open-nomad/website/source/docs/job-specification/spread.html.md

layout	page_title	sidebar_current	description
docs	spread Stanza - Job Specification	docs-job-specification-spread	The "spread" stanza is used to spread placements across a certain node attributes such as datacenter. Spread may be specified at the job, group, or task levels for ultimate flexibility. More than one spread stanza may be specified with relative weights between each.

spread Stanza

Placement	job -> **spread** job -> group -> **spread** job -> group -> task -> **spread**

The spread stanza allows operators to influence the placement of allocations in a task group. Operators can use spread to increase failure tolerance of their application. The operator can specify a node attribute such as datacenter, availability zone, or even rack in a physical datacenter to spread the allocations over. By default, when using spread the scheduler will attempt to place allocations equally among the available values of the given target.

Nodes are scored according to how closely they match the desired target percentage defined in the spread stanza. Spread scores are combined with other scoring factors such as bin packing.

A job or task group can have more than one spread criteria, with weights to express relative preference.

Spread criteria are treated as a soft preference by the Nomad scheduler. If no nodes match a given spread criteria, placement is still successful.

Spread may be expressed on attributes or client metadata. Additionally, spread may be specified at the job, group, or task levels for ultimate flexibility.

spread Parameters

• attribute (string: "") - Specifies the name or reference of the attribute to use. This can be any of the Nomad interpolated values.

• target (target: ) - Specifies one or more target percentages for each value of the attribute in the spread stanza. If this is omitted, Nomad will spread allocations evenly across all values of the attribute.

• weight (integer:0) - Specifies a weight for the spread stanza. The weight is used during scoring and must be an integer between 0 to 100. Weights can be used when there is more than one spread or affinity stanza to express relative preference across them.

target Parameters

• value (string:"") - Specifies a target value of the attribute from a spread stanza.

• percent (integer:0) - Specifies the percentage associated with the target value.

spread Examples

The following examples show different ways to use the spread stanza.

Even Spread Across Data Center

This example shows a spread stanza across the node's datacenter attribute. If we have two datacenters us-east1 and us-west1, and a task group of count = 10, Nomad will attempt to place 5 allocations in each datacenter.

spread {
  attribute = "${node.datacenter}"
  weight    = 100
}

Spread With Target Percentages

This example shows a spread stanza that specifies one target percentage. If we have three datacenters us-east1, us-east2 and us-west1, and a task group of count = 10 Nomad will attempt to place place 5 of the allocations in "us-east1", and then spread the rest among the other two datacenters.

spread {
  attribute = "${node.datacenter}"
  weight    = 100

  target "us-east1" {
    percent = 50
  }
}

This example shows a spread stanza that specifies target percentages for two different datacenters. If we have two datacenters us-east1 and us-west1, and a task group of count = 10, Nomad will attempt to place 6 allocations in us-east1 and 4 in us-west1.

spread {
  attribute = "${node.datacenter}"
  weight    = 100

  target "us-east1" {
    percent = 60
  }

  target "us-west1" {
      percent = 40
  }
}

Spread Across Multiple Attributes

This example shows spread stanzas with multiple attributes. Consider a Nomad cluster where there are two datacenters us-east1 and us-west1, and each datacenter has nodes with ${meta.rack} being r1 or r2. For the following spread stanza used on a job with count=12, Nomad will attempt to place 6 allocations in each datacenter. Within a datacenter, Nomad will attempt to place 3 allocations in nodes on rack r1, and 3 allocations in nodes on rack r2.

spread {
  attribute = "${node.datacenter}"
  weight    = 50
}
spread {
  attribute = "${meta.rack}"
  weight    = 50
}