druid/docs/content/design/overlord.md

---
layout: doc_page
title: "Overlord Node"
---

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# Overlord Node

### Configuration

For Overlord Node Configuration, see [Overlord Configuration](../configuration/index.html#overlord).

### HTTP Endpoints

For a list of API endpoints supported by the Overlord, please see the [API reference](../operations/api-reference.html#overlord).

### Overview

The Overlord node is responsible for accepting tasks, coordinating task distribution, creating locks around tasks, and returning statuses to callers. Overlord can be configured to run in one of two modes - local or remote (local being default).
In local mode Overlord is also responsible for creating Peons for executing tasks. When running the Overlord in local mode, all MiddleManager and Peon configurations must be provided as well.
Local mode is typically used for simple workflows.  In remote mode, the Overlord and MiddleManager are run in separate processes and you can run each on a different server.
This mode is recommended if you intend to use the indexing service as the single endpoint for all Druid indexing.

### Overlord Console

The Overlord console can be used to view pending tasks, running tasks, available workers, and recent worker creation and termination. The console can be accessed at:

```
http://<OVERLORD_IP>:<port>/console.html
```

### Blacklisted Workers

If the workers fail tasks above a threshold, the Overlord will blacklist these workers. No more than 20% of the nodes can be blacklisted. Blacklisted nodes will be periodically whitelisted.

The following vairables can be used to set the threshold and blacklist timeouts.

```
druid.indexer.runner.maxRetriesBeforeBlacklist
druid.indexer.runner.workerBlackListBackoffTime
druid.indexer.runner.workerBlackListCleanupPeriod
druid.indexer.runner.maxPercentageBlacklistWorkers
```

### Autoscaling

The Autoscaling mechanisms currently in place are tightly coupled with our deployment infrastructure but the framework should be in place for other implementations. We are highly open to new implementations or extensions of the existing mechanisms. In our own deployments, MiddleManager nodes are Amazon AWS EC2 nodes and they are provisioned to register themselves in a [galaxy](https://github.com/ning/galaxy) environment.

If autoscaling is enabled, new MiddleManagers may be added when a task has been in pending state for too long. Middle managers may be terminated if they have not run any tasks for a period of time.