Difference between High Availability and Fault Tolerance
Evidian SafeKit
What is the difference between high availability and fault tolerance?
Overview
This article explores the pros and cons of a high availability cluster versus a fault tolerant system by looking at hardware constraints, software failures, RTO, RPO...
The following comparative tables explain in detail the difference between a fault tolerant system and SafeKit, a software high availability cluster.
What is high availability?
A high availability cluster is based on two servers with restart of the critical application in the event of hardware or software failures. There are 2 types of clusters: hardware clusters and software clusters.
Hardware clusters are based on shared disks resulting in dependencies between servers and their connections to shared disk arrays.
Software clusters like Evidian SafeKit are based on real-time data replication and are hardware-agnostic: they can be deployed on physical or virtual servers or in the cloud.
What is fault tolerance?
A fault tolerant system relies on either specialized hardware or specialized hypervisor to detect a hardware failure and instantly switch to a redundant hardware component without application restart.
Fault-tolerant systems only deal with hardware failures and not software failures, by far the most common reason for system downtime.
Pros and cons of high availability and fault tolerance
Software high availability cluster
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Fault-tolerant system
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Product | |
SafeKit on Windows and Linux | Fault tolerant products |
Hardware / hypervisor | |
No dedicated server, no dedicated hypervisor.
Works with the standard and free hypervisor of Windows, Hyper-V, included in Windows kernel for servers and PCs. Works with the standard and free hypervisor KVM (Kernel-based Virtual Machine) integrated in mainline Linux kernel. Each server can be the failover server of the other one for multiple applications. |
Dedicated hardware or dedicated hypervisor.
The secondary server is dedicated to the execution of the same application synchronized at the instruction level. |
Software failure | |
Software failure supported with restart in another OS environment. | Software exception on both servers at the same time on the same OS. |
Smooth upgrage/fix of application and OS | |
Yes
Smooth upgrade/fix of application and OS possible server by server. N and N+1 versions can coexist. |
No
Same application and OS image on both servers. |
RTO/RPO | |
The recovery time with SafeKit (RTO) depends on the time to detect and to restart the application (about 1 minute).
The data loss with SafeKit (RPO) is zero as the replication is synchronous. |
The recovery time (RTO) of a fault tolerant system is zero.
The application is not restarted in case of failure and continue its execution on the secondary server. The data loss (RPO) is also zero. |
Flexibility | |
Can run on any type of server with standard Windows and Linux OS | Depends on specific hardware or on specific hypervisors |
Suited for | |
Software editors which want to add a simple high availability option to their application | Environment where hardware failures is the main concern |
Evidian SafeKit mirror cluster with real-time file replication and failover |
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3 products in 1 More info > |
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Very simple configuration More info > |
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Synchronous replication More info > |
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Fully automated failback More info > |
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Replication of any type of data More info > |
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File replication vs disk replication More info > |
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File replication vs shared disk More info > |
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Remote sites and virtual IP address More info > |
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Quorum and split brain More info > |
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Active/active cluster More info > |
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Uniform high availability solution More info > |
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RTO / RPO More info > |
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Evidian SafeKit farm cluster with load balancing and failover |
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No load balancer or dedicated proxy servers or special multicast Ethernet address |
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All clustering features |
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Remote sites and virtual IP address |
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Uniform high availability solution |
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Software clustering vs hardware clustering
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Shared nothing vs a shared disk cluster |
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Application High Availability vs Full Virtual Machine High Availability
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High availability vs fault tolerance
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Synchronous replication vs asynchronous replication
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Byte-level file replication vs block-level disk replication
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Heartbeat, failover and quorum to avoid 2 master nodes
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Virtual IP address primary/secondary, network load balancing, failover
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Partners, the success with SafeKit
This platform agnostic solution is ideal for a partner reselling a critical application and who wants to provide a redundancy and high availability option easy to deploy to many customers.
With many references in many countries won by partners, SafeKit has proven to be the easiest solution to implement for redundancy and high availability of building management, video management, access control, SCADA software...
Building Management Software (BMS)
Video Management Software (VMS)
Electronic Access Control Software (EACS)
SCADA Software (Industry)
Network load balancing and failover |
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Windows farm | Linux farm |
Generic Windows farm > | Generic Linux farm > |
Microsoft IIS > | - |
NGINX > | |
Apache > | |
Amazon AWS farm > | |
Microsoft Azure farm > | |
Google GCP farm > | |
Other cloud > |
Advanced clustering architectures
Several modules can be deployed on the same cluster. Thus, advanced clustering architectures can be implemented:
- the farm+mirror cluster built by deploying a farm module and a mirror module on the same cluster,
- the active/active cluster with replication built by deploying several mirror modules on 2 servers,
- the Hyper-V cluster or KVM cluster with real-time replication and failover of full virtual machines between 2 active hypervisors,
- the N-1 cluster built by deploying N mirror modules on N+1 servers.
User's Guide
Application Modules
Release Notes
Presales documentation
Introduction
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- Demonstration
- Examples of redundancy and high availability solution
- Evidian SafeKit sold in many different countries with Milestone
- 2 solutions: virtual machine or application cluster
- Distinctive advantages
- More information on the web site
- SafeKit training
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- Cluster of virtual machines
- Mirror cluster
- Farm cluster
Installation, Console, CLI
- Install and setup / pptx
- Package installation
- Nodes setup
- Upgrade
- Web console / pptx
- Cluster configuration
- Configuration tab
- Control tab
- Monitor tab
- Advanced Configuration tab
- Troubleshooting
- Command line / pptx
- Cluster administration
- Module administration
- Control commands
- Troubleshooting
Advanced configuration
- Mirror module / pptx
- Mirror's states in action
- start_prim / stop_prim scripts
- userconfig.xml
- Heartbeat (<hearbeat>)
- Virtual IP address (<vip>)
- Real-time file replication (<rfs>)
- How real-time file replication works?
- Troubleshooting
- Farm module / pptx
- Farm's states in action
- start_both / stop_both scripts
- userconfig.xml
- Farm heartbeats (<farm>)
- Virtual IP address (<vip>)
- Troubleshooting
Advanced configuration
- Checkers / pptx
- Checkers in action
- userconfig.xml
- errd checker
- intf and ip checkers
- custom checker
- splitbrain checker for a mirror module
- tcp, ping, module checkers
- Troubleshooting
Support
- Support tools / pptx
- How to analyze snapshots?
- Best practises
- Evidian support / pptx
- Get permanent license key
- Register on support.evidian.com
- Call desk