August 29th, 2025

Machine translation is used partially for this article. See the Japanese version for the original article.

Introduction

We have built a multi-region HA cluster using the Oracle Cloud DNS resource on the Oracle Cloud Infrastructure (hereinafter called "OCI") environment. The Oracle Cloud DNS resource is one of the features added in EXPRESSCLUSTER X 5.2, and it switches the connection destination by rewriting the A record of the DNS zone in the OCI DNS service.
In this article, we will introduce the basic configuration method of the OCI DNS service and the Oracle Cloud DNS resource in a multi-region environment.

EXPRESSCLUSTER X 5.2 includes enhancements in cloud support, usability, and Infrastructure as Code (IaC) features. If you are interested, please refer to the following:

[Reference]
Documentation - Product Introduction

-> Presentation

-> EXPRESSCLUSTER X 5.2 New Enhancement

Contents

1. What is the Oracle Cloud DNS Resource?

The Oracle Cloud DNS resource provides the capability to switch connection destinations using a virtual hostname (DNS name) by rewriting the A record of a zone managed by the OCI DNS service. This resource is available for both EXPRESSCLUSTER X for Windows and Linux.

The OCI DNS service offers both public and private DNS zones. Public DNS zones are used for name resolution over the internet, while private DNS zones are used for name resolution from within a single Virtual Cloud Network (hereinafter called "VCN").

The Oracle Cloud DNS resource supports both public and private DNS zones. By using the OCI CLI, it rewrites the A record where the desired virtual hostname is registered in each zone, thereby achieving connection destination switching.

In addition, Oracle Cloud DNS monitoring resources will be added along with the addition of Oracle Cloud DNS resources. This monitor resource periodically runs OCI CLI commands or queries to check whether the A record has been rewritten successfully. This ensures the DNS switch is working as intended.

2. Benefits of Using the Oracle Cloud DNS Resource

With the addition of the Oracle Cloud DNS resource in EXPRESSCLUSTER X 5.2, we can now easily switch HA cluster connection destinations using a virtual hostname, not only in a single region but also across multiple regions.

In past blog articles, we have introduced how to build a multi-region HA cluster using Dynamic DNS resources in an OCI environment. However, it was necessary to build a DNS server and consider the availability of the DNS server.

The Oracle Cloud DNS resource allows us to build a multi-region HA cluster more easily because it uses the OCI DNS service, which is a managed service. We no longer need to manage our own DNS servers.

There is also the Oracle Cloud Virtual IP resource, which uses a virtual IP address for the connection destination, as a connection destination switching resource dedicated to the OCI environment. However, its use requires the Oracle Cloud Load Balancer. As of June 2025, the Oracle Cloud Load Balancer cannot be created across multiple VCNs, and the backend server must be an instance within the VCN. Therefore, we recommend using the Oracle Cloud DNS resource to build an HA cluster in multiple regions.

3. HA Cluster Configuration

We will build a "Multi-Region Mirror Disk HA Cluster Based on DNS Name Control" using OCI's Ashburn and Phoenix regions.

The configuration is as follows:

As mentioned earlier, we will use the OCI CLI to rewrite the A record of the OCI DNS service in the Oracle Cloud DNS resource. Therefore, the instances for the HA cluster that run the OCI CLI need to be able to communicate with the API endpoint of the OCI DNS service. This time, we add a NAT Gateway so that the instances for the HA cluster can communicate with the OCI DNS service API endpoint via the Internet. If you want to build more securely, you can also build a closed HA cluster within OCI by using a Service Gateway for the OCI DNS service.

3.1 Setting the OCI Environment

3.1.1 VCN Settings

The settings for the VCN, subnets, and dynamic routing gateway are as follows:

■Ashburn region

■Phoenix region

Please refer to the following for information on how to configure the dynamic routing gateway and remote peering.

[Reference]
We Tried Building an HA Cluster Across Regions Using VCN Peering on Oracle Cloud Infrastructure (Windows/Linux)

-> 3. HA Cluster Configuration Procedure

-> 3.1 Creating Remote VCN Peering

3.1.2 OCI DNS Service Settings

The OCI DNS service configuration is as follows:

■Ashburn region

■Phoenix region

3.1.3 Creating Instances for the HA Cluster

The instances for the HA cluster configuration are as follows:

■Ashburn region

■Phoenix region

To enable rewriting OCI DNS service records, we will enable OCI CLI to run from the instances forming the HA cluster, referring to the following:

[Reference]
Command Line Interface (CLI) - Quickstart

The following policies are required to run OCI CLI. Please refer to the startup guide for details.

[Reference]
Documentation - Manuals

• EXPRESSCLUSTER X 5.3 > EXPRESSCLUSTER X 5.3 for Windows > Getting Started Guide

  -> 6 Notes and Restrictions

  -> 6.2 Before installing EXPRESSCLUSTER

  -> 6.2.24 Policy setting in the OCI environment
• EXPRESSCLUSTER X 5.3 > EXPRESSCLUSTER X 5.3 for Linux > Getting Started Guide

  -> 6 Notes and Restrictions

  -> 6.3 Before installing EXPRESSCLUSTER

  -> 6.3.29 Policy setting in the OCI environment

3.2 EXPRESSCLUSTER Settings

In this verification, we are using EXPRESSCLUSTER X 5.3 (Internal Ver. Windows: 13.30, Linux: 5.3.0-1).
The configuration of EXPRESSCLUSTER is as follows.

In OCI, consistent device paths are supported for block volumes attached to instances. When attaching a block volume to an instance, we select a device path to ensure consistency even after the instance is rebooted.

[Reference]
Connecting to Volumes With Consistent Device Paths

3.2.1 Example Setting of the Oracle Cloud DNS Resources

An example setting of the Oracle Cloud DNS resource is as follows:
On the "Info" screen of the resource definition for the group, we select "Oracle Cloud DNS resource" and click "Next".

On the "Dependency" and "Recovery Operation" screens, leave the default settings unchanged and click "Next".

Next, configure the settings on the "Details" screen. One of the input fields on the "Details" screen is "TTL". The TTL setting value determines the cache lifetime of the OCI DNS service. In environments where failover occurs in a short period of time, the remaining TTL cache may prevent successful name resolution when resources are started on the failover destination. Therefore, consider shortening the TTL value according to the failover time. In this example, we set the default value of 300 seconds.
Configure the "Details" screen as follows:

■Common

■server01

■server02

After configuring the above settings, click "Finish" to create the Oracle Cloud DNS resource.

For more details on configuring the Oracle Cloud DNS resource, please refer to the following reference guide:

[Reference]
Documentation - Manuals

• EXPRESSCLUSTER X 5.3 > EXPRESSCLUSTER X 5.3 for Windows > Reference Guide

  -> 3 Group resource details

  -> 3.28 Understanding Oracle Cloud DNS resources
• EXPRESSCLUSTER X 5.3 > EXPRESSCLUSTER X 5.3 for Linux > Reference Guide

  -> 3 Group resource details

  -> 3.24 Understanding Oracle Cloud DNS resources

4. Checking the Operation

We will perform the following steps to check the operation of the HA cluster:

Through the above checking the operation, we checked that the connection destination using the virtual hostname switches before and after the failover.

Conclusion

This time, we introduced how to build a multi-region HA cluster using the Oracle Cloud DNS resource on the OCI environment. By utilizing the Oracle Cloud DNS resource, you no longer need to prepare a DNS server, making it easier to build a multi-region HA cluster.

If you consider introducing the configuration described in this article, you can perform a validation with the trial module of EXPRESSCLUSTER. Please do not hesitate to contact us if you have any questions.