Optimistic concurrency in Firebase - Mini Project: Build & Apply

Optimistic Concurrency Control with Firebase

📖 Scenario: You are building a simple app where multiple users can update a shared document in Firebase Firestore. To avoid overwriting each other's changes, you will implement optimistic concurrency control using a version number.

🎯 Goal: Create a Firestore document with a version field, read and update it safely by checking the version to prevent conflicts.

📋 What You'll Learn

Create a Firestore document with a content string and a version number

Read the document and store its version

Write an update function that only updates if the version matches

Increment the version on successful update

💡 Why This Matters

🌍 Real World

Optimistic concurrency control is used in collaborative apps where multiple users edit shared data to prevent overwriting each other's changes.

💼 Career

Understanding concurrency control in cloud databases like Firestore is essential for backend and full-stack developers working on real-time applications.

1

Create initial Firestore document

Create a Firestore document in the collection sharedDocs with the ID doc1. The document should have two fields: content set to "Initial content" and version set to 1.

Firebase

// Your code here to create the Firestore document with content and version

Need a hint?

Use setDoc with doc(db, 'sharedDocs', 'doc1') and set content and version fields.

2

Read document and store version

Write code to read the Firestore document doc1 from the sharedDocs collection. Store the document data in a variable called docData and the version field in a variable called currentVersion.

Firebase

import { getFirestore, doc, setDoc, getDoc } from 'firebase/firestore';

const db = getFirestore();

await setDoc(doc(db, 'sharedDocs', 'doc1'), {
  content: 'Initial content',
  version: 1
});

// Your code here to read the document and store version

Need a hint?

Use getDoc to read the document, then extract version from the data.

3

Implement optimistic concurrency update logic

Write a function called updateContentIfVersionMatches that takes newContent and expectedVersion as parameters. It should read the document doc1, check if the version matches expectedVersion, and if so, update the content to newContent and increment the version by 1. Use Firestore transactions to ensure atomicity.

Firebase

import { getFirestore, doc, setDoc, getDoc, runTransaction } from 'firebase/firestore';

const db = getFirestore();

await setDoc(doc(db, 'sharedDocs', 'doc1'), {
  content: 'Initial content',
  version: 1
});

const docRef = doc(db, 'sharedDocs', 'doc1');
const docSnap = await getDoc(docRef);

const docData = docSnap.data();
const currentVersion = docData.version;

// Your code here to write updateContentIfVersionMatches function

Need a hint?

Use runTransaction to read and update the document atomically. Check if version matches expectedVersion before updating.

4

Call update function with concurrency check

Call the function updateContentIfVersionMatches with newContent set to "Updated content" and expectedVersion set to the previously stored currentVersion variable.

Firebase

import { getFirestore, doc, setDoc, getDoc, runTransaction } from 'firebase/firestore';

const db = getFirestore();

await setDoc(doc(db, 'sharedDocs', 'doc1'), {
  content: 'Initial content',
  version: 1
});

const docRef = doc(db, 'sharedDocs', 'doc1');
const docSnap = await getDoc(docRef);

const docData = docSnap.data();
const currentVersion = docData.version;

async function updateContentIfVersionMatches(newContent, expectedVersion) {
  await runTransaction(db, async (transaction) => {
    const docSnapshot = await transaction.get(docRef);
    if (!docSnapshot.exists()) {
      throw 'Document does not exist!';
    }
    const data = docSnapshot.data();
    if (data.version !== expectedVersion) {
      throw 'Version mismatch';
    }
    transaction.update(docRef, {
      content: newContent,
      version: expectedVersion + 1
    });
  });
}

// Your code here to call updateContentIfVersionMatches

Need a hint?

Call updateContentIfVersionMatches with the exact string 'Updated content' and the variable currentVersion.