MID Server selection sequence for Discovery

The Discovery application follows this sequence to find a MID Server.

MID Server auto-selection

These steps are followed when you select Auto-Select MID Server for the MID Server selection method on the Discovery form.
  1. Discovery looks for a MID Server with the Discovery application that also has an appropriate IP range configured.
  2. If no MID Servers meet these criteria, it looks for a MID Server that has the ALL application that also has an appropriate IP range configured.
  3. If more than one MID Servers meet the above criteria, Discovery chooses the first MID Server with the status of Up. If more than one MID Servers are Up, it randomly picks one.
  4. If none are Up, it uses the default MID Server specified for the Discovery application, assuming it is Up.
  5. If no default MID Server is specified for the Discovery application, it uses the default MID Server specified for the ALL application, assuming it is Up.
  6. If no default MID Server is specified for the Discovery application, Discovery cycles through the same steps above, but it looks for MID Servers with the status of Paused or Upgrading.
    Note: When a MID Server is paused or upgrading, it does not actually process commands until it returns to the status of Up.

MID Server clusters

These steps are followed when you select Specific MID Cluster for the MID Server selection method on the Discovery form, and the cluster is a load balancing cluster:
  1. Discovery uses the first MID Server in the cluster that Discovery finds with the status of Up.
  2. If more than one MID Servers are Up, Discovery randomly picks one. If it cannot find any MID Servers, Discovery looks for MID Servers in the cluster with the status of Paused or Upgrading.
These steps are followed if the cluster is a failover cluster:
  1. Discovery uses the MID Server with the lowest Order value that also has the status of Up.
  2. If no MID Servers are found, Discovery looks for MID Servers in the cluster with the status of Paused or Upgrading, choosing the one with the lowest Order value.
Note: Discovery ignores the default MID Server for the Discovery and ALL applications when selecting a MID Server from the cluster.

Port scan (Shazzam) phase

During the port scan phase, Discovery collects all of the target IP addresses and splits them equally between MID Servers matching the criteria mentioned above (meaning the MID Servers are qualified to do the port scan work). The Shazzam batch size, which you configured on the Discovery schedule, determines the number of IP addresses that each Shazzam probe can scan. This helps determine how much work each MID Server does during the port scan phase.

For example, if there are 16,000 IP addresses to scan among three qualified MID Servers, and you use the default Shazzam batch size of 5000, two MID Servers each handle 5000 IP address scans (1 Shazzam probe each), and the other MID Server handles 6000 IP address scans by launching two Shazzam probes.