How to Use Template Stanzas Instead of Dynamic Blocks for Nomad Variable Data – HashiCorp Help Center

Overview

Nomad Variables are a convenient way to store configuration data, but they cannot be used inside dynamic blocks. This article demonstrates both the failure case (with real errors) and the working alternative using template stanzas.

Pre-requisites

Docker installed and running. Refer this.
Nomad installed and running. Refer this.
Consul installed and running. Refer this.

1. Non‑Working Setup: Dynamic Blocks with Variables

variable "my_structured_var" {
  type = map(string)
  default = {
    app1 = "value1"
    app2 = "value2"
  }
}

job "example" {
  datacenters = ["dc1"]

  group "app" {
    dynamic "task" {
      for_each = var.my_structured_var

      labels = {
        name  = task.key
        value = task.value
      }

      content {
        driver = "docker"
        config {
          image   = "busybox:latest"
          command = "echo"
          args    = ["${task.key}=${task.value}"]
        }
      }
    }
  }
}

Error Received

When running:

nomad job run example.nomad

Nomad fails with:

Error parsing job file from example.nomad:
example.nomad:16,16-17: Invalid expression; A static list expression is required.

Why This Happens

Dynamic blocks in Nomad require static lists/maps at parse time.
Nomad Variables are resolved later at runtime, so they cannot be used in for_each.
This is the expected error and confirms the limitation.

2. Working Setup: Template Stanza with Consul KV

Instead of dynamic blocks, use a template stanza. Templates are evaluated at runtime, so they can access Nomad Variables or Consul KV data.

job "tetris" {
  datacenters = ["dc1"]

  group "games" {
    count = 3

    network {
      port "web" {
        to = 80
      }
    }

    task "tetris" {
      driver = "docker"

      config {
        image = "bsord/tetris"
        ports = ["web"]
      }

      resources {
        cpu    = 50
        memory = 256
      }

      template {
        data = <<EOH
        {{ with key "app/config" | parseJSON }}
        Feature flag: {{ .feature_flag }}
        API URL: {{ .api_url }}
        {{ end }}
        EOH

        destination = "local/config.json"
      }
    }
  }
}

Setup

Store config in Consul KV:

consul kv put app/config '{"feature_flag": true, "api_url": "https://example.com"}'

Run the job:
```
nomad job run tetris.nomad.hcl
```

Inspect the rendered file:

nomad alloc exec <alloc_id> cat local/config.json

Expected Output

Feature flag: true
API URL: https://example.com

3. Recommended Pattern: Consul KV for Shared Config

For structured configuration shared across jobs, use Consul KV:

Better suited for larger data.
Supports ACLs and scoping.
Works seamlessly with template stanzas.

4. References

https://developer.hashicorp.com/nomad/docs/concepts/variables

https://developer.hashicorp.com/consul/docs/automate/kv