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Failover Clustering. An Effective Solution for High Availability and Disaster Recovery

A failover cluster is, essentially, a redundant and performance-optimized backup system for digital architectures. This powerful technology is heavily employed in industries that require running mission-critical applications, systems, and services. In this article, we explore how failover clustering works, the different types, their challenges, and practical applications.

Different Failover Cluster Types

There are two main types of failover clusters. High Availability (HA) and Continuous Availability (CA). The choice between these types largely depends on the organization’s infrastructure, hardware and software, system sophistication, and the importance of digital operations.

High Availability Clusters

HA clusters are designed to minimize downtime and ensure uninterrupted access to applications and services. These clusters use shared storage and clustered roles to replicate data and workloads across multiple servers, allowing another server in the cluster to take over with minimal disruption in case of server failure.

Continuous Availability Clusters

In contrast, CA clusters aim to provide zero downtime for applications and services. These clusters employ synchronous replication to maintain data availability on all servers in the cluster. Consequently, if a server fails, another server takes on its workload immediately without any interruption to users.

Other Failover Cluster Types

Besides HA and CA clusters, there are several other types of failover clusters.

  1. Stretch clusters. These clusters span over two or more data centers, using synchronous replication and high-speed, low-latency connections for excellent reliability and recovery design.
  2. Geo-distributed clusters. Employed to provide global or large regional service for apps and systems, geo-distributed clusters have nodes spread across multiple geographic regions, with asynchronous replication to replicate data across different areas.
  3. Hybrid failover clusters. These clusters combine physical servers with virtual machines (VMs) or private and public cloud infrastructures, offering increased security and flexibility.

How Failover Clusters Work

At their core, failover cluster technology relies on shared storage, clustered roles, synchronous replication, heartbeat monitoring, and quorum voting.

  • Shared storage. Ensures that all servers have access to the same data.
  • Clustered roles. Applications and services that can be moved between nodes in the cluster without disruption.
  • Heartbeat monitoring. Constantly sends out a “heartbeat” from each node advertising its status and availability over a dedicated network link.
  • Quorum voting. Ensures that there is always a majority of operational servers or nodes in the cluster.
  • Synchronous replication. Ensures that data is always available on all servers in the cluster (typically used for CA clusters).

Practical Applications of Failover Clusters

Failover clusters are valuable for these specific use cases.

  1. Ongoing availability of mission-critical applications. Fault-tolerant systems are crucial for online transaction processing (OLTP) systems, airlines reservation systems, electronic stock trading, and ATM banking.
  2. Disaster recovery. Clusters can help businesses recover from disasters quickly and minimize downtime by switching applications and services to another data center without disrupting users.
  3. Data and analytics. Failover cluster characteristics are beneficial for managing and analyzing large amounts of data across different industries.
  4. Edge computing. Clusters can be used for deploying and managing edge computing applications and services.

Challenges of Failover Clusters

Despite their many advantages, failover clusters also come with some downsides.

  • Cost. The costs of implementing failover clusters with proprietary hardware and software can be substantial.
  • Management. Building sophisticated failover clusters requires highly skilled personnel.

Nevertheless, with the rise of cloud computing and virtualization, the costs associated with hardware, software, and management have significantly decreased.

In conclusion, failover clustering is an effective option for organizations seeking high availability and disaster recovery solutions. By understanding the various cluster types and their applications, companies can make informed decisions and harness the power of this technology to maintain business continuity and mitigate risks.

Hyper-V Delivers Failover Clustering. A Summary

Hyper-V has been making strides in creating an effective and reliable environment for various virtual servers. Managing a set of servers can be challenging without an automated failover system. Now, Hyper-V leverages the Failover Clustering feature in Windows Server 2019, Windows Server 2016, and Azure Stack HCI, offering a seamless workflow experience.

Cluster Sets and Azure-aware Clusters

Cluster sets, a feature exclusive to Windows Server 2019, lets you scale the number of servers in a single Software-Defined Datacenter (SDDC) solution. This is achieved by loosely grouping compute, storage, and hyper-converged clusters. The advantage? You can live-migrate virtual machines between clusters within the set.

Now, failover clusters can automatically detect when they’re running in Azure IaaS virtual machines and optimize the configuration to achieve high availability. Understanding the Azure environment, the clusters proactively failover and log Azure planned maintenance events.

Domain and USB Witness

Hyper-V, by virtue of Failover Clustering, grants clusters the flexibility to move dynamically across Active Directory domains. This aids in domain consolidation and enables clusters to be created by hardware partners and subsequently joined to the customer’s domain.

Moreover, the new feature allows the use of a USB drive connected to a network switch as a witness in quorum determination for a cluster. This expands the File Share Witness to support any SMB2-compliant device.

Cluster Infrastructure and Updating Enhancements

The Cluster Shared Volumes (CSV) cache is now enabled by default to boost virtual machine performance. Cluster Aware Updating (CAU) is more integrated and considerate of Storage Spaces Direct, ensuring data resynchronization on each node and inspecting updates for intelligent restarts.

Security Tightening

Intra-cluster communication for CSV and Storage Spaces Direct now employs certificates to provide the best security. Failover Clusters have ceased using NTLM authentication and instead rely on Kerberos and certificate-based authentication only.

Cluster Operating System Rolling Upgrades

This is a major breakthrough in Windows Server 2016. This functionality lets an administrator upgrade the operating system of the cluster nodes from Windows Server 2012 R2 to a newer version without halting the Hyper-V or the Scale-Out File Server workloads. The cluster functions at a Windows Server 2012 R2 level until all nodes are updated to Windows Server 2016.

What’s Changed?

The up-gradation of a Hyper-V failover cluster is now smooth with zero downtime. Previously, migrating clusters involved taking the entire existing cluster offline, reinstalling the new operating system for each node and then bringing the cluster back online – a cumbersome process with unavoidable downtime.

With Windows Server 2016, the concept of “mixed mode” is introduced, as nodes are running either Windows Server 2012 R2 or Windows Server 2016, and there’s no need to take the cluster offline at any point.

Events such as SQL Server Always On Availability Groups, storage-agnostic block-level synchronous replication between servers, and Cloud Witness, a new type of Failover Cluster quorum witness, are now possible. This not only enhances disaster recovery features but also extends failover clusters to metropolitan distances.

Therefore, Hyper-V delivering Failover Clustering contributes significantly to a seamless IT workflow, robust security, and dependable server management.

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