---
layout: "docs"
page_title: "affinity Stanza - Job Specification"
sidebar_current: "docs-job-specification-affinity"
description: |-
  The "affinity" stanza allows restricting the set of eligible nodes.
  Affinities may filter on attributes or metadata. Additionally affinities may
  be specified at the job, group, or task levels for ultimate flexibility.
---

# `affinity` Stanza

<table class="table table-bordered table-striped">
  <tr>
    <th width="120">Placement</th>
    <td>
      <code>job -> **affinity**</code>
      <br>
      <code>job -> group -> **affinity**</code>
      <br>
      <code>job -> group -> task -> **affinity**</code>
    </td>
  </tr>
</table>

The `affinity` stanza allows operators to express placement preference for a set of nodes. Affinities may
be expressed on [attributes][interpolation] or [client metadata][client-meta].
Additionally affinities may be specified at the [job][job], [group][group], or
[task][task] levels for ultimate flexibility.

Placing the same affinity at both the job level and at the group level is redundant
since affinities are applied hierarchically. The job affinities will affect
all groups (and tasks) in the job.

Nomad will use affinities when computing scores for placement. Nodes that match affinities will
have their scores boosted. Affinity scores are combined with other scoring factors such as bin packing.
Operators can use weights to express relative preference across multiple affinities. If no nodes match a given affinity,
placement is still successful. This is different from [constraints][constraint] where placement is
restricted only to nodes that meet the constraint's criteria.

## `affinity` Parameters

- `attribute` `(string: "")` - Specifies the name or reference of the attribute
  to examine for the affinity. This can be any of the [Nomad interpolated
  values](/docs/runtime/interpolation.html#interpreted_node_vars).

- `operator` `(string: "=")` - Specifies the comparison operator. The ordering is
  compared lexically. Possible values include:

    ```text
    =
    !=
    >
    >=
    <
    <=
    regexp
    set_contains_all
    set_contains_any
    version
    ```

    For a detailed explanation of these values and their behavior, please see
    the [operator values section](#operator-values).

- `value` `(string: "")` - Specifies the value to compare the attribute against
  using the specified operation. This can be a literal value, another attribute,
  or any [Nomad interpolated
  values](/docs/runtime/interpolation.html#interpreted_node_vars).

- `weight` `(integer:0)` - Specifies a weight for the affinity. The weight is used
  during scoring and must be an integer between -100 to 100. Negative weights act as
  anti affinities, causing nodes that match them to be scored lower. Weights can be used
  when there is more than one affinity to express relative preference across them.

### `operator` Values

This section details the specific values for the "operator" parameter in the
Nomad job specification for affinities. The operator is always specified as a
string, but the string can take on different values which change the behavior of
the overall affinity evaluation.

```hcl
affinity {
  operator = "..."
}
```

- `"regexp"` - Specifies a regular expression affinity against the attribute.
  The syntax of the regular expressions accepted is the same general syntax used
  by Perl, Python, and many other languages. More precisely, it is the syntax
  accepted by RE2 and described at in the [Google RE2
  syntax](https://golang.org/s/re2syntax).

    ```hcl
    affinity {
      attribute = "..."
      operator  = "regexp"
      value     = "[a-z0-9]"
      weight    = 50
    }
    ```

- `"set_contains_all"` - Specifies a contains affinity against the attribute. The
  attribute and the list being checked are split using commas. This will check
  that the given attribute contains **all** of the specified elements.

    ```hcl
    affinity {
      attribute = "..."
      operator  = "set_contains"
      value     = "a,b,c"
      weight    = 50
    }
    ```
- `"set_contains"`  - Same as `set_contains_all`

- `"set_contains_any"` - Specifies a contains affinity against the attribute. The
  attribute and the list being checked are split using commas. This will check
  that the given attribute contains **any** of the specified elements.

    ```hcl
    affinity {
      attribute = "..."
      operator  = "set_contains"
      value     = "a,b,c"
      weight    = 50
    }
    ```

- `"version"` - Specifies a version affinity against the attribute. This
  supports a comma-separated list of values, including the pessimistic
  operator. For more examples please see the [go-version
  repository](https://github.com/hashicorp/go-version) for more specific
  examples.

    ```hcl
    affinity {
      attribute = "..."
      operator  = "version"
      value     = ">= 0.1.0, < 0.2"
      weight    = 50
    }
    ```

## `affinity` Examples

The following examples only show the `affinity` stanzas. Remember that the
`affinity` stanza is only valid in the placements listed above.

### Kernel Data

This example adds a preference for running on nodes which have a kernel version
higher than "3.19".

```hcl
affinity{
  attribute = "${attr.kernel.version}"
  operator  = "version"
  value     = "> 3.19"
  weight    = 50
}
```

### Operating Systems

This example adds a preference to running on nodes that are running Ubuntu
14.04

```hcl
affinity {
  attribute = "${attr.os.name}"
  value     = "ubuntu"
  weight    = 50
}

affinity {
  attribute = "${attr.os.version}"
  value     = "14.04"
  weight    = 100
}
```

### Meta Data

The following example adds a preference to running on nodes with specific rack metadata

```hcl
affinity {
  attribute = "${meta.rack}"
  value     = "rack1"
  weight    = 50
}
```

The following example adds a preference to running on nodes in a specific datacenter.

```hcl
affinity {
  attribute = "${node.datacenter}"
  value     = "us-west1"
  weight    = 50
}
```

### Cloud Metadata

When possible, Nomad populates node attributes from the cloud environment. These
values are accessible as filters in affinities. This example adds a preference to run this
task on nodes that are memory-optimized on AWS.

```hcl
affinity {
  attribute = "${attr.platform.aws.instance-type}"
  value     = "m4.xlarge"
  weight    = 50
}
```

[job]: /docs/job-specification/job.html "Nomad job Job Specification"
[group]: /docs/job-specification/group.html "Nomad group Job Specification"
[client-meta]: /docs/configuration/client.html#meta "Nomad meta Job Specification"
[task]: /docs/job-specification/task.html "Nomad task Job Specification"
[interpolation]: /docs/runtime/interpolation.html "Nomad interpolation"
[node-variables]: /docs/runtime/interpolation.html#node-variables- "Nomad interpolation-Node variables"
[constraint]: /docs/job-specification/constraint.html "Nomad Constraint job Specification"

### Placement Details
Operators can run `nomad alloc status -verbose` to get more detailed information on various
factors, including affinities that affect the final placement.

#### Example Placement Metadata
The following is a snippet from the CLI output of `nomad alloc status -verbose <alloc-id>` showing scoring metadata.

```
Placement Metrics
Node                                  binpack  job-anti-affinity  node-reschedule-penalty  node-affinity  final score
30bd48cc-d760-1096-9bab-13caac424af5  0.225    -0.6               0                        1              0.208
f2aa8b59-96b8-202f-2258-d98c93e360ab  0.225    -0.6               0                        1              0.208
86df0f74-15cc-3a0e-23f0-ad7306131e0d  0.0806   0                  0                        0              0.0806
7d6c2e9e-b080-5995-8b9d-ef1695458b52  0.0806   0                  0                        0              0.0806
```

The placement score is affected by the following factors.

- `bin-packing` - Scores nodes according to how well they fit requirements. Optimizes for using minimal number of nodes.
- `job-anti-affinity` - A penalty added for additional instances of the same job on a node, used to avoid having too many instances
  of a job on the same node.
- `node-reschedule-penalty` - Used when the job is being rescheduled. Nomad adds a penalty to avoid placing the job on a node where
  it has failed to run before.
- `node-affinity` - Used when the criteria specified in the `affinity` stanza matches the node.