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Session affinity

When you enable session affinity, your load balancer directs all requests from a particular end user to a specific endpoint. This continuity preserves information about the user session — such as items in their shopping cart — that might otherwise be lost if requests were spread out among multiple servers.

Session affinity can also help reduce network requests, leading to savings for customers with usage-based billing.

How it works

Session affinity automatically directs requests from the same client to the same endpoint:

  1. When a client makes its first request, Cloudflare sets a __cflb cookie on the client (to track the associated endpoint).
  2. Subsequent requests by the same client are forwarded to that endpoint for the duration of the cookie and as long as the endpoint remains healthy.
  3. If the cookie expires or the endpoint becomes unhealthy, Cloudflare sets a new cookie tracking the new failover endpoint.
    flowchart LR
      accTitle: Session affinity process
      accDescr: Session affinity directs requests from the same client to the same server.
     A[Client] --Request--> B{<code>__cflb</code> cookie set?}
     B -->|Yes| C[Route to previous endpoint]
     C --> O2
     B ---->|No| E[Follow normal routing]
     E --> O2
     E --Set <code>__cflb</code> cookie--> A
     subgraph P1 [Pool 1]
        O1[Endpoint 1]
        O2[Endpoint 2]
     end

All cookie-based sessions default to 23 hours unless you set a custom session Time to live (TTL).

The session cookie is secure when Always Use HTTPS is enabled. Additionally, HttpOnly is always enabled for the cookie to prevent cross-site scripting attacks.