Multi-tenancy patterns in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Multi-tenancy Isolation Levels

In Firebase multi-tenancy, which isolation level best ensures that data from one tenant cannot be accessed by another tenant?

A Shared database with tenant ID filtering in queries

B Single Firestore collection with tenant data mixed

C Shared Firebase project with different Firestore collections per tenant

D Separate Firebase projects for each tenant

💡 Hint

Think about physical separation of data and access control.

Using separate Firebase projects for each tenant provides the strongest isolation because each tenant's data and resources are completely separated at the project level.

❓ Architecture

intermediate

Choosing a Multi-tenancy Data Model in Firestore

You want to design a Firestore database for multiple tenants. Which data model allows easy querying of tenant-specific data while minimizing data duplication?

A One collection per tenant with duplicated structure

B Single collection with tenant ID field in each document

C Separate Firestore projects per tenant

D Single collection with tenant data mixed without tenant ID

💡 Hint

Consider how queries filter data efficiently.

A single collection with a tenant ID field allows filtering by tenant while keeping the structure consistent and avoiding duplication.

❓ security

advanced

Securing Tenant Data with Firebase Security Rules

Given a Firestore collection 'orders' where each document has a 'tenantId' field, which Firebase security rule correctly restricts read access to documents belonging only to the authenticated tenant?

match /orders/{orderId} {
  allow read: if ... ;
}

A request.auth.token.tenantId == resource.data.tenantId

B request.auth.uid == resource.data.tenantId

C resource.data.tenantId == 'tenant123'

D request.auth.token.email == resource.data.tenantId

💡 Hint

Check how tenant identity is passed in the authentication token.

The tenant ID should be included in the user's auth token and matched against the document's tenantId field to enforce access control.

❓ service_behavior

advanced

Behavior of Firebase Functions in Multi-tenant Setup

In a multi-tenant Firebase app using a single project, what happens if a Cloud Function does not check tenant identity before processing a request?

A Function runs only for the first tenant that called it

B Function automatically restricts access to the calling tenant's data

C Function processes requests for all tenants without restriction

D Function throws an authentication error

💡 Hint

Consider default behavior without explicit tenant checks.

Without explicit tenant checks, the function will process all requests regardless of tenant, risking data leaks.

✅ Best Practice

expert

Optimizing Cost and Performance in Firebase Multi-tenancy

You manage a Firebase app serving multiple tenants with varying usage patterns. Which multi-tenancy pattern best balances cost efficiency and performance?

A Single Firebase project with tenant ID filtering and Firestore rules

B Separate Firebase projects per tenant regardless of size

C Single Firebase project with duplicated collections per tenant

D Shared Firebase project with no tenant separation

💡 Hint

Think about managing resources and scaling efficiently.

A single Firebase project with tenant ID filtering and security rules allows sharing resources while isolating data logically, optimizing cost and performance.

Practice

1. What is the main purpose of multi-tenancy in Firebase applications?

easy

A. To allow multiple users to share the same app while keeping their data separate

B. To increase the speed of database queries

C. To reduce the cost of Firebase hosting

D. To enable offline data access for users

Solution

1. Step 1: Understand multi-tenancy concept

   Multi-tenancy means many users use one app but their data stays separate.

2. Step 2: Identify the purpose in Firebase context

   Firebase apps use multi-tenancy to keep user data isolated while sharing the app.

3. Final Answer:

   To allow multiple users to share the same app while keeping their data separate -> Option A

4. Quick Check:

   Multi-tenancy = separate data for shared app [OK]

Hint: Multi-tenancy means shared app, separate data [OK]

Common Mistakes:

• Confusing multi-tenancy with performance optimization
• Thinking multi-tenancy is about offline access
• Assuming multi-tenancy reduces hosting costs

2. Which Firestore data structure is best for separating tenant data in a multi-tenant Firebase app?

easy

A. A single collection with tenant ID as a field in each document

B. Separate collections for each tenant grouped by tenant ID

C. One document per tenant with all data nested inside

D. Storing all tenant data in one document without tenant ID

Solution

1. Step 1: Review Firestore data organization

   Firestore uses collections and documents to organize data.

2. Step 2: Choose best pattern for tenant separation

   Grouping data by tenant ID in separate collections keeps tenant data isolated and easier to secure.

3. Final Answer:

   Separate collections for each tenant grouped by tenant ID -> Option B

4. Quick Check:

   Tenant data separated by collections = Separate collections for each tenant grouped by tenant ID [OK]

Hint: Use collections grouped by tenant ID for clear separation [OK]

Common Mistakes:

• Putting all tenant data in one document
• Not using tenant ID to separate data
• Using a single collection without grouping

3. Given this Firestore security rule snippet for multi-tenancy:

match /tenants/{tenantId}/data/{docId} {
  allow read, write: if request.auth.token.tenant == tenantId;
}

What happens if a user with tenant ID 'tenantA' tries to read data from 'tenantB'?

medium

A. Access is granted because tenant IDs are ignored

B. Access is granted only for read but not write

C. Access is denied only if the document does not exist

D. Access is denied because tenant IDs do not match

Solution

1. Step 1: Understand the security rule condition

   The rule allows access only if the user's tenant ID matches the tenantId in the path.

2. Step 2: Apply the condition to the user and requested tenant

   User's tenant ID is 'tenantA' but tries to access 'tenantB' data, so condition fails.

3. Final Answer:

   Access is denied because tenant IDs do not match -> Option D

4. Quick Check:

   Tenant ID mismatch blocks access [OK]

Hint: Tenant ID in token must match path tenant ID [OK]

Common Mistakes:

• Assuming access is granted without tenant ID check
• Thinking read access is allowed but write is denied
• Ignoring tenant ID in security rules

4. You wrote this Firestore rule for multi-tenancy:

match /tenants/{tenantId}/data/{docId} {
  allow read, write: if request.auth.token.tenantId == tenantId;
}

But users report access errors. What is the likely problem?

medium

A. The token field name is incorrect; it should be 'tenant' not 'tenantId'

B. The path variable 'tenantId' is not defined in the rule

C. Firestore does not support tenant-based security rules

D. The rule allows access to all tenants without restriction

Solution

1. Step 1: Check token field name in the rule

   The rule uses 'request.auth.token.tenantId' but the token field is usually 'tenant'.

2. Step 2: Understand impact of wrong token field

   If the token field name is wrong, the condition always fails, causing access errors.

3. Final Answer:

   The token field name is incorrect; it should be 'tenant' not 'tenantId' -> Option A

4. Quick Check:

   Correct token field name fixes access [OK]

Hint: Match token field name exactly in rules [OK]

Common Mistakes:

• Using wrong token field name
• Assuming Firestore ignores token field names
• Not defining path variables correctly

5. You want to design a Firebase app supporting multiple tenants with isolated data and secure access. Which combination of patterns is best?

hard

A. Use a single Firestore collection for all tenants and filter data in the app code

B. Use Firebase Realtime Database instead of Firestore for multi-tenancy

C. Create separate Firestore collections per tenant and enforce tenant ID checks in security rules

D. Store all tenant data in one document and use client-side filtering for access control

Solution

1. Step 1: Evaluate data separation methods

   Separate collections per tenant keep data isolated and easier to manage.

2. Step 2: Apply security best practices

   Security rules must check tenant ID to prevent unauthorized access.

3. Step 3: Compare options

   Create separate Firestore collections per tenant and enforce tenant ID checks in security rules combines data separation and security rules, ensuring isolation and protection.

4. Final Answer:

   Create separate Firestore collections per tenant and enforce tenant ID checks in security rules -> Option C

5. Quick Check:

   Separate collections + security rules = best multi-tenancy [OK]

Hint: Combine data separation with security rules for best isolation [OK]

Common Mistakes:

• Relying on client-side filtering only
• Using single collection without security rules
• Choosing Realtime Database without need