Cost optimization (read/write reduction) in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

Understanding Firestore Read Costs

You have a Firestore collection with 1000 documents. You run a query that returns 10 documents. How many document reads will be billed for this query?

A 1 document read

B 1000 document reads

C 10 document reads

D 0 document reads

💡 Hint

Think about how Firestore charges for queries based on documents returned, not total documents in the collection.

Firestore charges for each document returned by a query. Since 10 documents are returned, 10 document reads are billed.

❓ Architecture

intermediate

Improving Firestore Write Performance with Batched Writes

You need to update 50 documents in Firestore. Which approach reduces network calls and improves performance?

A Update each document individually with separate write calls

B Read all documents first, then update them one by one

C Delete and recreate the entire collection

D Use a batched write to update all 50 documents in one operation

💡 Hint

Consider how Firestore handles batched writes and network calls.

Batched writes group multiple updates into a single atomic operation, reducing network overhead and improving performance, though write costs per document remain the same.

❓ security

advanced

Minimizing Unnecessary Firestore Reads with Security Rules

You want to prevent users from reading documents they shouldn't access to reduce unnecessary read costs. Which security rule snippet correctly restricts reads to only documents where the user's UID matches the document's owner field?

Firebase

match /documents/{docId} {
  allow read: if request.auth.uid == resource.data.owner;
}

A allow read: if request.auth.uid == resource.data.owner;

B allow read: if request.auth.uid != resource.data.owner;

C allow read: if true;

D allow read: if request.auth.uid == null;

💡 Hint

Think about how to restrict access to only authorized users.

Option A allows reads only if the authenticated user's UID matches the document's owner field, preventing unauthorized reads and reducing costs.

🧠 Conceptual

advanced

Optimizing Firestore Data Structure for Read Efficiency

You have a Firestore app where users frequently read profile data and their recent activity. Which data structure approach reduces read operations and costs?

A Denormalize recent activity data inside the user profile document

B Store profile and recent activity in separate collections and read them separately

C Embed recent activity as a subcollection inside each user profile document

D Store all recent activity in a global collection and query by user ID

💡 Hint

Consider how denormalization can reduce the number of reads.

Denormalizing recent activity inside the user profile document allows fetching all needed data in a single read, reducing read operations and costs.

✅ Best Practice

expert

Implementing Client-side Caching to Reduce Firestore Reads

You want to minimize Firestore read costs by caching data on the client side. Which approach best balances data freshness and cost savings?

A Disable caching and always fetch fresh data from Firestore

B Use Firestore's offline persistence with periodic manual refreshes

C Cache data indefinitely on the client and never refresh

D Cache data only in memory and clear cache on every app restart

💡 Hint

Think about how Firestore's offline persistence works and how to keep data reasonably fresh.

Firestore's offline persistence caches data locally and syncs changes automatically. Periodic manual refreshes ensure data freshness while reducing read costs.