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Equalizer Overview

not aware. What Equalizer

does

know is that a specific client has been load balanced to a specific server in one of

its virtual clusters. With this knowledge, Equalizer can track that information and send that client back to the same

server they were connected the first time.

Layer 7 Persistence

Equalizer provides server or connection persistence using cookies in Layer 7 HTTP and HTTPS clusters, and

using the client IP address in Layer 4 TCP and UDP clusters. The following sections explain connection

persistence provided by Equalizer, and its relationship to session persistence.

When a request from a client that has not previously connected to this cluster is received by Equalizer, it is load

balanced according to the current server load values as described in "Load Balancing" on page 33.

However, when a client has existing persistence to a server, Equalizer attempts to put the client back on that

server.

Equalizer can use cookies or a server’s IP address to maintain a persistent session between a client and a

particular server. A cookie, used in Equalizer HTTP and HTTPS clusters contains the identity of the server that

should be used. When a client connects to the cluster for the first time, Equalizer injects this cookie into the

response data. The client’s browser is then responsible for presenting this cookie back to Equalizer. If Equalizer

finds this cookie in the client’s request, it connects to the server listed.

Source IP persistence or sticky persistence can be used in all kinds of Equalizer clusters. When a client connects

for the first time, a “sticky record” is created within Equalizer’s memory. For every subsequent connection,

Equalizer finds this record and uses the information stored there to choose the same server.

EQ/OS 10 features “fallback persistence” where Equalizer provide a secondary persistence option where if, for

example, a cookie response is not received, a secondary, or “fallback” option can be used. As an example, if two

persist methods are listed (e.g., Cookie 1:Cluster IP, Server IP /Port and Source IP)- if a cookie is found- the cookie

will be used, otherwise the Source IP will be embedded in the response header back to the client.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.

Layer 4 Persistence

For Layer 4 TCP and UDP clusters, Equalizer supports IP address based persistent connections. With the sticky

connection feature enabled, Equalizer identifies clients by their IP addresses when they connect to a cluster.

Equalizer then 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 it to an available server based on the current load balancing policy.

To correctly handle sticky connections from ISPs that use multiple proxy servers to direct user connections,

Equalizer supports sticky network aggregation, which uses only the network portion of a client's IP address to

maintain a persistent 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 intercluster stickiness for a cluster,

Equalizer checks the cluster for a sticky record as it receives each connection request, just like it does for ordinary

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