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README.md

Instances Across Availability Zones

Spread OpenStack compute instances (Nova) across a list of availability zones, one instance per zone, using for_each. This is the foundation of a fault-tolerant tier: if a zone goes down, the instances in the others keep serving.

Primary search phrase: Terraform OpenStack availability zones

Architecture

flowchart LR
    TF[Terraform] -->|for_each AZs| VM[openstack_compute_instance_v2]
    DN[data: networking_network_v2] -->|network id| VM
    DI[data: images_image_v2] -->|image id| VM
    DF[data: compute_flavor_v2] -->|flavor id| VM
    VM -->|az = nova| Z1[(Zone nova)]
    VM -->|az = az2| Z2[(Zone az2)]
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for_each = toset(var.availability_zones) creates one instance per zone, keyed by the zone name. Each instance sets availability_zone = each.value and is named <prefix>-<az>, so the resources have stable addresses — adding or removing a zone never disturbs the others.

Usage

export OS_CLOUD=openstack          # or set `cloud` in terraform.tfvars
cp terraform.tfvars.example terraform.tfvars
terraform init
terraform plan
terraform apply

List the zones available to you with openstack availability zone list.

Inputs

Name Description Type Default
cloud clouds.yaml entry to use string "openstack"
availability_zones Zones to spread instances across (one per zone) list(string) ["nova", "az2"]
instance_name_prefix Prefix for instance names string "web"
flavor_name Flavor (size) string "m1.small"
image_name Glance image to boot string "ubuntu-22.04"
network_name Tenant network to attach string "private"
key_pair_name Existing key pair for SSH (optional) string ""
security_group_names Security groups list(string) ["default"]
tags Instance tags list(string) see variables.tf

Outputs

Name Description
instance_ids_by_az Map of availability zone => instance UUID
access_ips_by_az Map of availability zone => first IPv4 address
network_id Network the instances are attached to

Best practices

  • Why this approach: for_each over a set of zone names gives each instance a stable, human-readable key. Removing one zone from the list destroys only that instance and leaves the rest untouched — the key behaviour you want for HA, and exactly where count falls short.
  • Common mistakes: Using count for per-AZ placement (reordering the list renumbers and replaces instances); assuming the default nova zone exists on every cloud (it may not — always check openstack availability zone list); expecting cross-AZ scheduling to also spread anti-affinity — for that, add a server group via scheduler_hints.
  • Scaling considerations: This creates exactly one instance per zone. For multiple instances per zone, combine with a server group or nest counts; mind per-zone capacity and project quotas.
  • Performance considerations: Keep latency-sensitive components together but zone-redundant; instances in different AZs may have slightly different network paths. Match the flavor to the workload.
  • Cost considerations: Each zone runs its own billable instance. Tag everything (done here) for per-zone cost attribution and destroy idle stacks.

Security considerations

  • All instances share security_group_names; define least-privilege groups explicitly rather than relying on default — see security/security-group.
  • Inject SSH access via a managed key pair, never passwords, and keep secrets out of user-data; use application credentials or a secrets manager.
  • Spreading across zones increases availability but also the surface area — keep images patched uniformly across zones.

Troubleshooting

Symptom Likely cause Fix
No valid host was found A listed AZ has no capacity for the flavor Pick a different AZ/flavor; check openstack hypervisor stats show
Quota exceeded Project instance/cores/RAM quota hit across zones Raise quota or shorten availability_zones (quotas examples)
The requested availability zone is not available Zone name typo or zone not exposed to your project openstack availability zone list; fix the list
Image <name> not found Wrong image_name or image not visible to the project openstack image list; check image visibility
Network <name> not found Wrong network_name or no access openstack network list
Provider auth errors Bad/missing clouds.yaml or OS_CLOUD See provider configuration

Cleanup

terraform destroy

Further reading