Load balancing is an essential concept in system design, ensuring that incoming traffic is efficiently distributed across multiple servers or resources. One critical aspect of load balancing is deciding whether to use stateless or stateful methods. In this article, we will break down the differences between these two types and understand their applications.
Stateless vs. Stateful Load Balancing
Imagine you are managing a coffee shop with multiple baristas. Customers come in, place their orders, and wait for their coffee. You have two ways to manage the process:
- Stateless Approach:
- Every customer goes to the next available barista without remembering who served them last time.
- Stateful Approach:
- Regular customers always go to the same barista who knows their favorite order.
These two approaches reflect how stateless and stateful load balancing work in system design. Let’s dive deeper.
What is Stateless Load Balancing?
Stateless load balancing does not keep track of client sessions. Each incoming request is treated independently, without considering any previous interactions. The load balancer makes routing decisions based solely on data like the client’s IP address or request URL.
Example:
A search engine handles thousands of queries per second. The load balancer directs each search request to the next available server without retaining any information about previous queries.
Pros:
- High Performance: No overhead from maintaining session data.
- Scalability: Easy to add or remove servers without disrupting ongoing connections.
- Simplicity: Ideal for stateless applications like search engines or static content delivery.
Cons:
- No Session Persistence: A client might be directed to different servers on subsequent requests.
- Less Suitable for Stateful Apps: Applications that require maintaining user sessions, like login states, might face issues.
What is Stateful Load Balancing?
Stateful load balancing keeps track of the client’s session, ensuring that all requests from a specific client go to the same server. This is crucial when maintaining user sessions or persistent data between interactions.
Example:
A banking application requires users to log in. Once authenticated, the user should remain connected to the same server for the duration of the session to maintain security and consistency.
Pros:
- Session Persistence: Maintains continuity for user-specific data.
- Improved User Experience: Critical for applications like shopping carts or user profiles.
Cons:
- Complex Management: Maintaining session consistency across multiple servers can be challenging.
- Scalability Issues: Adding or removing servers may break session consistency.
Types of Stateful Load Balancing:
- Source IP Affinity:
- Clients are assigned to a specific server based on their IP address.
- Example: A client on a mobile network might be reassigned if the IP changes.
- Session Affinity:
- Uses session identifiers like cookies to ensure the same client always hits the same server.
- Example: An e-commerce site where the user’s shopping cart is maintained on the same server.
Fun Analogy:
Think of it as a hotel check-in process:
- Stateless Load Balancer: Guests are assigned to any available room without remembering which room they stayed in previously.
- Stateful Load Balancer: Regular guests are always assigned to their preferred room.
Mnemonic to Remember:
“Some Servers Retain Sessions, Some Don’t.”
- S: Stateless Load Balancing
- S: Stateful Load Balancing
- R: Retain Session
- S: Session Affinity
- S: Source IP Affinity
- D: Don’t Retain Session
When to Choose Each Type?
- Choose Stateless Load Balancing when:
- The application is stateless (e.g., serving static content).
- High scalability and performance are required.
- Choose Stateful Load Balancing when:
- The application requires session persistence (e.g., user authentication).
- Maintaining a consistent user experience is essential.
Final Thoughts:
Both stateless and stateful load balancing have their merits, and the choice depends on the application’s requirements. Stateless methods offer simplicity and performance, while stateful techniques ensure session consistency. Understanding the difference helps you make an informed decision when designing systems that need to handle varied workloads.
🚀 System Design for TPM Series – Part 5: Stateless vs. Stateful Load Balancing 🚀
Explore the fundamental differences between stateless and stateful load balancing and when to use each!
Read here:
https://github.com/ANSANJAY/System-Design-Fundamentals/blob/main/Stateless%20vs%20Stateful%20Load%20Balancing.md
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