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Load Balancing
Cloudflare Docs
Load Balancing
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  1. Products
  2. Load Balancing
  3. Concepts
  4. Session affinity

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.

​​ Types

Session affinity specifies the type of session affinity the load balancer should use unless specified as "none" or "" (default).

On the first request to a proxied load balancer, a cookie is generated, encoding information of which endpoint the request will be forwarded to. Subsequent requests, by the same client to the same load balancer, will be sent to the endpoint the cookie encodes for the duration of the cookie and as long as the endpoint remains healthy. If the cookie has expired or the endpoint is unhealthy, a new endpoint is calculated and used.

​​ How does it work?

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.

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.

This behaves similar to cookie except the initial endpoint selection is stable and based on the client’s IP address.

On the first request to a proxied load balancer, a session key based on the configured HTTP headers is generated. The session key encodes the request headers used for storing which endpoint the request will be forwarded to during the load balancer session state. Subsequent requests to the load balancer with the same headers will be sent to the same endpoint, for the duration of the session and as long as the endpoint remains healthy. If the session has been idle for the duration of session affinity TTL seconds or the endpoint is unhealthy, then a new endpoint is calculated and used.

​​ Control how headers are used

By default, at least one of the HTTP headers that you configure for session affinity by HTTP header must be present on requests sent to your load balancer in order for header-based sessions to be created. If a client adds or removes HTTP headers on their requests and they have already established a session, a new session will be created based on the new HTTP headers found in subsequent requests as long as they are specified in your configuration.

If you would like to require all of your configured HTTP headers to be present on requests in order for sessions to be created, then set session_affinity_attributes.require_all_headers to true via the Cloudflare API or toggle Require all headers to enabled in the Cloudflare dashboard when editing your load balancer.

​​ Enabling Session Affinity from the Cloudflare dashboard

Enable Session Affinity when you create or edit a load balancer, during the Hostname step.

If you enable Session Affinity, choose one of the following options:

  • By Cloudflare cookie only: Sets a __cflb cookie to track the associated endpoint.
  • By Cloudflare cookie and Client IP fallback: Sets a __cflb cookie, but also uses the client IP address when no session affinity cookie is provided.
  • By HTTP header.

​​ Enabling Session Affinity via the Cloudflare API

Session affinity is a property of load balancers, which you can set with the following endpoints:

Customize the behavior of session affinity by using the session_affinity, session_affinity_ttl, and session_affinity_attributes parameters.

To enable session affinity by HTTP header, set the session_affinity value to header and add your HTTP header names to session_affinity_attributes.headers.

For more details on API commands in context, refer to Create a load balancer with the API.

​​ Endpoint Drain

Drain or remove all traffic from an endpoint without affecting any active customers using endpoint drain. For more details on endpoint drain, refer to Performing planned maintenance.

​​ Zero-Downtime Failover

Zero-Downtime Failover automatically sends traffic to endpoints within a pool during transient network issues. This helps reduce errors shown to your users when issues occur in between active health monitors.

You can enable one of three options:

  • None: No failover will take place and errors may show to your users.
  • Temporary: Traffic will be sent to other endpoint(s) until the originally pinned endpoint is available.
  • Sticky: The session affinity cookie is updated and subsequent requests are sent to the new endpoint moving forward as needed.