Limits is a system attribute and designates which resources are available on a system, including shared memory segments and semaphores.

Prerequisites is a service group attribute and helps manage application requirements. For example, a database may require three shared memory segments and 10 semaphores. VCS Load policy determines which systems meet the application criteria and then selects the least-loaded system.

If the prerequisites defined for a service group are not met on a system, the service group cannot be brought online on the system.

When configuring these attributes, define the service group's prerequisites first, then the corresponding system limits. Each system can have a different limit and there is no cap on the number of group prerequisites and system limits. Service group prerequisites and system limits can appear in any order.

You can also use these attributes to configure the cluster as N-to-1 or N-to-N. For example, to ensure that only one service group can be online on a system at a time, add the following entries to the definition of each group and system:

    Prerequisites = { GroupWeight = 1 }
    Limits = { GroupWeight = 1 }

System limits and group prerequisites work independently of FailOverPolicy. Prerequisites determine the eligible systems on which a service group can be started. When a list of systems is created, HAD then follows the configured FailOverPolicy.