---
layout: docs
page_title: Node Selector Strategy
description: Learn about the Autoscaler's node selection strategies.
---

# Node Selector Strategy

The node selector strategy is a mechanism the Nomad Autoscaler uses to identify
nodes for termination when performing horizontal cluster scale-in actions. It
is exposed as a target configuration option so that operators can modify its
behaviour based on requirements.

### `least_busy` Node Selector Strategy

The `least_busy` strategy is the default node selector strategy used if the
`node_selector_strategy` configuration option is omitted. When used, the Nomad
Autoscaler will sort nodes based on their overall CPU and memory allocation;
picking those with the lowest values. In the event a number of nodes have the
same allocated percentage value, the selection will be random according to Golang's
sort implementation.

The `least_busy` strategy suits workloads that can tolerate migrations but
allows you to minimise such events.

### `empty` Node Selector Strategy

The `empty` strategy will only consider nodes eligible for termination if they
have zero non-terminal allocations. The terminal status of an allocation is
determined using [these Nomad SDK allocation functions][nomad_api_terminal_alloc].
The strategy will skip any node that does not meet this requirement. This can
result in scaling actions terminating a smaller number of nodes than desired. In
the event no nodes are found to be empty, the Nomad Autoscaler will decline to
perform the scaling action.

The `empty` strategy is ideal for batch workloads, ensuring allocations are not
interrupted by scaling.

~> **Note:** [system][system_scheduler] jobs will prevent this strategy from
scaling-in nodes. Please use the [`empty_ignore_system`](#empty_ignore_system)
strategy if you have system jobs in your cluster.

### `empty_ignore_system` Node Selector Strategy

The `empty_ignore_system` strategy is similar to `empty`, but it will not
consider allocations from [system][system_scheduler] jobs. A node with only
terminal allocations and allocations from system jobs is considered empty under
this strategy.

### `newest_create_index` Node Selector Strategy

The `newest_create_index` strategy is the simplest strategy and uses the order
in which nodes are returned by the [Nomad SDK][nomad_api_node_stub_sort]. The
strategy is ideal for workloads and environments which can handle migrations
due to scaling. It is also the least computationally intensive selector strategy.

[nomad_api_terminal_alloc]: https://github.com/hashicorp/nomad/blob/14568b3e002868fc5c83ee7d158a78394c1ea9c1/api/allocations.go#L422-L442
[nomad_api_node_stub_sort]: https://github.com/hashicorp/nomad/blob/14568b3e002868fc5c83ee7d158a78394c1ea9c1/api/nodes.go#L797-L810
[system_scheduler]: /nomad/docs/schedulers#system
