Session Management in Horizontal Scaling
Session management becomes more complex in a horizontally scaled environment. Here are some best practices:
Session Affinity
Sticky sessions (also known as Session Affinity) involve associating a user’s session with a specific server for the duration of their session. When a user makes an initial request, the load balancer determines which server to send them to, and subsequent requests from that user are directed to the same server.
Session persistence is typically configured on the load balancer itself. Most modern load balancers provide settings to enable and configure session affinity. When using session persistence, it’s important to ensure that the load balancer itself is redundant and scalable. This prevents it from becoming a single point of failure.
It’s important to consider session timeout settings. If a session lasts too long, it can lead to uneven server load distribution. Conversely, if it’s too short, users may experience session expirations.
However, it can also introduce some challenges, especially if a server becomes unavailable. In this case, the load balancer needs to have a mechanism to handle failover or session reassignment.
Techniques for Implementing Session Persistence:
Cookie-Based Persistence: The load balancer sets a cookie on the client’s browser containing a unique identifier. This identifier is used to associate subsequent requests with a specific server.
URL Rewriting: In some cases, the load balancer can append a session ID to the URL, allowing it to determine which server to route the request to.
IP-Based Affinity: The load balancer can use the client’s IP address to ensure requests from the same client are sent to the same server.
Session State Externalization
Store session data in a centralized and easily accessible location, such as a distributed cache or a database, rather than on individual servers. This ensures session continuity even if a user’s request is routed to a different server.
Without externalization, if a user’s subsequent request is directed to a different server, the session data would be lost, leading to inconsistencies and potentially disrupting the user experience.
Choose a storage solution that can handle the expected load and provides low-latency access to session data. In-memory caches like Redis or Memcached are often preferred for session state externalization due to their high-speed read and write operations.
Considerations
Serialization and Deserialization: The session data needs to be serialized (converted into a format that can be stored) and deserialized (converted back into usable data) when read from or written to the centralized storage. If session data contains sensitive information, ensure it is encrypted when stored in the external storage system.
Data Consistency: Depending on the storage solution, additional measures may be needed to ensure data consistency. For example, distributed caches often have mechanisms for handling concurrent access.
Benefits:
Session Continuity: Externalizing session state ensures that even if a user’s request is directed to a different server, their session data remains intact, providing a seamless and consistent user experience.
Load Balancing Flexibility: Load balancers can distribute requests more freely because they don’t need to consider session affinity. This allows for better resource utilization across the server pool.
Failover and Redundancy: If a server becomes unavailable or fails, the session data is still accessible from other servers. This enhances system resilience.
Use Stateless Authentication Tokens
When dealing with horizontal scaling, it’s indeed advisable to lean towards stateless application design. Stateless applications don’t rely on server-side session storage, which can be problematic in a horizontally scaled environment where requests might be handled by different servers.
Stateless authentication tokens, like JWT, are one of the techniques that facilitate stateless application design, specifically for handling authentication. A JWT typically contains information about the user (such as their user ID or username) and any relevant claims (permissions, roles, etc.) that is passed between the client and the server with each request. Since the token itself contains all the necessary information for authentication, the server does not need to store any session data. This makes it easier to scale horizontally.