The specific procedure for setting priority is covered in the procedures for "Modifying the Default Cluster for Fast Cluster Set Up" and "Defining One or More Clusters." The following section discusses general priority issues and gives a comprehensive example of cluster coordination:
Priority is indicated by an integer value that is set in the SyncServers registry. The lower the value, the higher the priority, so the server assigned a value of 0 has the highest possible priority. The highest acceptable value, and so the lowest priority value, is 65,535.
Priority values are used only to select between Sync Servers in the same status (either between a group of Sync Backups or between a group of Sync Alternates). Only the order in which Netscape Application Servers are started, not priority, determines which server should be the Sync Primary and which Sync Servers will start out as Sync Backups or Sync Alternates.
A Sync Local is not assigned a priority because it is not eligible to become a Sync Server, so a Sync Local cannot become a Sync Primary, Sync Backup, or Sync Alternate.
Which Sync Server becomes the Sync Primary in a cluster is determined simply by which Sync Server is started before any of the other servers. The next Sync Servers that start, up to the value in MaxBackups, become Sync Backups. When the Sync Primary fails, the Sync Backup with the highest priority, which is the lowest integer value, becomes the new Sync Primary.
When a Sync Backup becomes a Sync Primary, the number of Sync Backups falls below the value of MaxBackups. To restore the number of Sync Backups, the Sync Alternate with the highest priority becomes a Sync Backup.
Example: Coordination Within a Seven-Server Cluster
The following example illustrates cluster coordination through server roles, and the part that priority plays in determining those roles. As you trace the role changes through the example, keep in mind that server fallibility has been purposely exaggerated to provide many scenarios.
Although not required, you can ease cluster maintenance by assigning the highest priority to the Netscape Application Server that you will start as the Sync Primary, and the next highest priorities (in descending order) to the Sync Backups. Be aware that the cluster in this example does not do this. Also, notice that this cluster does not follow the recommended practice of starting the servers in priority order.
Assume a seven-server cluster with Netscape Application Servers that are numbered 0 to 6. Servers 0 through 4 are Sync Servers that are assigned the same priorities as their server numbers (for example, server 0 has a priority of zero). Servers 5 and 6 are Sync Locals. MaxBackups for the cluster is set to two.
Server 3 fails and goes down. Between the two Sync Backups, server 4 and server 1, server 1 has the higher priority (lower integer value) and it becomes the new Sync Primary. This leaves server 4 as the only Sync Backup.
Because MaxBackups is set to two, one of the Sync Alternates is converted to a Sync Backup. Server 0 becomes the new Sync Backup because it has a higher priority than the other remaining Sync Alternate, server 2. At this point:
Server 3 comes back online. It becomes a Sync Alternate. Even though it was originally a Sync Primary, the synchronizer now sees it as just another Sync Server, so the server does not resume its Sync Primary role. At this point:
Server 0 fails. Server 2 becomes a Sync Backup because it has the higher priority (lower integer value) among the Sync Alternates. At this point:
Server 0 comes back online and becomes a Sync Alternate. Server 1, the Sync Primary, fails. Among the Sync Backups, server 2 has a higher priority than server 4, so server 2 becomes the new Sync Primary. Server 0 becomes a Sync Backup. At this point:
Server 2 fails. Server 0 becomes the Sync Primary and server 3 becomes a Sync Backup. At this point:
Server 3 fails. Even though only one Sync Backup remains, neither server 5 nor server 6 is considered because neither is a Sync Server. At this point:
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