---
title: "Replicated instances in two data centres, with DNS failover"
description: "Efficient Cloud Instances replicated between two Noraina data centres, with NorainaDNS health checks that move traffic to the second site when the first fails."
url: https://www.noraina.cloud/architectures/replicated-instances-dns-failover/
language: en
---
# Replicated instances in two data centres, with DNS failover

For companies that need an application to keep running if a whole data centre fails, without building their own disaster recovery site. Instances run in one of our data centres and replicate every five minutes to a second one; NorainaDNS health checks send users to the second site when the first stops answering.

## How the pieces fit

1. **Efficient Cloud Instances** run the application in a primary data centre, for example Equinix DB3 in Dublin. Their disks replicate every five minutes to a second site you choose, for example Templus in Barcelona.
2. **NorainaDNS** serves the application's domain. Its records are failover records: the primary answer is the address in the first site, with a health check that reaches the application directly; the secondary answer is the address in the second site.
3. When the first site fails, the replica is started in the second site, and NorainaDNS answers with the secondary address as soon as the primary health check fails.
4. **Efficient Cloud Backup** keeps immutable copies outside our platform, for the failures replication cannot fix, such as deleted data or ransomware.

Stateless parts of the application, such as web front ends, can run in both sites at once with weighted records, so only the stateful part waits for the failover.

## Failing back

Once the replica has taken writes, the original site has to be resynchronised before traffic returns to it. We agree with you how failback is triggered, so the health check alone never sends users back to stale data.

## What you get

- Production and replica in any two of our five sites: London, Dublin, Paris (two sites) and Barcelona.
- Recovery point of five minutes and recovery time of fifteen minutes from the decision to fail over.
- DNS that follows the health of each site, managed from the console, the API or Terraform.
- Failover tests on a schedule with a signed report, as evidence for business continuity controls in ISO 27001, ENS, NIS2 and DORA.

## Cost

NorainaDNS starts at $24 a month, with $8 a month for each additional health check; see the [NorainaDNS pricing](https://www.noraina.cloud/products/efficient-cloud-dns/#pricing). Instances and backup are quoted for your configuration: [tell us what you run](https://www.noraina.cloud/contact/) and we send a quote the same week.

## Frequently asked questions

### How do users reach the second data centre after a failover?

The application's DNS records are failover records in NorainaDNS, with a health check on the primary site. When the health check fails, NorainaDNS answers with the address of the second site, so users reach the replica without anyone editing DNS during the incident.

### How much data can be lost when failing over to the second data centre?

Disks replicate every five minutes, so the recovery point objective is five minutes. The recovery time objective is fifteen minutes from the decision to fail over.

### Is replication to a second data centre a backup?

No. Replication copies every change, including a deletion or ransomware encryption, to the second site within minutes. That is why this architecture adds Efficient Cloud Backup, with immutable copies outside our platform that can be restored to a point in time.

## Compliance

Our services support these controls and produce evidence for them. Certification depends on your whole management system and is granted by an accredited auditor, not by a supplier.

- **ISO 27001**: 5.29, 5.30, 8.14, 8.13, 8.24, 5.33 (https://www.noraina.cloud/compliance/iso-27001/)
- **ENS**: op.cont.2, op.cont.3, op.cont.4, mp.info.6, mp.s.4 (https://www.noraina.cloud/compliance/ens/)
- **NIS2**: 21.2.c, 21.2.h (https://www.noraina.cloud/compliance/nis2/)
- **DORA**: art.11, art.12 (https://www.noraina.cloud/compliance/dora/)
- **GDPR**: 32.1.b, 32.1.c, 32.1.a (https://www.noraina.cloud/compliance/gdpr/)

Last updated: 2026-10-08