SOLID for Firebase Apps

The default Firebase tutorial puts every read, write, and rule check directly in a screen component. That works until your second screen needs the same query, or until you want to test a payment flow without spending real money. This skill is what to do then — applied specifically to Firebase + React Native (the web SDK story is similar).

Minimum viable example

// lib/services/todos.ts — owns Firestore for one collection. SRP.
import firestore from "@react-native-firebase/firestore";

export const TodosService = {
  list(uid: string) {
    return firestore().collection("users").doc(uid).collection("todos");
  },
  add(uid: string, title: string) {
    return TodosService.list(uid).add({
      title, done: false,
      createdAt: firestore.FieldValue.serverTimestamp(),
    });
  },
  toggle(uid: string, id: string, done: boolean) {
    return TodosService.list(uid).doc(id).update({ done });
  },
};

// lib/hooks/useTodos.ts — the only thing components import. DIP.
import { useEffect, useState } from "react";
import { TodosService } from "../services/todos";

export function useTodos(uid: string) {
  const [todos, setTodos] = useState<unknown[]>([]);
  useEffect(() => {
    const unsub = TodosService.list(uid)
      .orderBy("createdAt", "desc")
      .onSnapshot((snap: any) =>
        setTodos(snap.docs.map((d: any) => ({ id: d.id, ...d.data() })))
      );
    return unsub;
  }, [uid]);
  return todos;
}

The component uses const todos = useTodos(uid) — that's all it needs to know. Firebase is one import away from being swapped.

When SOLID earns its keep in a Firebase app

Honest evaluation before the patterns:

The patterns below are progressive — apply them in order as the app grows. Don't start with a Repository pattern on day 1.

1. SRP — Single Responsibility

Bad — one component owns auth, Firestore, UI, navigation, validation:

function TodosScreen() {
  const uid = auth().currentUser!.uid;
  const [todos, setTodos] = useState([]);
  const [draft, setDraft] = useState("");

  useEffect(() => {
    return firestore().collection("users").doc(uid).collection("todos")
      .orderBy("createdAt", "desc")
      .onSnapshot((snap) => setTodos(snap.docs.map(d => ({ id: d.id, ...d.data() }))));
  }, [uid]);

  async function add() {
    if (!draft.trim()) return;
    await firestore().collection("users").doc(uid).collection("todos").add({
      title: draft.trim(),
      done: false,
      createdAt: firestore.FieldValue.serverTimestamp(),
    });
    setDraft("");
  }

  return <View>{/* ... */}</View>;
}

Good — three files, each with one job:

// app/(app)/todos.tsx — presentation only
function TodosScreen() {
  const uid = useUid();              // auth concern
  const todos = useTodos(uid);       // data concern
  const { add, toggle } = useTodoActions(uid);
  return <TodosList todos={todos} onAdd={add} onToggle={toggle} />;
}

Why it matters in Firebase apps: the SDK is stateful (snapshot listeners, auth state, persistence). Mixing state-having code with stateless UI logic makes the component hard to reason about and impossible to memoize.

2. OCP — Open/Closed

You should be able to add behavior to your Firestore writes (validation, audit logs, optimistic updates) without modifying every call site.

Bad — every screen that creates a post duplicates the same fields:

await firestore().collection("posts").add({
  ...data,
  createdAt: firestore.FieldValue.serverTimestamp(),
  authorId: auth().currentUser!.uid,
  schemaVersion: 2,
});

Good — a factory the service owns, screens are closed to schema changes:

const PostsService = {
  async create(data: { title: string; body: string }) {
    return firestore().collection("posts").add({
      ...data,
      createdAt: firestore.FieldValue.serverTimestamp(),
      authorId: auth().currentUser!.uid,
      schemaVersion: 2,
    });
  },
};

When schemaVersion becomes 3, you change one file. The same pattern works for Cloud Functions middleware (auth check + app check + validation as composable wrappers).

3. LSP — Liskov Substitution

Firestore's strength here is typed converters — they let you treat a DocumentSnapshot<Post> interchangeably with any other typed snapshot.

import { type FirestoreDataConverter } from "firebase/firestore";

type Post = { authorId: string; title: string; body: string; createdAt: number };

export const postConverter: FirestoreDataConverter<Post> = {
  toFirestore: (post) => post,
  fromFirestore: (snap) => snap.data() as Post,
};

const postsRef = collection(db, "posts").withConverter(postConverter);
// snap.data() is now typed as Post throughout the app

Substitutability matters for swapping storage implementations later. If a Post always has the same shape coming out of fromFirestore regardless of where you query it from, downstream code never has to type-check.

4. ISP — Interface Segregation

Don't make hooks depend on more of the Firebase SDK than they need.

Bad:

import { auth, firestore, storage, messaging } from "../lib/firebase";
// Hook only needs the current uid but transitively pulls in everything

Good:

// lib/auth.ts — only exposes what most code needs
export function useUid(): string | null {
  const [uid, setUid] = useState<string | null>(null);
  useEffect(() => auth().onAuthStateChanged((u) => setUid(u?.uid ?? null)), []);
  return uid;
}

Now hooks import useUid from lib/auth, not auth itself. The web bundle is smaller (better tree-shaking) and the dependency surface is narrower.

Same for Cloud Functions — split a giant index.ts into per-feature files re-exported from index.ts. Firebase deploys only redeploy changed functions when bundles are distinct.

5. DIP — Dependency Inversion

The big one. Hooks and components should depend on abstractions, not on Firebase directly. This is what makes tests possible without an emulator AND what lets you migrate backends.

Two levels depending on scale:

Level 1 (small app) — a service module as the seam

// lib/services/todos.ts — the only file that imports firestore
import firestore from "@react-native-firebase/firestore";

export const TodosService = {
  list: (uid: string) => firestore().collection("users").doc(uid).collection("todos"),
  add:    (uid: string, title: string) => /* ... */,
  toggle: (uid: string, id: string, done: boolean) => /* ... */,
};

Hooks and components import TodosService, never firestore directly. For tests, mock the module. For backend migration, rewrite this one file.

Level 2 (real testability needed) — a Repository interface

// lib/repositories/todos.ts
export interface TodosRepository {
  watchByUser(uid: string, cb: (todos: Todo[]) => void): () => void;
  add(uid: string, title: string): Promise<string>;
  toggle(uid: string, id: string, done: boolean): Promise<void>;
}

// lib/repositories/firestoreTodos.ts
export const firestoreTodos: TodosRepository = { /* ... */ };

// lib/repositories/inMemoryTodos.ts
export function createInMemoryTodos(): TodosRepository { /* ... */ }

Hooks accept the repo via context (a TodosProvider) — production wires firestoreTodos, tests wire createInMemoryTodos(). Now you can run E2E tests in milliseconds with no Firebase emulator at all.

The cost: more indirection. The benefit: tests run 100× faster, you can swap Firebase for Supabase in one file, and your domain logic stops depending on an external SDK's release cadence.

Use Level 1 by default. Promote to Level 2 when the tests-against-emulator pain exceeds the indirection pain.

Cross-cutting: Cloud Functions

The same principles apply, with one Firebase-specific twist: avoid initializing the Admin SDK in every file.

// functions/src/lib/admin.ts — initialized once
import { initializeApp, getApps } from "firebase-admin/app";
if (getApps().length === 0) initializeApp();
export { getFirestore } from "firebase-admin/firestore";
export { getAuth } from "firebase-admin/auth";

// functions/src/posts/archive.ts — depends on the abstraction
import { onCall, HttpsError } from "firebase-functions/v2/https";
import { getFirestore } from "../lib/admin";

export const archivePosts = onCall(async (request) => {
  if (!request.auth) throw new HttpsError("unauthenticated", "");
  await getFirestore().collection("posts")/* ... */;
});

Each function file is a single responsibility (one operation, one route). The admin SDK is initialized in one place.

Common mistakes

• Premature Repository pattern. Wrapping every Firestore call in a IUserRepository interface for a 3-screen app is overhead the project can't repay. Start with direct SDK calls in a service module; promote to interfaces when you need tests.
• God hooks. A useApp() that returns user, todos, posts, notifications, and feature flags. Split per concern; let consumers choose what to subscribe to.
• Service classes with state. class TodosService { private cache = ... } is an anti-pattern — Firestore already manages cache. Use module-level functions, not classes-with-state.
• Wrapping onSnapshot listeners in promises. They're streams, not promises. The repository abstraction returns an Unsubscribe function, not Promise<Todo[]>.
• Exporting raw Firebase types from your service. If TodosService.list() returns a FirebaseFirestoreTypes.CollectionReference, every consumer is coupled to Firebase. Map to your own Todo type at the service boundary.
• DIP all the way down to security rules. Rules are not application logic; they're a backend security layer. Don't try to abstract them into a generic permission system — express the security model directly in firestore.rules as the firebase-firestore skill recommends.
• Class hierarchies for Firebase wrappers. A BaseRepository<T> parent class with UsersRepository extends BaseRepository<User> is a Java reflex. In TypeScript, prefer composition with generic factory functions: function createRepository<T>(collectionName: string): Repository<T>.
• Skipping SOLID for "just an MVP". Trap: the MVP becomes the codebase. Adopt SRP and ISP from day 1 (they're free). Defer OCP/LSP/DIP until they pay for themselves.

See references/testing-with-repositories.md for a complete worked example with Vitest + inMemoryTodos.