Session Persistence for Stateful Workloads: Sticky Sessions in an Anycast Network

The architecture of modern cloud-native platforms ideally follows the principle of statelessness. Requests are distributed across a global Anycast network, and it doesn’t matter which backend system in a distant data center processes the request, as all instances access the same data base. This design is perfect for modern web APIs or static websites.

However, the reality in established corporate and industrial structures often looks different. Numerous stateful applications exist here: long-lived TCP connections from IoT sensors in production plants, traditional ERP systems, complex terminal sessions, or legacy databases. These systems expect a client to consistently communicate with the same backend server throughout the duration of its session. If this connection breaks or the next data packet lands on a neighboring server, the session context is lost, and the application fails with an error.

The technological challenge is to reliably guarantee this Session Persistence (Sticky Sessions) in a geographically distributed Anycast network without sacrificing the overall system’s resilience and elasticity.

The Architectural Dilemma: Anycast vs. Statefulness

To understand the problem, one must consider how Anycast routing works. Anycast means that multiple servers worldwide are reachable under exactly the same IP address. The Border Gateway Protocol (BGP) of the internet dynamically decides which Point of Presence (PoP) represents the shortest and fastest path for the respective user.

If the global routing situation on the internet changes—for instance, because a major network operator is maintaining a line or a peer node is overloaded—BGP can switch the path for a user in the middle of an active session.

[ Client in operation ]
               |
      +--------+--------+
      | (BGP rerouting in the middle of the session)
      v                 v
[ Edge PoP Frankfurt ] [ Edge PoP Paris ]
      |                 |
      v                 v
[ Backend Server 1 ]  [ Backend Server 2 ] <--- "Who are you? I don't know your session!" (Error)

If the edge infrastructure operates purely passively here, the next TCP packet suddenly lands at a different PoP and thus on a completely different backend server. For stateful legacy or industrial applications, this is a programmed failure.

The Solution: IP-Based Affinity and Connection Tracking on Layer 4

Since a Layer-4 load balancer does not decrypt the data stream, it cannot read HTTP cookies to recognize a session ID. The solution for sticky sessions on the transport layer is therefore based on IP-based session affinity coupled with high-performance Connection Tracking (Conntrack).

The system uses a three-stage mechanism to fix the connection like invisible cement:

1. Mathematical Source Hashing (Consistent Hashing)

When the very first TCP packet (SYN) arrives at the edge, the load balancer calculates a hash value from the client’s source IP. This mathematical value firmly determines which backend in the pool the connection is handed over to. By using Consistent Hashing, this assignment remains stable even when new backend servers are added to or removed from the pool in the background.

2. Live Connection Tracking in the Kernel

Once the connection is established, the load balancer enters the combination of source IP, source port, destination IP, and destination port into an ultra-fast in-memory table. As long as this TCP session is active, all subsequent data packets of this specific stream are passed directly to the same backend server without recalculation.

3. Cross-PoP Failover Management

If BGP actually forces the user to switch to another physical edge PoP in the middle of the session due to a massive internet disruption, the extended security mechanisms of an integrated platform take effect. The load balancer at the new PoP recognizes that it is an existing, stateful connection, evaluates the IP hash, and routes the packet over the internal backbone exactly to the backend system where the session originally started.

Economic Value: Gentle Modernization Instead of Expensive Refactoring

Enabling stable session persistence at a modern Anycast edge infrastructure offers companies tangible economic and strategic advantages:

• Protection of Legacy Investments: Companies do not have to completely rewrite old but perfectly functioning core applications (e.g., in logistics or ERP) for the cloud at a cost of millions (Refactoring). The edge infrastructure captures the statefulness and makes the legacy systems fit for modern cloud operations.
• Stability for IoT and Industrial Workloads: Production facilities and sensors often keep a single TCP session open for hours or days to stream telemetry data. Sticky sessions prevent short network fluctuations on the internet from leading to data interruptions in monitoring.
• Easy Scaling Despite State: Even if the application remains stateful at its core, the backend pool can be elastically expanded in the background. The load balancer distributes new sessions evenly (Weighted Round-Robin) across the new servers, while existing sessions continue uninterrupted on their assigned systems.

Conclusion: The Edge as a Bridge Between Worlds

Digitalization in medium-sized businesses rarely calls for radical clear-cuts but rather for intelligent bridges. A modern IT infrastructure should not force developers and companies to abandon functioning software architectures just because the network is becoming more global. The combination of Anycast performance and intelligent Layer-4 session persistence proves that the uncompromising resilience of a global network and the strict stability requirements of stateful enterprise workloads are not mutually exclusive. They form the foundation for a risk-free and gradual modernization of the digital value chain.

FAQ: Session Persistence in Enterprise Use

What happens to sticky sessions when a backend server needs scheduled maintenance?

For this case, the platform supports what is known as Connection Draining (controlled draining). When a backend server is put into maintenance mode for an update, the load balancer no longer directs new sessions to this system. However, existing, active sticky sessions are allowed to complete their connection on this server over a defined transition period. Only when the last session is cleanly closed is the server physically shut down for the update.

Can IP affinity lead to uneven load (imbalance) in the cluster?

Yes, this risk exists in specific scenarios, known as the Mega-Proxy Problem. If thousands of employees of a large customer all access your application through the same central company gateway (and thus with exactly the same public source IP), the load balancer calculates the same hash value for all. The result: All traffic from this large customer lands on a single backend server, while the other backends remain idle. In such specific environments, the edge architecture must be adjusted so that, in addition to the IP hash, other transport characteristics (such as TCP port ranges) are included in the calculation to split the traffic more finely.

How long is a sticky session stored in the system if the user is inactive?

This can be precisely defined via a configurable Timeout Rule. If a client does not send any data packets over the line for a certain period (e.g., 30 minutes), the Conntrack entry in the load balancer’s memory is automatically deleted to free up resources. If the client reconnects afterward, it is treated as a new connection, the hash value is recalculated, and it is assigned to the currently freest backend server.