Immutability for safety in LLD - Architecture Diagram

System Overview - Immutability for safety

This system demonstrates how immutability helps keep data safe and consistent. It ensures that once data is created, it cannot be changed, preventing accidental or malicious modifications. The system supports multiple users reading data safely while updates create new versions without altering existing ones.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
Immutable Service
  |
  +----------------+
  |                |
  v                v
Immutable Store   Cache
(Database with     (Read-only
append-only data)  cache for
                   fast reads)

Components

User

client

Initiates requests to read or write data

Load Balancer

load_balancer

Distributes incoming requests evenly to API Gateway instances

API Gateway

api_gateway

Routes requests to the Immutable Service and handles authentication

Immutable Service

service

Processes requests, enforces immutability by creating new data versions instead of modifying existing ones

Immutable Store

database

Stores data in an append-only manner to preserve history and prevent changes to existing data

Cache

cache

Provides fast read access to frequently requested immutable data

Request Flow - 11 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Immutable Service

Immutable Service → Cache

Cache → Immutable Service

Immutable Service → Immutable Store

Immutable Store → Immutable Service

Immutable Service → Cache

Immutable Service → API Gateway

API Gateway → Load Balancer

Load Balancer → User

Failure Scenario

Component Fails:Immutable Store

Impact:New writes fail because data cannot be appended; reads may still succeed if cache has data

Mitigation:Use database replication and failover to a standby immutable store; cache serves stale data during failover

Architecture Quiz - 3 Questions

Which component ensures that data once written cannot be changed?

A Load Balancer

B Immutable Service

C Cache

D API Gateway

Design Principle

This architecture uses immutability to ensure data safety by never modifying existing data. Instead, new versions are appended, preserving history and preventing accidental changes. Caching immutable data improves read performance without risking stale or inconsistent data.

Practice

1. What is the main benefit of using immutability in system design?

easy

A. It allows faster data processing by skipping checks.

B. It makes data changeable by multiple users at the same time.

C. It prevents data from being changed after creation, improving safety.

D. It reduces the size of data stored in memory.

Solution

1. Step 1: Understand immutability meaning

   Immutability means data cannot be changed once created.

2. Step 2: Identify safety benefit

   This prevents accidental or concurrent changes, improving safety.

3. Final Answer:

   It prevents data from being changed after creation, improving safety. -> Option C

4. Quick Check:

   Immutability = Prevents changes [OK]

Hint: Immutability means no changes allowed after creation [OK]

Common Mistakes:

• Thinking immutability allows data changes
• Confusing immutability with performance optimization
• Assuming immutability reduces memory size

2. Which of the following code snippets correctly creates an immutable data structure in a low-level design context?

easy

A. Using a constant object or final class with no setters.

B. Using a regular class with public fields that can be changed.

C. Using a mutable list that allows adding or removing items.

D. Using a global variable that can be updated anytime.

Solution

1. Step 1: Identify immutable structure traits

   Immutable means no changes allowed after creation, so no setters or public mutable fields.

2. Step 2: Match code snippet to traits

   Constant object or final class with no setters fits immutability.

3. Final Answer:

   Using a constant object or final class with no setters. -> Option A

4. Quick Check:

   Immutable = constant, no setters [OK]

Hint: Immutable means no setters or public mutable fields [OK]

Common Mistakes:

• Choosing mutable lists or global variables
• Confusing final keyword with mutable fields
• Ignoring setters in class design

3. Consider this pseudo-code snippet for an immutable user profile object:

user = ImmutableUser(name='Alice', age=30)
user.age = 31
print(user.age)

What will be the output?

medium

A. 31

B. None

C. 30

D. Error: Cannot modify immutable object

Solution

1. Step 1: Understand immutability effect on assignment

   Immutable objects do not allow changing fields after creation.

2. Step 2: Analyze the assignment line

   Trying to assign user.age = 31 will cause an error because the object is immutable.

3. Final Answer:

   Error: Cannot modify immutable object -> Option D

4. Quick Check:

   Immutable object modification = Error [OK]

Hint: Immutable objects throw error on field change [OK]

Common Mistakes:

• Assuming value silently changes
• Assuming old value prints without error
• Ignoring immutability enforcement

4. You have a mutable shared configuration object causing race conditions in a concurrent system. Which fix uses immutability to solve this?

medium

A. Add locks around every access to the mutable object.

B. Replace the shared object with an immutable configuration instance passed by value.

C. Allow threads to modify the shared object but reset it periodically.

D. Use global variables to store configuration for faster access.

Solution

1. Step 1: Identify immutability benefit in concurrency

   Immutable objects prevent race conditions by disallowing changes.

2. Step 2: Choose solution using immutability

   Replacing shared mutable object with immutable instance passed by value avoids conflicts.

3. Final Answer:

   Replace the shared object with an immutable configuration instance passed by value. -> Option B

4. Quick Check:

   Immutability fixes race conditions [OK]

Hint: Immutable shared data avoids race conditions [OK]

Common Mistakes:

• Relying only on locks without immutability
• Allowing mutable shared state
• Using global variables increases risk

5. In a complex system, you want to safely share user session data across multiple services without accidental modification. Which design approach best uses immutability for safety?

hard

A. Create immutable session objects and pass copies to each service.

B. Use a single mutable session object shared globally with synchronization.

C. Store session data in a database and allow services to update it directly.

D. Send session data as plain text strings and let services parse and modify.

Solution

1. Step 1: Understand immutability in distributed systems

   Immutable objects prevent accidental changes when shared across services.

2. Step 2: Evaluate design options

   Passing immutable session copies ensures safety without synchronization overhead.

3. Final Answer:

   Create immutable session objects and pass copies to each service. -> Option A

4. Quick Check:

   Immutable copies for safe sharing [OK]

Hint: Pass immutable copies to avoid accidental changes [OK]

Common Mistakes:

• Using mutable shared objects with locks
• Allowing direct database updates without control
• Parsing and modifying plain text increases errors