Specifications

Introducing Equalizer
Equalizer Installation and Administration Guide 3
gateways, and can even load-balance certain types of NFS server cluster that provide a single-
system image.
Equalizer does not support Active Content Verification for UDP clusters.
Maintaining Persistent Sessions
The persistence of session data is important when the client and server need to refer to data
previously generated during the same session. For example, a web-based shopping cart application
may depend on persistent session information between the client and server; that is, the details in
the shopping cart potentially need to persist across many individual TCP connections before the
data is no longer needed and the transaction is complete. Equalizer supports two mechanisms for
maintaining persistent sessions: cookie-based and IP-address-based persistence.
Cookie-Based Persistence
Equalizer can use cookie-based persistence for HTTP and HTTPS clusters that support Layer 7 load
balancing. In cookie-based persistence, Equalizer “stuffs” a cookie into the server’s response header
on its way back to the client. This cookie uniquely identifies the server to which the client was just
connected. The client includes (sends) the cookie in subsequent requests to the Equalizer. Equalizer
uses the information in the cookie to route the requests back to the same server.
Equalizer can direct requests from a particular client to the same server, even if the connection is to
a different virtual cluster. For example, if a user switches from an HTTP cluster to an HTTPS
cluster, the persistent cookie will still be valid if the HTTPS cluster contains a server with the same
IP address.
If the server with which a client has a persistent session is unavailable, Equalizer automatically
selects a different server. Then, the client must establish a new session; Equalizer stuffs a new
cookie in the next response.
IP-Address Based Persistence
For generic TCP and UDP clusters that support Layer 4 load balancing, Equalizer supports IP-
address based persistent sessions. With the sticky connections feature enabled, Equalizer identifies
clients by their IP addresses when they connect to a cluster. Equalizer routes requests received from
a particular client during a specified period of time to the same server in the cluster.
A sticky timer measures the amount of time that has passed since there was a connection from a
particular IP address to a specific cluster. The sticky time period begins to expire as soon as there
are no longer any active connections between the client and the selected cluster. Equalizer resets the
timer whenever a new connection occurs. If the client does not establish any new connections to the
same cluster, the timer continues to run until the sticky time period expires. At expiration, Equalizer
handles any new connection from that client like any other incoming connection and routes to an
available server based on the selected load-balancing criteria.
To correctly handle sticky connections from ISPs that use multiple proxy servers to direct user
connections, Equalizer supports sticky network aggregation with which only the network portion of
a client's IP address maintains a sticky connection. Sticky network aggregation directs the user to
the same server no matter which proxy he or she connects through.
You can also configure Equalizer to ensure that it directs requests from a particular client to the
same server even if the incoming connection is to a different virtual cluster. When you enable inter-
cluster stickiness for a cluster, Equalizer checks the cluster for a sticky record as it receives each
connection request, just like it does for ordinary sticky connections. If Equalizer does not find a