165 lines
5.4 KiB
Plaintext
165 lines
5.4 KiB
Plaintext
---
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layout: docs
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page_title: spread Stanza - Job Specification
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sidebar_title: spread
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description: >-
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The "spread" stanza is used to spread placements across a certain node
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attributes such as datacenter.
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Spread may be specified at the job or group levels for ultimate flexibility.
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More than one spread stanza may be specified with relative weights between
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each.
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---
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# `spread` Stanza
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<Placement
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groups={[
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['job', 'spread'],
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['job', 'group', 'spread']
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]}
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/>
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The `spread` stanza allows operators to increase the failure tolerance of their
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applications by specifying a node attribute that allocations should be spread
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over. This allows operators to spread allocations over attributes such as
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datacenter, availability zone, or even rack in a physical datacenter. By
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default, when using spread the scheduler will attempt to place allocations
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equally among the available values of the given target.
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```hcl
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job "docs" {
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# Spread allocations over all datacenter
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spread {
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attribute = "${node.datacenter}"
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}
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group "example" {
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# Spread allocations over each rack based on desired percentage
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spread {
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attribute = "${meta.rack}"
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target "r1" {
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percent = 60
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}
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target "r2" {
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percent = 40
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}
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}
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}
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}
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```
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Nodes are scored according to how closely they match the desired target percentage defined in the
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spread stanza. Spread scores are combined with other scoring factors such as bin packing.
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A job or task group can have more than one spread criteria, with weights to express relative preference.
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Spread criteria are treated as a soft preference by the Nomad scheduler.
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If no nodes match a given spread criteria, placement is still successful.
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Spread may be expressed on [attributes][interpolation] or [client metadata][client-meta].
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Additionally, spread may be specified at the [job][job] and [group][group] levels for ultimate flexibility. Job level spread criteria are inherited by all task groups in the job.
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## `spread` Parameters
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- `attribute` `(string: "")` - Specifies the name or reference of the attribute
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to use. This can be any of the [Nomad interpolated
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values](/docs/runtime/interpolation#interpreted_node_vars).
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- `target` <code>([target](#target-parameters): <required>)</code> - Specifies one or more target
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percentages for each value of the `attribute` in the spread stanza. If this is omitted,
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Nomad will spread allocations evenly across all values of the attribute.
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- `weight` `(integer:0)` - Specifies a weight for the spread stanza. The weight is used
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during scoring and must be an integer between 0 to 100. Weights can be used
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when there is more than one spread or affinity stanza to express relative preference across them.
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## `target` Parameters
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- `value` `(string:"")` - Specifies a target value of the attribute from a `spread` stanza.
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- `percent` `(integer:0)` - Specifies the percentage associated with the target value.
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## `spread` Examples
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The following examples show different ways to use the `spread` stanza.
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### Even Spread Across Data Center
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This example shows a spread stanza across the node's `datacenter` attribute. If we have
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two datacenters `us-east1` and `us-west1`, and a task group of `count = 10`,
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Nomad will attempt to place 5 allocations in each datacenter.
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```hcl
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spread {
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attribute = "${node.datacenter}"
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weight = 100
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}
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```
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### Spread With Target Percentages
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This example shows a spread stanza that specifies one target percentage. If we
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have three datacenters `us-east1`, `us-east2`, and `us-west1`, and a task group
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of `count = 10`, Nomad will attempt to place place 5 of the allocations in "us-east1",
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and will spread the remaining among the other two datacenters.
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```hcl
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spread {
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attribute = "${node.datacenter}"
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weight = 100
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target "us-east1" {
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percent = 50
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}
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}
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```
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This example shows a spread stanza that specifies target percentages for two
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different datacenters. If we have two datacenters `us-east1` and `us-west1`,
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and a task group of `count = 10`, Nomad will attempt to place 6 allocations
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in `us-east1` and 4 in `us-west1`.
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```hcl
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spread {
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attribute = "${node.datacenter}"
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weight = 100
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target "us-east1" {
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percent = 60
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}
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target "us-west1" {
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percent = 40
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}
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}
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```
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### Spread Across Multiple Attributes
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This example shows spread stanzas with multiple attributes. Consider a Nomad cluster
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where there are two datacenters `us-east1` and `us-west1`, and each datacenter has nodes
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with `${meta.rack}` being `r1` or `r2`. With the following spread stanza used on a job with `count=12`, Nomad
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will attempt to place 6 allocations in each datacenter. Within a datacenter, Nomad will
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attempt to place 3 allocations in nodes on rack `r1`, and 3 allocations in nodes on rack `r2`.
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```hcl
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spread {
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attribute = "${node.datacenter}"
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weight = 50
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}
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spread {
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attribute = "${meta.rack}"
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weight = 50
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}
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```
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[job]: /docs/job-specification/job 'Nomad job Job Specification'
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[group]: /docs/job-specification/group 'Nomad group Job Specification'
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[client-meta]: /docs/configuration/client#meta 'Nomad meta Job Specification'
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[task]: /docs/job-specification/task 'Nomad task Job Specification'
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[interpolation]: /docs/runtime/interpolation 'Nomad interpolation'
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[node-variables]: /docs/runtime/interpolation#node-variables- 'Nomad interpolation-Node variables'
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[constraint]: /docs/job-specification/constraint 'Nomad Constraint job Specification'
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