Help Center/ Elastic Load Balance/ User Guide (Ankara Region)/ Backend Server Group/ Key Features/ Sticky Session
Updated on 2024-04-18 GMT+08:00
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Sticky Session

Sticky sessions ensure that requests from a client always get routed to the same backend server before a session elapses.

Here is an example that describes how sticky session works. Assume that you have logged in to a server. After a while, you send another request. If sticky sessions are not enabled, the request may be routed to another server, and you will be asked to log in again. If sticky sessions are enabled, all your requests are processed by the same server, and you do not need to repeatedly log in.

Differences Between Sticky Sessions at Layer 4 and Layer 7

The following table describes the differences of sticky sessions at Layer 4 at Layer 7.

Table 1 Sticky session comparison

OSI Layer

Listener Protocol

Sticky Session Type

Scenarios Where Sticky Sessions Become Invalid

Layer 4

TCP or UDP

Source IP address: The source IP address of each request is calculated using the consistent hashing algorithm to obtain a unique hashing key, and all backend servers are numbered. The system allocates the client to a particular server based on the generated key. This allows requests from the same IP address are forwarded to the same backend server.
  • Source IP addresses of the clients change.
  • The session stickiness duration has been reached.

Layer 7

HTTP or HTTPS
  • Load balancer cookie: The load balancer generates a cookie after receiving a request from the client. All subsequent requests with the cookie are routed to the same backend server.

    The load balancer itself does not generate cookies.

  • Application cookie: The application deployed on the backend server generates a cookie after receiving the first request from the client. All subsequent requests with the same cookie are routed to the same backend server.
  • If requests sent by the clients do not contain a cookie, sticky sessions will not take effect.
  • Requests from the clients exceed the session stickiness duration.
  • If you set Load Balancing Algorithm to Source IP hash, you do not need to manually enable and configure Sticky Session. Source IP hash allows requests from the same client to be directed to the same server.
  • If you set Load Balancing Algorithm to Weighted round robin or Weighted least connections, you need to manually enable and configure Sticky Session.

Constraints and Limitations

  • If you use Direct Connect or VPN to access ELB, you must select Source IP hash as the load balancing algorithm and disable sticky sessions for ELB.
  • Dedicated load balancers support Source IP address and Load balancer cookie.
  • For HTTP and HTTPS listeners, enabling or disabling sticky sessions may cause few seconds of service interruption.
  • If you enable sticky sessions, traffic to backend servers may be unbalanced. If this happens, disable sticky sessions and check the requests received by each backend server.
Parent topic: Key Features