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The rebalance mode used for this resource is FULL-AUTO mode. Thus, administrators don’t need to maintain a list of controller hosts in the ideal state. This new resource is auto-rebalancing. Once a controller is shutdown, it will be removed from the lead controller resource. Thus, it’s easy for administrators to add/remove/swap controller hosts with least effort. Plus, this resource leverages CRUSH-ed rebalance strategy, which provides more even partition distribution so that all the Pinot controllers can be in master state evenly across all the partitions. The default delayed rebalance time is set to 5 minutes, which means if some Pinot controller host goes offline, Helix cluster will wait at most 5 minutes for this controller to recover. If timeout is met, Helix cluster will elect a new master. The benefit of having this 5-minute delay is that it won't be too often to switch leadership (e.g. when restarting all the controller hosts one by one, it won't take 5 minutes to finish the restart). Plus, since some periodic tasks run more frequently, 5 minutes isn't a too long number so that Helix cluster can take action to elect a new leader shortlytimely.

Controller Modes

There’re 3 different modes to run controllers, i.e. dual mode, Helix only, and Pinot only. 

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Enable the new resource. The way of enabling it is to change the setting "RESOURCE_ENABLED" from false to true in ZNode CONFIGS/RESOURCE/leadControllerResource in ZK.
Here is the value of leadControllerResource before enabling the resource:

{
 "id" : "leadControllerResource",
 "simpleFields" :{ "RESOURCE_ENABLED" : "false" },
 "mapFields" : {},
 "listFields" : {}
}

Once the setting "RESOURCE_ENABLED" turned enabled, all All the dual-mode controllers will be immediately registered as masters/slaves in the new resource. Periodic tasks and real-time segment completion will immediately be distributed. The following criteria must be met in order to test the robustness of this feature before we move on to the next step. It could take days, weeks or months depending on the installation:

1. All LLC and HLC tables have completed at least one segment and started new ones.
2. All tables are accounted for in all the periodic tasks (no table is ignored).
3. At least one round of rolling restart of pinot controllers is done, and criteria 1 and 2 are verified after the restart.
4. If any of these criteria goes wrong, disable the lead controller resource and everything comes back to the original state. 

If you want to keep Helix controller and Pinot controller running in the same hardware, you can stop at this step. If you want to have these two controllers run in separate hardware, please follow the following steps below.

Step 2

After verifying everything working fine, we can add 1 or more Helix-only controllers to the cluster, so that they can be the candidates of the Helix cluster leadership.

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Disable lead controller resource. All the controller workload will be done by Helix leader.

API Design

In order to get the information of table assignments for better debugging purpose, the following APIs are needed: 

Get the leaders for all the tables in the cluster 

GET /leader/tables 

{
  "leadControllerResourceEnabled": true,
  "leadControllerEntryMap": {
    "leadControllerResource_0": {
      "tableNames": [
        "testTable1_OFFLINE"
      ],
      "leadControllerId": "Controller_172.25.124.150_9000"
    },
    ...
    "leadControllerResource_23": {
      "tableNames": ["testTable2_REALTIME"],
      "leadControllerId": "Controller_172.25.124.150_9008"
    }
  }
}

{
  "leadControllerResourceEnabled": false,
  "leadControllerEntryMap": {}
}

Given a table name, return whether lead controller resource is enabled, the partition id and lead controller instance id 

GET /leader/tables/{tableName} 

{
  "leadControllerResourceEnabled": true,
  "leadControllerEntryMap": {
    "leadControllerResource_7": {
      "tableNames": [
        "testTable_OFFLINE"
      ],
      "leadControllerId": "Controller_172.25.124.150_9000"
    }
  }
}

{
  "leadControllerResourceEnabled": false,
  "leadControllerEntryMap": {}
}

Test Plans and Schedule

Once the final plan has been adjusted and finalized, we can do the following steps. 

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