High Availability

This topic describes what High Availability means from the standpoint of an AppLogic grid. When a grid has high availability:

• there are sufficient available resources (CPU, memory, and bandwidth) to restart appliances in the event of a failure of any one server in the grid
• the running applications do not have any degraded volumes (all volumes are in the OK state)

The current high availability state is shown on the Grid Dashboard as well as in the output of the 'grid info' shell command. The high availability state may be one of the following:

ok

There is high availability on the grid. The high availability state is ok if there are enough available resources to restart appliances in the event of the failure of any one server or if there are no applications currently running.

unavailable

High availability is not available. When the high availability state is unavailable, there will be a message shown on the dashboard describing the reason(s) why high availability is unavailable. See Dashboard Notification Messages for a description of the high availability messages. HA may be unavailable for any of the following reasons:

• Running applications have 1 or more degraded volumes.
• There is not enough available resources on the grid to restart appliances/applications in case one of the servers fail.
• All of the servers are currently disabled.
• There is only one server in the grid.

checking ...

The grid controller is currently checking the high availability state of the grid.

disabled

High availability check is disabled.

The grid checks for high availability upon the following controller events (See Controller Notifications for details):

• post component start
• post component stop
• post component restart
• post server enable
• post server disable
• server is registered (added to the grid)
• server is deregistered (removed from the grid)

In addition, the high availability state may be checked manually by the user by executing the ha check command from the command shell.

-- BeckyH - 02 May 2008