One parameter of special significance in determining the behavior of the data collection device (DCD) cluster is the minimum master eligible devices (MMED) setting. All of the devices in the cluster (including the primary and secondary console devices) are eligible to be the master device.

When a device is added or removed from the DCD cluster, the system performs a calculation to determine the optimum default value. You can override the default value to suit your requirements.

This setting determines how many DCDs in the cluster must be online for the cluster to continue to process alert data. If your goal is to keep operating regardless of device failures, it would seem like the obvious choice would be to set this number to as low a value as possible. However, you should keep in mind a couple of factors:

• The BIG-IQ^® console is counted as a device in the cluster, so a cluster size of 1 does not make sense.
• Similarly, a cluster size of 2 (a DCD and the BIG-IQ console) is not a good idea. Because the DCD cluster logic uses multiple DCDs to ensure the reliability of your data, you need at least two logging devices to get the best data integrity.
• It might also seem like a good idea to set the MMED value to a higher value (for example, one less than the number in the entire cluster), but actually, best practice is to not specify a value larger than the number of devices in one zone. If there is a communications failure, the devices in each zone compose the entire cluster, and if the MMED is set to a lower value, both clusters would stop processing data.

Here are some of the most common failure scenarios that can occur to a data collection device cluster, and how the cluster responds to that scenario.

This scenario is a little more complex than the case where data collection devices fail and needs a little more discussion to understand. However, it's also much less likely to occur. The cluster behavior in this scenario is controlled by the MMEN setting. In a two data center data collection device (DCD) cluster scenario, the MMEN setting is 3 and there is one master node located in one of the two data centers. The admin doesn't control which node is the master, but the master node is identified on the Logging Configuration screen. For this example, let's assume that the master node is in the Seattle data center. If communication goes down between the data centers, the Seattle data center continues to function as before because with four nodes (the console and three DCDs) it satisfies the MMEN setting of 3. In the Boston data center, a new master node is elected because without communication between the two data centers, communication with the master node is lost. Since there are also four master eligible nodes in the Boston data center, it satisfies the MMEN setting too. The Boston data center elects a new master and forms its own cluster. So, initially, BIG-IP^® devices in both data centers are sending alerts to the DCDs in their clusters, and a master node in each zone is controlling that zone.

When communication resumes, the original cluster does not reform on its own, because both data centers have formed their own independent clusters. To reform the original cluster, you restart the master node for one of the clusters.

• If you restart the master node in the Boston data center, the cluster logic sees that the Seattle data center already has an elected master node, so the Boston cluster joins the Seattle cluster instead of forming its own. The Seattle master node then syncs its data with the Boston nodes in the cluster. The data sync overwrites the Boston data with the Seattle data. The result is that Boston data received during the communication failure is lost.
• If instead, you restart the master node in the Seattle data center, the cluster logic would see that the Boston data center already has an elected master node, so the Seattle cluster would join the Boston cluster. The Boston master node would then sync its data with the nodes in the Seattle cluster. That data sync would overwrite the Seattle data with the Boston data. In this case, the result is that Seattle data received during the communication failure is lost.

To preserve as much data as possible, instead of just reforming the original cluster by restarting one of the clusters, we recommend you perform the following two precautionary steps.

1. When a communication failure occurs, change the target DCDs for the BIG-IP devices in the zone that did not include the original master node (Boston, in our example) to one of the DCDs in the zone that housed the original master node (Seattle, in our example).
2. When communication is restored, in the zone that did not include the original master node (Boston, in our example), use SSH to log in to the master node as root, and then type bigstart restart elasticsearch , and press Enter. Restarting this service removes this node from the election process just long enough so that the original (Seattle) master node can be elected.

After you perform these two steps, all alerts are sent to nodes in the zone that contained the original master node. Then when communication is restored, the DCDs in the zone in which the master node was restarted (Boston) rejoin the cluster. The resulting data sync overwrites the Boston data with the Seattle data. The Seattle data center has the data that was collected before, during, and after the communications failure. The result is that all of the data for the original cluster is saved and when the data is synced, all alert data is preserved.

1. Install and configure a BIG-IQ console node in both data centers.
2. Configure the two console nodes so that one is the HA primary and the other is the HA secondary.
3. For the console node configured as the HA primary, specify the zone in which that device resides.
4. Deploy the DCD in each data center.
5. On the primary console node, add all of the DCDs to the cluster. As you add devices, specify the zone name appropriate for the data center in which each node is physically located.
6. Initiate an HA failover of the primary BIG-IQ console to the secondary BIG-IQ console.
   When the BIG-IQ HA primary fails over to the secondary, the logging configuration information propagates to both zones and a new primary is designated.
7. On the (newly designated) primary BIG-IQ console, specify the name of the zone in which that console resides.
8. Initiate an HA failover of the (new) primary BIG-IQ console to the secondary BIG-IQ console.
   The BIG-IQ console that was originally the HA primary is once again the primary.
9. Set the minimum master capable devices on the primary console. To display the HA properties screen, click System > BIG-IQ HA and then select the device.
   The Zone field is located at the bottom of the screen.

1. At the top of the screen, click System, and then, on the left, click BIG-IQ DATA COLLECTION and select BIG-IQ Data Collection Cluster.
   • Under Summary, you can view information detailing how much data is stored, as well as how the data is stored.
   • Under Configuration, you can access the screens that control DCD cluster performance.
2. On the left, click Configuration > Cluster Settings .
   The Cluster Settings screen opens.
3. To change this setting, click Override.
   The button text changes to Update.
4. In the Minimum Master Eligible Devices field, specify the new minimum number of healthy devices for this DCD cluster, and click Update.
   The system updates the setting.
5. When you are satisfied with the minimum number of devices setting, click Cancel to close the screen.

1. On the BIG-IQ Data Collection Devices screen, remove the node from the DCD cluster.
   The system recalculates and resets the Minimum Master Eligible Nodes setting.
2. If your DCD cluster configuration does not use the default value, override the Minimum Master Eligible Nodes setting to its previous value.
3. Perform the maintenance on the DCD.
4. Add the node back into the DCD cluster.
   The system recalculates and resets the Minimum Master Eligible Nodes setting.
5. If your DCD cluster configuration does not use the default value, override the Minimum Master Eligible Nodes setting to its previous value.

1. At the top of the screen, click System, and then, on the left, click BIG-IQ DATA COLLECTION > BIG-IQ Data Collection Devices .
   The BIG-IQ Data Collection Devices screen opens to list the data collection devices in the cluster.
2. Under Device Name, select the DCD that you want to revise.
3. On the DCDs properties page, click Edit to change the zone for the DCD.
4. Use SSH to log in to DCD as root.
5. Type bigstart restart elasticsearch. and press Enter.
6. Repeat the last two steps for each DCD, and for each BIG-IQ ^®system in the cluster.
   Note: As you run this command on each DCD, it momentarily stops processing DCD data, so the data routes to another node in the cluster and no data is lost.