Adding an extra mid tier to a cluster

When an increase in demand requires extra capacity in a cluster, you can add an extra mid tier to the cluster. The newly added mid tier must seamlessly start serving requests and share the load along with other mid tiers in the cluster. Suppose that you have four mid tiers and eight AR System servers operating in a cluster, serving requests from 2,400 users. If you find that each mid tier is serving more than 75 percent of its capacity and you expect the load to further increase, you can add a fifth mid tier and distribute the load equally among all the mid tiers. 

• • Before you begin
  • To add the extra (n+1) mid tier

Before you begin

• Plan your deployment strategy. For example, you might have to decide whether to deploy n+1 or n+2 mid tiers to the cluster.
• Ensure that you have a separate computer. If you are deploying n+2 mid tiers, ensure that you have two computers available.
• Ensure that the computers have the same configuration as that of other mid tiers in the cluster in terms of memory, CPU usage, disk space, and so on.

• Ensure that the computers have the same cluster configuration. 

  Notes

  Ensure that the extra mid tier is already set up in the cluster but is not running, and that you have deleted the midTierInstallationDirectory/cache folder before starting the mid tier.

• To avoid the extra time required to copy the cache folder, ensure that you already have a "good copy" of the preloaded cache in the n+1 mid tier (for example, in the /opt/Preload_Cache folder). Ensure that you store the good copy of the cache on a local drive and not on a shared drive, which can delay copying the cache folder. 

• Add the cache directory path to the arsystem.ehcache.midTierBackupCacheDir property in the config.properties file of the n+1 mid tier. For example:

  arsystem.ehcache.midTierBackupCacheDir = /opt/Preload_Cache

  Note

  If you are using a Centralized Configuration Server (CCS), ensure that this property (Cache Backup Directory) is set correctly from the Cache Settings page from any of the mid tiers in the cluster. Ensure the availability of the good copy of the cache in this folder on all mid tiers. For more information, see Backing-up-the-mid-tier-cache.

• If you are using Centralized Configuration Server, copy the ccs.properties file in the midTierInstallationDirectory/WEB-INF/classes folder from other mid tier.

To add the extra (n+1) mid tier

1. 1. 1. Start the n+1 mid tier:
         1. Start the mid tier to connect to the CCS server by using ccs.properties and refresh the local copy of config.properties. This action also copies the cache backup from the backup directory and starts using it automatically.
         2. After the n+1 mid tier is started, verify the cache folder located in the midTierInstallationDirectory/cache folder of the n+1 mid tier. The size of this cache folder must be same as the good cache copy stored in the /opt/Preload_Cache folder.
      2. Ensure that the n+1 mid tier is added as a node to the appropriate f5 load balancer pool.
         Otherwise, f5 does not balance the load across the n+1 mid tier.
      3. Verify whether the newly added mid tier is serving requests by:

1. 1. 1. • • Looking at the status of the relevant node in f5 load balancer 
           • Reviewing the number of views generated in the Cache Advanced page of the newly added mid tier (For example: http://localhost:portnumber/arsys/shared/config/config_cache_adv.jsp)

The newly added mid tier can now handle all requests seamlessly without causing delays and performance issues.