebase-firestore")
    implementation("com.google.firebase:firebase-crashlytics")
}

The main services

Firestore in practice

Firestore stores data as documents (like JSON objects) inside collections. For example, a collection orders holds one document per order. You can read once, or listen for changes and get updates in real time, which is how chat apps show new messages instantly.

data class Order(val item: String = "", val qty: Int = 0, val status: String = "placed")

val db = Firebase.firestore

// Write
db.collection("orders").add(Order("Veg biryani", 2))

// Listen for real-time changes (wrap in a Flow in a repository)
db.collection("orders").whereEqualTo("status", "placed")
    .addSnapshotListener { snapshot, error ->
        val orders = snapshot?.toObjects(Order::class.java).orEmpty()
    }

Security rules: never skip this

Firestore and Storage are accessed directly from the app, so security rules on Firebase's servers decide who can read and write what. The "test mode" offered when you create a database allows anyone to read and write for a limited time. Before real users arrive, write rules such as "users can read and write only their own documents". Leaving open rules has exposed many apps' user data.

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId}/orders/{orderId} {
      allow read, write: if request.auth != null && request.auth.uid == userId;
    }
  }
}

Pricing

Firebase has a free plan (Spark) with generous limits for learning and small apps, and a pay-as-you-go plan (Blaze) for growth; some features, such as Cloud Functions, need Blaze. Phone authentication SMS and heavy database usage cost money at scale. Check the current pricing page, set budget alerts, and design queries to read only what you need.

Common mistakes

• Leaving Firestore in test mode with open rules.
• google-services.json for a different applicationId: the app cannot connect.
• Reading whole collections when the screen needs ten items: slow and expensive. Use queries and limits.
• Not declaring Firebase data collection in the Data safety form.