What are Sticky Sessions in Load Balancing?

Last Updated : 27 Jun, 2024

Load balancing is a crucial technique that ensures that incoming network traffic is distributed across multiple servers to optimize resource use, maximize throughput, minimize response time, and avoid overload on any single server. One of the important concepts within load balancing is "sticky sessions," also known as session persistence.

Important Topics to Understand What are Sticky Sessions in Load Balancing

What are Sticky Sessions?

Sticky sessions refer to the method of binding a user's session to a specific server within a load-balanced environment. When a user initiates a session by making their first request, they are assigned to a particular server. For the duration of the session, all subsequent requests from that user are routed to the same server. This ensures that session data stored on the server remains consistent and accessible throughout the user's interaction with the web application.

In simple words Sticky Session is a method that makes it possible for the load balancer to identify the request and redirect the user to the same server after receiving their request.

Importance of Sticky Sessions

Sticky sessions are crucial for applications that rely on session data stored on individual servers. They provide a consistent user experience by ensuring that all interactions within a session are handled by the same server. This consistency is vital for applications where the session state is maintained on the server, such as shopping carts in e-commerce sites, user dashboards, and personalized content delivery.

Below are the key reasons why sticky sessions are important:

Consistent User Experience: Sticky sessions ensure that all interactions within a user’s session are handled by the same server, providing a seamless and consistent experience. This is crucial for applications where session data is stored locally on the server and needs to be consistently accessed throughout the session.
Session Data Integrity: Applications like shopping carts in e-commerce platforms rely heavily on session data to keep track of user activities, such as items added to the cart. Sticky sessions prevent data inconsistencies by directing all related requests to the same server.
Improved Performance: By maintaining session data on a single server, sticky sessions reduce the need for session data to be synchronized across multiple servers. This minimizes overhead and can lead to faster response times and better overall performance.
Simplified Architecture: Implementing sticky sessions can simplify the application architecture by eliminating the need for complex distributed session management solutions. This can make development and maintenance easier and reduce potential points of failure.
Personalized Services: For applications providing personalized content or services, such as user dashboards, ensuring that the same server handles all requests allows the application to efficiently manage and deliver personalized data without needing to repeatedly fetch and synchronize data.

How Do Sticky Sessions Work?

In Client server interaction , a group of client interact with the server for a particular time span also called as a session. During the Session the data is stored on the server end to ensure the authenticity of the user.

The working of Sticky Session can be illustrated by the following diagram:

Without using Sticky Session: When Sticky Session are not used the load balancer routes the request of same user to different servers.
With Sticky Session: When Sticky Session are used the load balancer routes the request of same client to same servers every time.

Techniques to Implement Sticky Sessions

Implementing sticky sessions can be achieved through various techniques, each with its own advantages and suitable use cases. Here are some common methods:

Cookies:
- The load balancer sets a cookie in the user's browser during their first request. When a user initiates a session, the web application creates a session ID and stores it in a cookie. This cookie is then sent back to the user's browser.
- Subsequently, with every request, the browser sends this cookie back to the web server. The web server can then identify the user session and route the request to the appropriate server that holds the session data.
IP Address based:
- This technique utilizes the user's IP address for routing requests. The load balancer keeps track of which server a specific IP address was directed to previously.
- When a subsequent request arrives from the same IP, it's directed to the same server, maintaining session consistency.
- However, this method is less reliable because IP addresses can change, especially for mobile users or those behind proxies.
URL Parameters:
- In this technique, the session ID is embedded directly into the URL during the initial request. Subsequent requests from the same user will include this session ID in the URL, allowing the server to identify the session and retrieve the associated data.
- This method eliminates the need for separate cookies but can lead to longer and less user-friendly URLs.
- Additionally, security concerns arise as session data becomes visible within the URL.
Session Identifier in Request Headers:
- Custom headers can be used to pass session identifiers, enabling the load balancer to direct requests to the appropriate server based on these headers.

Benefits of Sticky Sessions

Sticky sessions provide several benefits, especially for applications that require session persistence:

Consistency: User sessions remain consistent because all requests are processed by the same server. This is particularly important for applications that store session data locally on the server.
Performance: Since the session data does not need to be repeatedly synchronized across servers, sticky sessions can improve performance by reducing the overhead associated with data sharing.
Simplicity: Sticky sessions can simplify the architecture of an application, as developers do not need to implement session-sharing mechanisms like distributed caches or databases.

Drawbacks of Sticky Sessions

Despite their benefits, sticky sessions have some notable drawbacks:

Scalability: Sticky sessions can lead to uneven load distribution. If certain users are assigned to specific servers, those servers might become overloaded while others remain underutilized. This imbalance can impact the overall performance and scalability of the system.
Reliability: If a server fails, all sessions associated with that server are disrupted, potentially leading to a poor user experience. Implementing failover mechanisms for sticky sessions can be complex.
State Management: Applications relying on sticky sessions must manage state on the server side, which can complicate deployment and scaling, especially in distributed environments.

Use Cases for Sticky Sessions

1. E-commerce Platforms

Shopping Carts: Sticky sessions ensure that all actions related to a shopping cart, such as adding or removing items, are directed to the same server. This prevents issues like lost items or inconsistencies in the cart's contents.
Checkout Process: During the checkout process, maintaining a session with the same server ensures that payment and shipping information is handled smoothly without interruptions or data loss.

2. Gaming Applications

Multiplayer Games: In online multiplayer games, maintaining a consistent session for each player is crucial. Sticky sessions ensure that game state, player progress, and interactions are consistently managed, providing a smooth gaming experience.
Real-Time Data: Real-time games that require frequent updates and interactions benefit from sticky sessions, as they reduce latency and ensure that player actions are accurately tracked.

3. Financial Services

Online Banking: Sticky sessions ensure that all transactions and account management tasks are consistently handled by the same server. This is crucial for maintaining security and data integrity in sensitive financial operations.
Trading Platforms: For trading platforms, sticky sessions help manage user sessions effectively, ensuring that trade data, user actions, and market updates are consistently processed and displayed.

Alternatives to Sticky Sessions

To address the limitations of sticky sessions, many modern applications use stateless designs and distributed session management:

Stateless Applications: By making applications stateless, session data is stored on the client-side (e.g., in cookies or local storage) or in a centralized session store like Redis or a database. This approach allows any server to handle any request, enhancing scalability and reliability.
Distributed Caching: Tools like Redis, Memcached, or other distributed caches can be used to store session data in a centralized manner, accessible by all servers in the load-balanced pool. This method allows the application to scale horizontally without being tied to sticky sessions.
Global Load Balancers: Advanced load balancing solutions, such as global load balancers, can provide more intelligent routing based on user geography, server health, and other factors, reducing the need for sticky sessions.

Conclusion

In this article we have discussed about sticky sessions and their importance in identifying the incoming request from the same client and routing the request of the same client to the same server by the help of the load balancer. Without using Sticky Session the request will be routed to different server each time.