How to implement redundant servers with a simple software (Windows/Linux)?
Evidian SafeKit
How to implement active/passive redundant servers with real-time replication and failover?
The SafeKit mirror cluster
In a mirror cluster, the SafeKit software is installed on two physical or virtual servers running Windows or Linux (on premise or in the Cloud). The primary server is the active server and runs a critical application. The secondary is a passive redundant server receiving in real-time all modifications made by the application inside its files. Clients are connected to a virtual IP address. If the primary server is down, SafeKit restarts automatically the critical application on the passive redundant server which becomes primary and switches the virtual IP address. When the failed server is restarted, it is automatically resynchronized and becomes the passive redundant server running as secondary.
How to implement active/active redundant servers with network load balancing and failover?
The SafeKit farm cluster
In a farm cluster, the SafeKit software is installed on redundant servers running Windows or Linux (on premise or in the Cloud). All redundant servers are active and run a same critical front-end application. Clients are connected to a virtual IP address. TCP sessions are load balanced between all redundant servers . If one server is down, SafeKit automatically reconfigures load balancing of TCP sessions between the remaining active servers. When the failed server is restarted, it is automatically reintegrated as an active redundant server and receives new TCP sessions.
All-in-one high availability
In one software product, SafeKit provides on Windows and Linux:
- load balancing
- synchronous real-time file replication
- automatic application failover
- automatic failback after a server failure
Save costs of network load balancers or dedicated proxy servers, shared disk or replicated SAN storage, enterprise editions of Operating Systems and databases, specific skills to maintain operational a cluster.
A complete solution
SafeKit solves:
- hardware failures (20% of problems), including the complete failure of a computer room,
- software failures (40% of problems), including restart of critical processes,
- and human errors (40% of problems) thanks to its ease of use and its web console.
A generic product
You can implement with the SafeKit product real-time replication and failover of any file directory and service, database, complete Hyper-V or KVM virtual machines, Docker, Podman, K3S, Cloud applications (see the module list).
Zero specific skills
No special computer skills are required to deploy a SafeKit high availability cluster.
Zero hardware extra costs
Forget about shared disks, replicated SAN storage... SafeKit is hardware independent and runs on your existing physical servers, or in virtual machines, or in the cloud.
Zero software extra costs
SafeKit works with the standard editions of Windows and Linux and does not require enterprise editions of databases.
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...
Step 1. Real-time replication
Server 1 (PRIM) runs the application. Clients are connected to a virtual IP address. SafeKit replicates in real time modifications made inside files through the network.
The replication is synchronous with no data loss on failure contrary to asynchronous replication.
You just have to configure the names of directories to replicate in SafeKit. There are no pre-requisites on disk organization. Directories may be located in the system disk.
Step 2. Automatic failover
When Server 1 fails, Server 2 takes over. SafeKit switches the virtual IP address and restarts the application automatically on Server 2.
The application finds the files replicated by SafeKit uptodate on Server 2. The application continues to run on Server 2 by locally modifying its files that are no longer replicated to Server 1.
The failover time is equal to the fault-detection time (30 seconds by default) plus the application start-up time.
Step 3. Automatic failback
Failback involves restarting Server 1 after fixing the problem that caused it to fail.
SafeKit automatically resynchronizes the files, updating only the files modified on Server 2 while Server 1 was halted.
Failback takes place without disturbing the application, which can continue running on Server 2.
Step 4. Back to normal
After reintegration, the files are once again in mirror mode, as in step 1. The system is back in high-availability mode, with the application running on Server 2 and SafeKit replicating file updates to Server 1.
If the administrator wishes the application to run on Server 1, he/she can execute a "swap" command either manually at an appropriate time, or automatically through configuration.
More information on power outage and network isolation in a cluster.
Redundancy at the application level
In this type of solution, only application data are replicated. And only the application is restared in case of failure.
With this solution, restart scripts must be written to restart the application.
We deliver application modules to implement redundancy at the application level. They are preconfigured for well known applications and databases. You can customize them with your own services, data to replicate, application checkers. And you can combine application modules to build advanced multi-level architectures.
This solution is platform agnostic and works with applications inside physical machines, virtual machines, in the Cloud. Any hypervisor is supported (VMware, Hyper-V...).
Redundancy at the virtual machine level
In this type of solution, the full Virtual Machine (VM) is replicated (Application + OS). And the full VM is restarted in case of failure.
The advantage is that there is no restart scripts to write per application and no virtual IP address to define. If you do not know how the application works, this is the best solution.
This solution works with Windows/Hyper-V and Linux/KVM but not with VMware. This is an active/active solution with several virtual machines replicated and restarted between two nodes.
- Solution for a new application (no restart script to write): Windows/Hyper-V, Linux/KVM
More comparison between VM HA vs Application HA
Why a replication of a few Tera-bytes?
Resynchronization time after a failure (step 3)
- 1 Gb/s network ≈ 3 Hours for 1 Tera-bytes.
- 10 Gb/s network ≈ 1 Hour for 1 Tera-bytes or less depending on disk write performances.
Alternative
- For a large volume of data, use external shared storage.
- More expensive, more complex.
Why a replication < 1,000,000 files?
- Resynchronization time performance after a failure (step 3).
- Time to check each file between both nodes.
Alternative
- Put the many files to replicate in a virtual hard disk / virtual machine.
- Only the files representing the virtual hard disk / virtual machine will be replicated and resynchronized in this case.
Why a failover ≤ 32 replicated VMs?
- Each VM runs in an independent mirror module.
- Maximum of 32 mirror modules running on the same cluster.
Alternative
- Use an external shared storage and another VM clustering solution.
- More expensive, more complex.
Why a LAN/VLAN network between remote sites?
- Automatic failover of the virtual IP address with 2 nodes in the same subnet.
- Good bandwidth for resynchronization (step 3) and good latency for synchronous replication (typically a round-trip of less than 2ms).
Alternative
- Use a load balancer for the virtual IP address if the 2 nodes are in 2 subnets (supported by SafeKit, especially in the cloud).
- Use backup solutions with asynchronous replication for high latency network.
Virtual IP address in a farm cluster
On the previous figure, the application is running on the 3 servers (3 is an example, it can be 2 or more). Users are connected to a virtual IP address.
The virtual IP address is configured locally on each server in the farm cluster.
The input traffic to the virtual IP address is received by all the servers and split among them by a network filter inside each server's kernel.
SafeKit detects hardware and software failures, reconfigures network filters in the event of a failure, and offers configurable application checkers and recovery scripts.
Load balancing in a network filter
The network load balancing algorithm inside the network filter is based on the identity of the client packets (client IP address, client TCP port). Depending on the identity of the client packet input, only one filter in a server accepts the packet; the other filters in other servers reject it.
Once a packet is accepted by the filter on a server, only the CPU and memory of this server are used by the application that responds to the request of the client. The output messages are sent directly from the application server to the client.
If a server fails, the SafeKit membership protocol reconfigures the filters in the network load balancing cluster to re-balance the traffic on the remaining available servers.
Stateful or stateless applications
With a stateful application, there is session affinity. The same client must be connected to the same server on multiple TCP sessions to retrieve its context on the server. In this case, the SafeKit load balancing rule is configured on the client IP address. Thus, the same client is always connected to the same server on multiple TCP sessions. And different clients are distributed across different servers in the farm.
With a stateless application, there is no session affinity. The same client can be connected to different servers in the farm on multiple TCP sessions. There is no context stored locally on a server from one session to another. In this case, the SafeKit load balancing rule is configured on the TCP client session identity. This configuration is the one which is the best for distributing sessions between servers, but it requires a TCP service without session affinity.
New application (real-time replication and failover)
New application (network load balancing and failover)
Database (real-time replication and failover)
- Microsoft SQL Server mirror
- PostgreSQL mirror
- MySQL mirror
- Oracle mirror
- MariaDB mirror
- Firebird mirror
Web (network load balancing and failover)
Full VM or container real-time replication and failover
Amazon AWS
Google GCP
Microsoft Azure
Other clouds
Physical security (real-time replication and failover)
Siemens (real-time replication and failover)
VM HA with the SafeKit Hyper-V or KVM module | Application HA with SafeKit application modules |
SafeKit inside 2 hypervisors: replication and failover of full VM | SafeKit inside 2 virtual or physical machines: replication and failover at application level |
Replicates more data (App+OS) | Replicates only application data |
Reboot of VM on hypervisor 2 if hypervisor 1 crashes Recovery time depending on the OS reboot VM checker and failover (Virtual Machine is unresponsive, has crashed, or stopped working) |
Quick recovery time with restart of App on OS2 if crash of server 1 Around 1 mn or less (see RTO/RPO here) Application checker and software failover |
Generic solution for any application / OS | Restart scripts to be written in application modules |
Works with Windows/Hyper-V and Linux/KVM but not with VMware | Platform agnostic, works with physical or virtual machines, cloud infrastructure and any hypervisor including VMware |
SafeKit with the Hyper-V module or the KVM module | Microsoft Hyper-V Cluster & VMware HA |
No shared disk - synchronous real-time replication instead with no data loss | Shared disk and specific extenal bay of disk |
Remote sites = no SAN for replication | Remote sites = replicated bays of disk across a SAN |
No specific IT skill to configure the system (with hyperv.safe and kvm.safe) | Specific IT skills to configure the system |
Note that the Hyper-V/SafeKit and KVM/SafeKit solutions are limited to replication and failover of 32 VMs. | Note that the Hyper-V built-in replication does not qualify as a high availability solution. This is because the replication is asynchronous, which can result in data loss during failures, and it lacks automatic failover and failback capabilities. |
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 More info > |
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All clustering features More info > |
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Remote sites and virtual IP address More info > |
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Uniform high availability solution More info > |
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Software clustering vs hardware clustering More info > |
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Shared nothing vs a shared disk cluster More info > |
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Application High Availability vs Full Virtual Machine High Availability More info > |
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High availability vs fault tolerance More info > |
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Synchronous replication vs asynchronous replication More info > |
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Byte-level file replication vs block-level disk replication More info > |
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Heartbeat, failover and quorum to avoid 2 master nodes More info > |
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Virtual IP address primary/secondary, network load balancing, failover More info > |
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To kickstart your journey with SafeKit, begin with the quick installation guides.
Packages (8.2)
- Windows (with Microsoft Visual C++ Redistributable)
- Windows (without Microsoft Visual C++ Redistributable)
- Linux
- Supported OS and last fixes
- Previous 7.5