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

Redundant servers with real-time file replication and failover

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

Redundant servers with network load balancing and failover

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.

SafeKit, a Simple High Availability Software Easy to Deploy

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 apply this generic product to your applications, databases, full Hyper-V or KVM virtual machines, Docker, Kubernetes, Cloud applications.

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.

SafeKit: an ideal solution for a partner application

This platform agnostic solution is ideal for a partner with a critical application and who wants to provide a redundancy and high availability option easy to deploy to many customers.

This clustering solution is also recognized as the simplest to implement by our partners.

How the SafeKit mirror cluster works?

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. 

File replication at byte level in a mirror cluster

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.

Failover in a mirror cluster

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 in a mirror cluster

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.

Return to normal operation in a mirror cluster

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.

Choose between redundancy at the application level or at the virtual machine level

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. Restart scripts must be written to restart the application. This solution is platform agnostic and works with applications inside physical machines, virtual machines, in the Cloud.

Application HA - redundancy at the application level

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.

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.

VM HA - redundancy at the virtual machine level

We deliver two modules for implementing redundancy at the virtual machine level: one for Hyper-V on Windows and one for KVM on Linux. Several VMs can be replicated and can run on both hypervisors with crossed replication and mutual takeover.

Typical usage with SafeKit

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 with a hardware clustering solution.
  • 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 < 25 replicated VMs?

  • Each VM runs in an independent mirror module.
  • Maximum of 25 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?

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.

How the SafeKit farm cluster works?

Virtual IP address in a farm cluster

How the Evidian SafeKit farm cluster implements network load balancing and failover

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.

SafeKit Modules for Plug&Play Redundancy and High Availability Solutions

SafeKit Modules for Plug&Play High Availability Solutions

Network load balancing and failover

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:

Demonstrations of Redundancy and High Availability Solutions

SafeKit Webinar

This webinar presents in 2 minutes Evidian SafeKit.

In this webinar, you will understand SafeKit mirror and farm clusters.

Microsoft SQL Server Cluster

This video shows a mirror module configuration with synchronous real-time replication and failover.

The file replication and the failover are configured for Microsoft SQL Server but it works in the same manner for other databases.

Free trial here

Apache Cluster

This video shows a farm module configuration with load balancing and failover.

The load balancing and the failover are configured for Apache but it works in the same manner for other web services.

Free trial here

Hyper-V Cluster

This video shows a Hyper-V cluster with full replications of virtual machines.

Virtual machines can run on both Hyper-V servers and they are restarted in case of failure.

Free trial here

SafeKit High Availability Differentiators against Competition

SafeKit Training

Introduction

  1. Overview / pptx

    • Features
    • Architectures
    • Distinctive advantages
  2. Competition / pptx

    • Hardware vs software cluster
    • Synchronous vs asynchronous replication
    • File vs disk replication
    • High availability vs fault tolerance
    • Hardware vs software load balancing
    • Virtual machine vs application HA

Installation, Console, CLI

  1. Install and setup / pptx

    • Package installation
    • Nodes setup
    • Cluster configuration
    • Upgrade
  2. Web console / pptx

    • Cluster configuration
    • Configuration tab
    • Control tab
    • Monitor tab
    • Advanced Configuration tab
  3. Command line / pptx

    • Silent installation
    • Cluster administration
    • Module administration
    • Command line interface

Advanced configuration

  1. Mirror module / pptx

    • userconfig.xml + restart scripts
    • Heartbeat (<hearbeat>)
    • Virtual IP address (<vip>)
    • Real-time file replication (<rfs>)
  2. Farm  module / pptx

    • userconfig.xml + restart scripts
    • Farm configuration (<farm>)
    • Virtual IP address (<vip>)
  3. Checkers / pptx

    • Failover machine (<failover>)
    • Process monitoring (<errd>)
    • Network and duplicate IP checkers
    • Custom checker (<custom>)
    • Split brain checker (<splitbrain>)
    • TCP, ping, module checkers

Support

  1. Support tools / pptx

    • Analyze snapshots
  2. Evidian support / pptx

    • Get permanent license key
    • Register on support.evidian.com
    • Call desk

Documentation

  1. Technical documentation

  2. Presales documentation