---
layout: docs
page_title: Scaling Policies
description: >
  Scaling policies describe the target resource desired state and how to
  perform calculations to ensure the current state reaches the desired.
---

# Nomad Autoscaler Scaling Policies

Nomad Autoscaler scaling policies can be configured via the [`scaling` block][jobspec_scaling_block]
or by configuration files stored on disk. The options available differ whether
you are performing horizontal application/cluster scaling or Dynamic Application
Sizing.

## Top Level Options

- `enabled` - A boolean flag that allows operators to administratively disable a
  policy from active evaluation.

- `min` - The minimum running count of the targeted resource. This can be 0 or any
  positive integer.

- `max` - The maximum running count of the targeted resource. This can be 0 or any
  positive integer.

## Task Group and Cluster Scaling `policy` Options

The following options are available when using the Nomad Autoscaler to perform
horizontal application scaling or horizontal cluster scaling.

- `cooldown` - A time interval after a scaling action during which no additional
  scaling will be performed on the resource. It should be provided as a duration
  (e.g.: `"5s"`, `"1m"`). If omitted the configuration value
  [policy_default_cooldown][policy_default_cooldown_agent] from the agent will
  be used.

- `evaluation_interval` - Defines how often the policy is evaluated by the
  Autoscaler. It should be provided as a duration (e.g.: `"5s"`, `"1m"`). If
  omitted the configuration value [default_evaluation_interval][eval_interval_agent]
  from the agent will be used.

- `on_check_error` - Defines how to handle errors during check evaluation.
  Possible values are `"fail"` or `"ignore"`. If set to `"fail"` the policy
  evaluation will stop if any [`check`](#check) returns an error and no
  scaling action will take place. If set to `"ignore"` any errors returned by a
  `check` will be ignored when computing the scaling action. This value may be
  overridden individually by setting [`on_error`](#on_error). Defaults to
  `"ignore"`.

- `target` - Defines where the autoscaling target is running. Detailed information
  on the configuration options can be found on the [Target Plugins][target_plugin_docs]
  page.

- `check` - Specifies one or more checks to be executed when determining if a
  scaling action is required.

## `check` Options

- `source` - The APM plugin that should handle the metric query. If omitted,
  this defaults to using the Nomad APM.

- `query` - The query to run against the specified APM. Currently this query
  should return a single value. Detailed information on the configuration options
  can be found on the [APM Plugins][apm_plugin_docs] page.

- `query_window` - Defines how far back to query the APM for metrics. It should
  be provided as a duration (e.g.: `"5s"`, `"1m"`). Defaults to `1m`. This
  value may not be supported by all APM plugins. Refer to the documentation of
  the specific APM plugin being used for more information.

- `group` - Specifies which checks should treated as correlated when the policy
  is evaluated. Refer to [Check Grouping][concepts_grouping] for more
  information.

- `on_error` - Defines how to handle errors during the `check` evaluation.
  Possible values are `"fail"` or `"ignore"`. If set to `"fail"` the policy
  evaluation will stop in case an error occurs and not scaling action will take
  place. If set to `"ignore"` the result of this `check` will not be taken into
  considation when computing the scaling action. If not set the value of
  [`on_check_error`](#on_check_error) will be used.

- `strategy` - The strategy to use, and it's configuration when calculating the
  desired state based on the current count and the metric returned by the APM.
  Detailed information on the configuration options can be found on the
  [Strategy Plugins][strategy_plugin_docs] page. Strategies for
  [Dynamic Application Sizing][das] are not allowed in this case.

### Example in a Job

A full example of a policy document that can be written into the Nomad task group
`scaling` block can be seen below.

```hcl
job "example" {
  group "app" {
    scaling {
      min     = 2
      max     = 10
      enabled = true

      policy {
        evaluation_interval = "5s"
        cooldown            = "1m"

        check "active_connections" {
          source = "prometheus"
          query  = "scalar(open_connections_example_cache)"

          strategy "target-value" {
            target = 10
          }
        }
      }
    }
  }
}
```

### Example in a File

An example of a policy document that can be placed in a file within the
`policy_dir` can be seen below. Multiple policies can be defined in the same
file using multiple `scaling` blocks.

```hcl
scaling "aws_cluster_policy" {
  enabled = true
  min     = 1
  max     = 2

  policy {
    cooldown            = "2m"
    evaluation_interval = "1m"

    check "cpu_allocated_percentage" {
      source = "prometheus"
      query  = "..."

      strategy "target-value" {
        target = 70
      }
    }

    check "mem_allocated_percentage" {
      source = "prometheus"
      query  = "..."

      strategy "target-value" {
        target = 70
      }
    }

    target "aws-asg" {
      dry-run             = "false"
      aws_asg_name        = "hashistack-nomad_client"
      node_class          = "hashistack"
      node_drain_deadline = "5m"
    }
  }
}

scaling "azure_cluster_policy" {
  enabled = true
  min     = 1
  max     = 2

  policy {
    ...
    target "azure-vmss" {
      resource_group      = "hashistack"
      vm_scale_set        = "clients"
      node_class          = "hashistack"
      node_drain_deadline = "5m"
    }
  }
}
```

## Task (DAS) `policy` Options

<EnterpriseAlert>
  This functionality only exists in Nomad Autoscaler Enterprise. This is not
  present in the open source version of Nomad Autoscaler.
</EnterpriseAlert>

The following options are available when using the Nomad Autoscaler Enterprise
to perform Dynamic Application Sizing recommendations for task resources. When
using the [`scaling` block][jobspec_scaling_block] for Dynamic Application
Sizing, the block requires a label to identify which resource it relates to. It
currently supports `cpu` and `mem` labels, examples of which can be seen below.

- `cooldown` - A time interval after a scaling action during which no additional
  scaling will be performed on the resource. It should be provided as a duration
  (e.g.: `"5s"`, `"1m"`). If omitted the configuration value
  [policy_default_cooldown][policy_default_cooldown_agent] from the agent will
  be used.

- `evaluation_interval` - Defines how often the policy is evaluated by the
  Autoscaler. It should be provided as a duration (e.g.: `"5s"`, `"1m"`). If
  omitted the configuration value [default_evaluation_interval][eval_interval_agent]
  from the agent will be used.

- `target` - Defines where the autoscaling target is running. Detailed information
  on the configuration options can be found on the [Target Plugins][target_plugin_docs]
  page.

- `check` - Specifies one check to be executed when determining if a recommendation
  is required. Only one check is permitted per scaling block within Dynamic
  Application Sizing.

## `check` Options

- `strategy` - The strategy to use, and it's configuration when calculating the
  desired state based on the current value and the available historic data. Detailed
  information on the configuration options can be found on the
  [Strategy Plugins][strategy_plugin_docs] page. Only [Dynamic Application Sizing][das]
  strategies are allowed.

### Example

The following examples are minimal blocks which can be used to configure CPU and
Memory based sizing recommendations for a Nomad job task.

```hcl
scaling "cpu" {
  policy {
    check "96pct" {
      strategy "app-sizing-percentile" {
        percentile = "96"
      }
    }
  }
}

scaling "mem" {
  policy {
    check "max" {
      strategy "app-sizing-max" {}
    }
  }
}
```

[concepts_grouping]: /nomad/tools/autoscaling/concepts/policy-eval/checks#check-grouping
[das]: /nomad/tools/autoscaling#dynamic-application-sizing
[policy_default_cooldown_agent]: /nomad/tools/autoscaling/agent#default_cooldown
[eval_interval_agent]: /nomad/tools/autoscaling/agent#default_evaluation_interval
[target_plugin_docs]: /nomad/tools/autoscaling/plugins/target
[strategy_plugin_docs]: /nomad/tools/autoscaling/plugins/strategy
[apm_plugin_docs]: /nomad/tools/autoscaling/plugins/apm
[jobspec_scaling_block]: /nomad/docs/job-specification/scaling
