Why Sticky sessions in HLD? - Purpose & Use Cases

The Big Idea

What if your favorite website forgot who you were every time you clicked a link?

The Scenario

Imagine a busy restaurant where customers are randomly sent to different waiters every time they order. Each waiter has to start from scratch, asking the same questions again and again.

The Problem

This random assignment causes confusion and delays. Customers get frustrated repeating their orders, and waiters waste time understanding preferences. The service becomes slow and error-prone.

The Solution

Sticky sessions solve this by always sending a customer to the same waiter. The waiter remembers previous orders, making service faster and smoother without repeating questions.

Before vs After

✗ Before

Load balancer sends requests randomly to any server.

✓ After

Load balancer routes requests from the same user to the same server (sticky session).

What It Enables

Sticky sessions enable consistent user experience by keeping user data tied to one server during their visit.

Real Life Example

Online shopping sites use sticky sessions to keep your shopping cart intact as you browse different pages without losing items.

Key Takeaways

Random request routing causes repeated work and confusion.

Sticky sessions keep user requests tied to one server.

This improves speed, consistency, and user satisfaction.

Practice

1. What is the main purpose of sticky sessions in system design?

easy

A. To keep a user connected to the same server during their session

B. To distribute user requests evenly across all servers

C. To store session data in a shared database

D. To encrypt user data during transmission

Solution

1. Step 1: Understand sticky sessions concept

   Sticky sessions ensure that a user's requests go to the same server to maintain session state.

2. Step 2: Compare with other options

   Options B and C describe load balancing and shared storage, not sticky sessions. To encrypt user data during transmission is about security, unrelated here.

3. Final Answer:

   To keep a user connected to the same server during their session -> Option A

4. Quick Check:

   Sticky sessions = same server connection [OK]

Hint: Sticky sessions keep user-server link constant [OK]

Common Mistakes:

• Confusing sticky sessions with load balancing
• Thinking session data is always shared
• Assuming sticky sessions encrypt data

2. Which method is commonly used to implement sticky sessions in a load balancer?

easy

A. Randomly assigning servers per request

B. Encrypting all session data

C. Caching data in a CDN

D. Using IP hashing to route requests

Solution

1. Step 1: Identify sticky session implementation methods

   IP hashing is a common way to ensure requests from the same user go to the same server.

2. Step 2: Eliminate unrelated options

   Encrypting data and caching in CDN do not control session routing. Random assignment breaks stickiness.

3. Final Answer:

   Using IP hashing to route requests -> Option D

4. Quick Check:

   Sticky sessions use IP hashing [OK]

Hint: IP hashing helps keep sessions sticky [OK]

Common Mistakes:

• Confusing encryption with session routing
• Thinking CDN caching manages sessions
• Assuming random server assignment keeps sessions sticky

3. Consider a load balancer using cookie-based sticky sessions. If a user's first request is routed to Server A, where will the second request go?

medium

A. Server B

B. Server A

C. Random server

D. Load balancer itself

Solution

1. Step 1: Understand cookie-based sticky sessions

   The load balancer sets a cookie to identify the server handling the user's session.

2. Step 2: Follow the cookie routing logic

   Subsequent requests with the cookie go to the same server, Server A in this case.

3. Final Answer:

   Server A -> Option B

4. Quick Check:

   Cookie sticky sessions route to same server [OK]

Hint: Cookie keeps user on original server [OK]

Common Mistakes:

• Assuming requests go to different servers
• Thinking load balancer handles requests directly
• Confusing random routing with sticky sessions

4. A system uses IP hashing for sticky sessions but users report inconsistent session behavior. What is the most likely cause?

medium

A. Users have dynamic IP addresses changing frequently

B. Session data is stored in a shared database

C. Load balancer is not configured for SSL

D. Cookies are disabled in user browsers

Solution

1. Step 1: Analyze IP hashing limitations

   IP hashing depends on stable IP addresses to route requests consistently.

2. Step 2: Identify cause of inconsistency

   Dynamic IPs cause requests to appear from different IPs, breaking stickiness.

3. Final Answer:

   Users have dynamic IP addresses changing frequently -> Option A

4. Quick Check:

   Dynamic IPs break IP hashing sticky sessions [OK]

Hint: Dynamic IPs break IP hashing stickiness [OK]

Common Mistakes:

• Blaming shared database for session routing
• Confusing SSL with session stickiness
• Assuming cookies affect IP hashing

5. You design a web app with multiple servers storing session data locally. To ensure users always reach the same server, which approach is best and why?

hard

A. Use IP hashing but allow fallback to any server if IP changes

B. Use a shared database for session storage and random load balancing

C. Use sticky sessions with cookie-based routing to bind users to servers

D. Disable sticky sessions and rely on client-side session storage

Solution

1. Step 1: Understand session storage setup

   Sessions stored locally require users to connect to the same server to access their data.

2. Step 2: Evaluate approaches for consistent routing

   Cookie-based sticky sessions reliably bind users to servers, ensuring session continuity.

3. Step 3: Compare other options

   Shared DB removes need for stickiness but contradicts local storage. IP hashing can fail with IP changes. Client-side storage shifts session responsibility.

4. Final Answer:

   Use sticky sessions with cookie-based routing to bind users to servers -> Option C

5. Quick Check:

   Local sessions need sticky cookie routing [OK]

Hint: Local sessions need cookie sticky routing [OK]

Common Mistakes:

• Choosing shared DB when sessions are local
• Relying on IP hashing despite IP changes
• Ignoring server session storage needs