Stateless vs Stateful Load Balancing
In this tutorial, we are going to discuss about Stateless vs Stateful Load Balancing. Stateless and stateful load balancing are two approaches used in distributing incoming network traffic across multiple servers to ensure efficient resource utilization, high availability, and scalability in web applications and services.
Stateless Load Balancing
In stateless load balancing, each request from a client is treated independently without the load balancer maintaining any knowledge of previous interactions. The decision of where to route a request is made solely based on information available in the current request.
Stateless load balancers operate without maintaining any information about the clients’ session or connection state. They make routing decisions based solely on the incoming request data, such as the client’s IP address, request URL, or other headers. Because stateless load balancers do not store session information, they can quickly and efficiently distribute incoming traffic without considering the clients’ history or past interactions with the application.
Stateless load balancer creates a hash of IP address of the client then uses this number to decide which server should the request be routed to. Hashing solely on the source IP doesn’t provide a good distribution as one client may create a lot of requests that will be sent to one server. But since client would be generating each request using a different source port, a combination of IP address and port is a more viable way to create a hash value.
Characteristics
- Stateless load balancers do not keep track of session state or any context related to previous requests.
- Every request is evaluated in isolation, and the load balancer determines the appropriate server to handle it based on predefined algorithms (e.g., round-robin, least connections, IP hash).
- Stateless load balancing is typically simpler to implement and scales well because there’s no overhead associated with maintaining session state.
Use Cases
- Stateless protocols and applications where each request is self-contained and can be handled independently, such as HTTP-based web services where sessions are managed using cookies or tokens.
- Services where maintaining session affinity is not critical or where session persistence can be managed by other means (e.g., server-side session management).
Example
Consider a web application that enables users to search for products according to their location. A stateless load balancer can allocate requests to servers based on the user’s geographic location, without retaining any session data.
Stateful Load Balancing
Stateful load balancing involves maintaining session state or context information about client-server interactions. The load balancer keeps track of the state of each session and uses this information to make routing decisions.
In contrast, stateful load balancing preserves session information between requests. The load balancer assigns a client to a specific server and ensures that all subsequent requests from the same client are directed to that server. This method is beneficial when requests pertain to a particular session and necessitate session data.
Stateful load balancing can be further categorized into two types:
- Source IP Affinity: This form of stateful load balancing assigns a client to a specific server based on the client’s IP address. While this approach ensures that requests from the same client consistently reach the same server, it may pose issues if the client’s IP address frequently changes, such as in mobile networks.
- Session Affinity: In this type of stateful load balancing, the load balancer allocates a client to a specific server based on a session identifier, such as a cookie or URL parameter. This method ensures that requests from the same client consistently reach the same server, regardless of the client’s IP address.
Characteristics
- Stateful load balancers maintain session affinity, ensuring that subsequent requests from the same client are directed to the same backend server that handled the initial request.
- Session state information can include data such as client IP addresses, session identifiers, connection status, etc.
- Stateful load balancers often provide features like session persistence, SSL termination, and application-layer security (e.g., WAF – Web Application Firewall).
Use Cases
- Applications that require maintaining session state, such as real-time communication systems (e.g., chat applications, online gaming) or applications with user sessions that span multiple requests (e.g., e-commerce websites with shopping carts).
- Protocols or applications where maintaining session affinity is essential for preserving the integrity of client sessions or for optimizing performance (e.g., maintaining cache consistency).
Example
Suppose a web application that requires users to log in to access their personal information. A stateful load balancer can guarantee that requests from the same user are routed to the same server, allowing session data such as login credentials to be available.
Choosing Between Stateless vs Stateful Load Balancing
- Stateless load balancing is often favored for its simplicity and scalability, especially in distributed systems where individual requests can be handled independently.
- Stateful load balancing is preferred when maintaining session state is necessary for preserving application functionality, ensuring data consistency, or optimizing performance.
Ultimately, the decision between stateless and stateful load balancing depends on the application or service’s requirements. Stateless load balancing is useful for applications capable of processing requests independently, while stateful load balancing is more appropriate for applications that depend on session data.
That’s all about Stateless vs Stateful Load Balancing in system design. If you have any queries or feedback, please write us at contact@waytoeasylearn.com. Enjoy learning, Enjoy system design..!!