High Availability (HA) Servers are built to survive hard failure that would otherwise take a service (or services) offline until repairs (or replacement) can made.  Often in such cases of hard failure not only is lengthy downtime unavoidable, it is usually accompanied by loss of data.

HA Servers avoid both downtime and loss of data through synchronization and redundancy.  Consider the typical HA Server configuration:

 
HA Server Diagram.

In this configuration we have two servers that share a virtual IP.  At any given time only one server is actively using the shared virtual IP.  As the active server processes incoming requests all local changes are synchronized (via the synchronizing connection) with the inactive server, this includes databases, configuration changes, and file system changes.  The secondary server monitors the livelihood of the primary, and at any indication of failure it takes over as primary.  The HA Servers use STONITH (Shoot The Other Node In The Head) to guarantee that when one switches from secondary to primary the old primary is not still active.

Features

• HA Servers are fault tolerant through the following situations:

• The power supply on the Primary failing.
• Disconnecting the Primary from the network.
• Unplugging the Primary's power cord.
• Turning the Primary server off.
• Pressing the reset switch on the Primary.

• In the event the primary needs to be taken down for maintenance the secondary can be brought up as the primary (takes less than 15 seconds), and the original primary can be taken off line until the maintenance is complete.
  
• A wide range and variety of services come on the HA Servers, customized to the specific need, including (but not limited to): 

• HTTP - web server
• HTTPS - secured web server
• FTP - FTP server
• SMB - Windows network protocol
• SMTP - email server
• POP3 - email clients
• DNS - domain name resolution
• NNTP - news servers
• IPSec/VPN - Virtual Private Network protocols
• ICMP - ping and routing protocols
• SSH - secured shell
• Telnet - remote login
• Finger - user identification service
• Whois - system identification service
• X400 - email services
• RPC - remote procedure call servers
• Kerberos - network authentication

Pricing starts at $2,500 per unit and depends on the number of services it must support (more services require faster hardware and more storage).