283 lines
8.1 KiB
Markdown
283 lines
8.1 KiB
Markdown
---
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layout: "docs"
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page_title: "constraint Stanza - Job Specification"
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sidebar_current: "docs-job-specification-constraint"
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description: |-
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The "constraint" stanza allows restricting the set of eligible nodes.
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Constraints may filter on attributes or metadata. Additionally constraints may
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be specified at the job, group, or task levels for ultimate flexibility.
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---
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# `constraint` Stanza
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<table class="table table-bordered table-striped">
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<tr>
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<th width="120">Placement</th>
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<td>
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<code>job -> **constraint**</code>
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<br>
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<code>job -> group -> **constraint**</code>
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<br>
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<code>job -> group -> task -> **constraint**</code>
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</td>
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</tr>
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</table>
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The `constraint` allows restricting the set of eligible nodes. Constraints may
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filter on [attributes][interpolation] or [client metadata][client-meta].
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Additionally constraints may be specified at the [job][job], [group][group], or
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[task][task] levels for ultimate flexibility.
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~> **It is possible to define irreconcilable constraints in a job.**
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For example, because all [tasks within a group are scheduled on the same client node][group],
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specifying different [`${attr.unique.hostname}`][node-variables] constraints at
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the task level will cause a job to be unplaceable.
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```hcl
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job "docs" {
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# All tasks in this job must run on linux.
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constraint {
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attribute = "${attr.kernel.name}"
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value = "linux"
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}
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group "example" {
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# All groups in this job should be scheduled on different hosts.
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constraint {
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operator = "distinct_hosts"
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value = "true"
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}
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task "server" {
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# All tasks must run where "my_custom_value" is greater than 3.
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constraint {
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attribute = "${meta.my_custom_value}"
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operator = ">"
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value = "3"
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}
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}
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}
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}
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```
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Placing constraints at both the job level and at the group level is redundant
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since constraints are applied hierarchically. The job constraints will affect
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all groups (and tasks) in the job.
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## `constraint` Parameters
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- `attribute` `(string: "")` - Specifies the name or reference of the attribute
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to examine for the constraint. This can be any of the [Nomad interpolated
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values](/docs/runtime/interpolation.html#interpreted_node_vars).
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- `operator` `(string: "=")` - Specifies the comparison operator. The ordering is
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compared lexically. Possible values include:
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```text
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=
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!=
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>
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>=
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<
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<=
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distinct_hosts
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distinct_property
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regexp
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set_contains
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version
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```
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For a detailed explanation of these values and their behavior, please see
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the [operator values section](#operator-values).
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- `value` `(string: "")` - Specifies the value to compare the attribute against
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using the specified operation. This can be a literal value, another attribute,
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or any [Nomad interpolated
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values](/docs/runtime/interpolation.html#interpreted_node_vars).
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### `operator` Values
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This section details the specific values for the "operator" parameter in the
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Nomad job specification for constraints. The operator is always specified as a
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string, but the string can take on different values which change the behavior of
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the overall constraint evaluation.
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```hcl
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constraint {
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operator = "..."
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}
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```
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- `"distinct_hosts"` - Instructs the scheduler to not co-locate any groups on
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the same machine. When specified as a job constraint, it applies to all groups
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in the job. When specified as a group constraint, the effect is constrained to
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that group. This constraint can not be specified at the task level. Note that
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the `attribute` parameter should be omitted when using this constraint.
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```hcl
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constraint {
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operator = "distinct_hosts"
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value = "true"
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}
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```
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The constraint may also be specified as follows for a more compact
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representation:
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```hcl
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constraint {
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distinct_hosts = true
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}
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```
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- `"distinct_property"` - Instructs the scheduler to select nodes that have a
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distinct value of the specified property. The `value` parameter specifies how
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many allocations are allowed to share the value of a property. The `value`
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must be 1 or greater and if omitted, defaults to 1. When specified as a job
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constraint, it applies to all groups in the job. When specified as a group
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constraint, the effect is constrained to that group. This constraint can not
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be specified at the task level.
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```hcl
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constraint {
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operator = "distinct_property"
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attribute = "${meta.rack}"
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value = "3"
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}
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```
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The constraint may also be specified as follows for a more compact
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representation:
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```hcl
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constraint {
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distinct_property = "${meta.rack}"
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value = "3"
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}
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```
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- `"regexp"` - Specifies a regular expression constraint against the attribute.
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The syntax of the regular expressions accepted is the same general syntax used
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by Perl, Python, and many other languages. More precisely, it is the syntax
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accepted by RE2 and described at in the [Google RE2
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syntax](https://golang.org/s/re2syntax).
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```hcl
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constraint {
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attribute = "..."
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operator = "regexp"
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value = "[a-z0-9]"
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}
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```
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- `"set_contains"` - Specifies a contains constraint against the attribute. The
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attribute and the list being checked are split using commas. This will check
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that the given attribute contains **all** of the specified elements.
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```hcl
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constraint {
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attribute = "..."
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operator = "set_contains"
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value = "a,b,c"
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}
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```
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- `"version"` - Specifies a version constraint against the attribute. This
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supports a comma-separated list of constraints, including the pessimistic
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operator. For more examples please see the [go-version
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repository](https://github.com/hashicorp/go-version) for more specific
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examples.
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```hcl
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constraint {
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attribute = "..."
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operator = "version"
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value = ">= 0.1.0, < 0.2"
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}
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```
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## `constraint` Examples
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The following examples only show the `constraint` stanzas. Remember that the
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`constraint` stanza is only valid in the placements listed above.
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### Kernel Data
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This example restricts the task to running on nodes which have a kernel version
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higher than "3.19".
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```hcl
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constraint {
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attribute = "${attr.kernel.version}"
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operator = "version"
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value = "> 3.19"
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}
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```
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### Distinct Property
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A potential use case of the `distinct_property` constraint is to spread a
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service with `count > 1` across racks to minimize correlated failure. Nodes can
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be annotated with which rack they are on using [client
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metadata][client-metadata] with values such as "rack-12-1", "rack-12-2", etc.
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The following constraint would assure that an individual rack is not running
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more than 2 instances of the task group.
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```hcl
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constraint {
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distinct_property = "${meta.rack}"
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value = "2"
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}
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```
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### Operating Systems
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This example restricts the task to running on nodes that are running Ubuntu
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14.04
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```hcl
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constraint {
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attribute = "${attr.os.name}"
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value = "ubuntu"
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}
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constraint {
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attribute = "${attr.os.version}"
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value = "14.04"
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}
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```
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### Cloud Metadata
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When possible, Nomad populates node attributes from the cloud environment. These
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values are accessible as filters in constraints. This example constrains this
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task to only run on nodes that are memory-optimized on AWS.
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```hcl
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constraint {
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attribute = "${attr.platform.aws.instance-type}"
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value = "m4.xlarge"
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}
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```
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### User-Specified Metadata
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This example restricts the task to running on nodes where the binaries for
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redis, cypress, and nginx are all cached locally. This particular example is
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utilizing node [metadata][meta].
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```hcl
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constraint {
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attribute = "${meta.cached_binaries}"
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set_contains = "redis,cypress,nginx"
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}
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```
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[job]: /docs/job-specification/job.html "Nomad job Job Specification"
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[group]: /docs/job-specification/group.html "Nomad group Job Specification"
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[client-meta]: /docs/agent/configuration/client.html#meta "Nomad meta Job Specification"
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[task]: /docs/job-specification/task.html "Nomad task Job Specification"
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[interpolation]: /docs/runtime/interpolation.html "Nomad interpolation"
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[node-variables]: /docs/runtime/interpolation.html#node-variables- "Nomad interpolation-Node variables"
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