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Zend Session Clustering

SC (Session Clustering), facilitates session management in cluster based environments. This module transparently shares session data between nodes. Session Clustering High Availability is an additional layer of functionality to for preventing information loss in case a server suddenly becomes unavailable.

Without SC, session information resides on different servers depending on where your Load Balancer routed incoming requests (at any given time) making continuity between requests difficult to follow. This is especially important in environments such as online stores that depend on continuity between request information, for example; to fill a shopping cart.

SC provides full continuity in a cluster. It doesn’t matter where the session resides because all servers share the information. This provides application transparency and an increase in application performance. SC also adds Linear Scalability providing a way to add new servers while running in order to expand your cluster such as cases where there are sudden traffic influxes. New servers added to the cluster start handling subsequent traffic immediately however, traffic that occurred before the server was added will not pass through the new server due to the fact that Load Balancers cannot recalculate distribution between nodes.

Using SC replaces the daemon that handles all incoming http requests with the SCD (Zend Session Manager) this new daemon creates a cookie if the request did not have a cookie or adds information to the cookie in the request. The information included in the cookie relates to the Cluster manager, the session key (the session's unique ID) and the backup server (only for HA). Subsequent requests will include this information (in the cookie) so the SCD will know where the session originated and which session and make sure that all new information coming from requests is managed from one master server.

Note:

Zend Session Clustering is disabled when running PHP in CLI mode.


	Related Links: Cluster Management