High Availability Architectures and Best Practices

What are the high availability architectures and the best practices?

Overview

This article explores the different high availability architectures and the best practices by given the pros and cons of each architecture.

The following comparative tables explain in detail the SafeKit high availability architecture and its best practices (SafeKit is a software high availability product).

What are the high availability architectures?

There are two types of high availability architectures: those for backend applications such as databases and those for frontend applications such as web services.

High availability architectures for backend are based on 2 servers sharing or replicating data with an automatic application failover in the event of hardware of software failures.

High availability architectures for frontend are based on a farm of servers (2 servers or more). The load balancing is made by hardware or software and distributes the TCP sessions to the available servers in the farm.

Moreover, you have to choose between high availability at the application level or at the virtual machine level.

What are the best practices?

This article explores the best practices in high availability architectures by comparing:

• software vs hardware clustering,
• shared nothing vs shared disk architecture,
• application vs virtual machine high availability,
• high availability vs fault tolerance,
• synchronous vs asynchronous replication,
• file vs disk replication,
• data replication techniques,
• RPO and RTO with examples,
• split brain ans quorum,
• virtual IP addresses.

Comparative tables on high availability architectures and best practices

Software clustering vs hardware clustering More info >
A simple software cluster with the SafeKit package just installed on two servers	Complex hardware clustering with external storage or network load balancers
Shared nothing vs a shared disk cluster More info >
SafeKit is a shared-nothing cluster: easy to deploy even in remote sites	A shared disk cluster is complex to deploy
Application High Availability vs Full Virtual Machine High Availability More info >
Application HA supports hardware failure and software failure with a quick recovery time (RTO around 1 mn or less). Application HA requires to define restart scripts per application and folders to replicate (SafeKit application modules).	Full virtual machines HA supports only hardware failure with a VM reboot and a recovery time depending on the OS reboot. No restart scripts to define with full virtual machines HA (SafeKit hyperv.safe or kvm.safe modules). Hypervisors are active/active with just multiple virtual machines.
High availability vs fault tolerance More info >
No dedicated server with SafeKit. Each server can be the failover server of the other one. Software failure with restart in another OS environment. Smooth upgrade of application and OS possible server by server (version N and N+1 can coexist)	Secondary server dedicated to the execution of the same application synchronized at the instruction level. Software exception on both servers at the same time. Smooth upgrade not possible
Synchronous replication vs asynchronous replication More info >
SafeKit implements real-time synchronous replication with no data loss in case of failure	With asynchronous replication, there is data loss on failure
Byte-level file replication vs block-level disk replication More info >
SafeKit implements real-time byte-level file replication and is simply configured with application directories to replicate even in the system disk	Block-level disk replication is complex to configure and requires to put application data in a special disk
Heartbeat, failover and quorum to avoid 2 master nodes More info >
To avoid 2 masters, SafeKit proposes a simple split brain checker configured on a router	To avoid 2 masters, other clusters require a complex configuration with a third machine, a special quorum disk, a special interconnect
Virtual IP address primary/secondary, network load balancing, failover More info >
No dedicated proxy servers and no special network configuration are required in a SafeKit cluster for virtual IP addresses	Special network configuration is required in other clusters for virtual IP addresses. Note that SafeKit offers a health check adapted to load balancers