🏔️ Oracle Cloud Regions, Availability Domains and Fault Domains

What regions, availability domains and fault domains are on Oracle Cloud, why most regions have just one domain, why your home region matters so much, and how the three levels compare with the other clouds

The short version

Oracle Cloud has three levels, one more than the others:

1. Region. A localized geographic area. Regions are independent, and may be separated by vast distances, across countries or continents.
2. Availability domain (AD). One or more data centers within a region. They are isolated and fault-tolerant: they do not share power, cooling or internal networking, so they are unlikely to fail at the same time. All the ADs of a region are connected by a low-latency, high-bandwidth network.
3. Fault domain (FD). A grouping of hardware and infrastructure inside an AD. Every AD has exactly three fault domains. Fault domains are meant to protect against hardware failures and planned maintenance: instances in different fault domains are on different hardware.

Regions

A region has an identifier in the form geography-city-number, and a short key. From Oracle's page:

• us-ashburn-1, key IAD.
• eu-frankfurt-1, key FRA.
• uk-london-1, key LHR.
• ap-tokyo-1, key NRT.

The identifier is what you use in commands and OCIDs, and the key shows up in some of the older names. Regions are grouped into realms, and your tenancy lives in one realm and can use only its regions: OC1 is the commercial realm (the great majority of regions), OC2, OC3 and OC4 are government realms, and there are dedicated realms.

Your home region, again

On the account page the home region is the permanent choice made at sign-up. It matters for regions in three ways:

• IAM changes are made there, and spread to the other regions.
• Always Free compute instances, Always Free Autonomous Databases and Always Free block volumes can only be created in the home region (volumes outside it incur charges).
• A trial, Always Free or pay-as-you-go account can subscribe to only one region. Pay-as-you-go can ask for more through a service limit request ("Regions", "Subscribed region count"). Accounts with a universal monthly credit can subscribe to every publicly released commercial region.

How to choose

Oracle's advice is short: deploy in the region where your application is most used, for performance. Deploy in several regions to survive a region-wide event (weather, earthquake) and to meet legal or jurisdictional rules, including tax domains. For a free account, add one more criterion that beginners meet the hard way: Arm capacity differs by region, and popular regions often show "out of host capacity" for the Always Free A1 shape (see the free VM guide). Since you cannot move later, think about it before you click.

Availability domains

Here Oracle is unlike the others, and honest about it:

• Most new regions have one availability domain. That was the launch strategy for new regions.
• A few mature regions have three. Oracle's examples: eu-frankfurt-1, uk-london-1, us-ashburn-1, us-chicago-1 and us-phoenix-1.

So on a typical new region, there is no multi-AD design: the protection against hardware failure is the fault domains inside the single AD, and protection against a bigger event is a second region. With three ADs you can spread across them as you would across zones on another cloud.

Names are per tenancy

The AD names you see are scrambled for each tenancy: your names have a prefix that belongs to your tenancy, like Uocm:PHX-AD-1. Oracle explains that this balances capacity: the same label PHX-AD-1 can be a different data center for different tenancies. To see your own, use the console (it lists the names when you create an instance) or the CLI (needs an account, from the documentation, not run here):

oci iam region list --output table
oci iam region-subscription list
oci iam availability-domain list --compartment-id TENANCY_OCID

The first lists all regions, the second the regions your tenancy has subscribed to, and the third the availability domains of the region you are working in, with your tenancy's prefix.

Fault domains

When you launch an instance, you may choose a fault domain, or let the system choose (Oracle says it assigns one automatically if you do not). Use them like this:

• Put the copies of one service in different fault domains. Two web servers in FAULT-DOMAIN-1 and FAULT-DOMAIN-2 of the same AD do not share the same hardware, so one hardware failure or one maintenance event does not take both.
• On a single-AD region, this is your only in-region spreading. It does not protect against the loss of the whole AD.

(The names FAULT-DOMAIN-1, -2 and -3 are what the console shows: from the documentation I read, there are three per AD.)

Side by side

• Region = region on every cloud. OCI regions are independent, and realms separate the commercial, government and dedicated clouds.
• Availability domain is close to an Azure availability zone, an AWS Availability Zone or a Google zone: separate data centers with their own power and cooling. But on Oracle most regions have only one, where AWS, Azure and Google say three or more in a region with zones.
• Fault domain has no exact counterpart. The nearest are Azure availability sets and AWS placement groups (hardware spreading inside a data center).
• Per-account name scrambling exists on Oracle (tenancy prefix) and, for older accounts, on AWS (zone names against zone IDs). Google and Azure use fixed names or numbers (Azure maps logical numbers per subscription).
• Home region is Oracle-specific. Region pairs exist only on Azure.

Common mistakes

• Choosing the home region casually.
• Assuming a new region has three availability domains. Most have one.
• Putting both copies of a service in one fault domain.
• Assuming AD-1 is the same place for everyone, or copying an AD name from a tutorial: use yours.
• Trying to create Always Free resources outside the home region.
• Not noticing that a trial account can subscribe to just one region.

The cheat sheet

• Region (us-ashburn-1, key IAD) → availability domain (1 in most regions, 3 in some mature ones such as ashburn, phoenix, chicago, london and frankfurt) → fault domain (exactly 3 per AD).
• Home region: permanent, IAM master, the only place for Always Free compute, databases and block volumes. Trial, free and pay-as-you-go accounts: one subscribed region.
• AD names carry a per-tenancy prefix. Get yours from the console or oci iam availability-domain list --compartment-id OCID.
• Fault domains = different hardware inside one AD. Spread the copies of a service across them.
• Choosing: where the users are, legal needs, multiple regions for region-wide events, and (free tier) Arm capacity.
• CLI: oci iam region list, oci iam region-subscription list, oci iam availability-domain list.

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