Can sessions be rebalanced when servers are added or removed?

Short answer

Not automatically. When a new Backend Service is added, the existing pinnings are preserved; the new server receives only new sessions and the load balances over time. The sessions of a server that is removed, put into Maintenance Mode or falls into the Error status in the health check, on the other hand, go to the other servers. The way to manually rebalance existing sessions is Rebalance on the Session Affinity Manager (SAM) tab; the New cookie, Current cookie and Hash methods have no such runtime tool.

Behavior by Method

Method	When a server is added	When a server is removed / goes down	Manual rebalancing
SAM	Existing sessions stay in place; the new server receives new sessions	The sessions of that server go to other servers	Rebalance, Clear All
New Cookie / Current Cookie	Existing cookies point to the same server	Clients whose cookie points to that server go to another server	None (cookie durations can be shortened)
Hash	Since the configuration is reloaded, the table is rebuilt; the mappings are created according to the new distribution	Same	None
Self-persistent algorithms (Source, URI, URL Param, HDR; RDP Cookie on L7 TCP)	Since the server list changes, some of the clients shift to a different server	Same	–

Gradual Rebalancing with SAM

1. The new Backend Service is added to the vService's list and the vService is saved with Save; the SAM table is preserved on an ordinary save.
2. On the vService's Session Affinity Manager (SAM) tab, Rebalance in the header of the list to which the new server was added (Default Backend Services or a conditional set) is clicked.
3. In the Backends field, the busy servers are selected, Percent is kept low (e.g. 20; default 100), and Selection, which appears while Percent is below 100, is determined; the Accept button in the form is clicked and the Rebalance Sessions? confirmation is given.
4. The selected sessions are deleted; these users are reassigned according to the algorithm on their next request, and some of them go to the new server.

Common Misconceptions

• "When a new server is added, the sessions are shared automatically." The existing pinnings are preserved; only new sessions are distributed.
• "Rebalance moves the sessions to the new server." It deletes the records; the new server is selected by the algorithm.
• "I balance by reloading the vService." Reload in the vService menu restarts the service and completely empties the SAM table; Rebalance is used for controlled balancing.
• "When a server goes down, the pinning is preserved." Requests are not sent to an unavailable server; the user goes to another server.

How it is done in the documentation

• HTTP (L7) vService · Rebalance
• HTTP (L7) vService · Session Affinity Manager (SAM)
• Backend Services · Maintenance Mode

Related questions

• What do Rebalance and Clear All do on the Session Affinity Manager (SAM) tab?
• Does saving a configuration change reset the persistence tables?
• Persistence is causing uneven load distribution; what can I do?