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Rolling Updates
===============

For rolling Druid cluster updates with no downtime, we recommend updating Druid nodes in the
following order:

1. Historical
2. Overlord (if any)
3. Middle Manager (if any)
4. Standalone Real-time (if any)
5. Broker
6. Coordinator

## Historical

Historical nodes can be updated one at a time. Each Historical node has a startup time to memory map
all the segments it was serving before the update. The startup time typically takes a few seconds to
a few minutes, depending on the hardware of the node. As long as each Historical node is updated
with a sufficient delay (greater than the time required to start a single node), you can rolling
update the entire Historical cluster.

## Overlord

Overlord nodes can be updated one at a time in a rolling fashion.

## Middle Managers

Middle Managers run both batch and real-time indexing tasks. Generally you want to update Middle
Managers in such a way that real-time indexing tasks do not fail. There are three strategies for
doing that.

### Rolling restart (restore-based)

Middle Managers can be updated one at a time in a rolling fashion when you set
`druid.indexer.task.restoreTasksOnRestart=true`. In this case, indexing tasks that support restoring
will restore their state on Middle Manager restart, and will not fail.

Currently, only realtime tasks support restoring, so non-realtime indexing tasks will fail and will
need to be resubmitted.

### Rolling restart (graceful-termination-based)

Middle Managers can be gracefully terminated using the "disable" API. This works for all task types,
even tasks that are not restorable.

To prepare a Middle Manager for update, send a POST request to
`<MiddleManager_IP:PORT>/druid/worker/v1/disable`. The Overlord will now no longer send tasks to
this Middle Manager. Tasks that have already started will run to completion.

To view all existing tasks, send a GET request to `<MiddleManager_IP:PORT>/druid/worker/v1/tasks`.
When this list is empty, you can safely update the Middle Manager. After the Middle Manager starts
back up, it is automatically enabled again. You can also manually enable Middle Managers by POSTing
to `<MiddleManager_IP:PORT>/druid/worker/v1/enable`.

### Autoscaling-based replacement

If autoscaling is enabled on your Overlord, then Overlord nodes can launch new Middle Manager nodes
en masse and then gracefully terminate old ones as their tasks finish. This process is configured by
setting `druid.indexer.runner.minWorkerVersion=#{VERSION}`. Each time you update your overlord node,
the `VERSION` value should be increased, which will trigger a mass launch of new Middle Managers.

The config `druid.indexer.autoscale.workerVersion=#{VERSION}` also needs to be set.

## Standalone Real-time

Standalone real-time nodes can be updated one at a time in a rolling fashion.

## Broker

Broker nodes can be updated one at a time in a rolling fashion. There needs to be some delay between
updating each node as brokers must load the entire state of the cluster before they return valid
results.

## Coordinator

Coordinator nodes can be updated one at a time in a rolling fashion.