I rad some article. Keepalived over GRE tunnel for failover on VPS environment

http://blog.killtheradio.net/how-tos/keepalived-haproxy-and-failover-on-the-cloud-or-any-vps-without-multicast/

but, I don't know how to configuration? and How to call this architecture?

only I Know that How to config Master/Backup configuration at keepalived.

What I want to know that How does work keepalived?

I want to design this....

                XMPP Server(EC2) 
                       |
-------------------------------------------------
keepalived Master(EC2) - keepalived Backup(EC2)
HAProxy #1               HAProxy#2      
-------------------------------------------------
                       |

Casandra#1   Casandra#2  Casandra#3  Casandra#4

Thanks! but What I want to know how to work on keepalived with unicast patche modul. ELB is expansive.
and this is first totaly design.
[Flow]
ELB --> XMPP Server --> ELB --> Casandra

                      ELB
                       |
XMPP#1        XMPP#2       XMPP#3       XMPP#4 
                       |
                      ELB
                       |
Casandra#1   Casandra#2  Casandra#3  Casandra#4

and change first design.
[Flow]
ELB --> XMPP Server --> HAProxy Master(Casandra Farm) --> Casandra

                      ELB
                       |
XMPP#1        XMPP#2      XMPP#3       XMPP#4 
                       |
-------------------------------------------------
keepalived Master(EC2) - keepalived Backup(EC2)
HAProxy#1                HAProxy#2      
-------------------------------------------------
                       |

Casandra#1   Casandra#2  Casandra#3  Casandra#4

this is second.
[Flow]
ELB --> HAProxy(XMPP Farm) --> XMPP Server --> HAProxy(Casandra Farm) --> Casanda
It's OK?

                      ELB
                       |
HAProxy#1   HAProxy#2   HAProxy#3   HAProxy#4
XMPP#1       XMPP#2     XMPP#3      XMPP#4 
                       |
Casandra#1   Casandra#2  Casandra#3  Casandra#4