Linux: The Simplest Load Balancing Cluster with Failover
Evidian SafeKit
The solution for Linux
Evidian SafeKit brings load balancing and failover to Linux.
This article explains how to implement quickly a Linux cluster without network load balancers, dedicated proxy servers or special MAC addresses. SafeKit is installed directly on the Linux servers.
A generic product
Note that SafeKit is a generic product on Windows and Linux.
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 all solutions).
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.
Virtual IP address in a farm cluster
On the previous figure, the Linux 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 Linux 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 Linux 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 Linux 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.
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.
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.
| 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 |
|
3 products in 1
More info >
 |
|
Very simple configuration
More info >
 |
|
Synchronous replication
More info >
 |
|
Fully automated failback
More info >
 |
|
Replication of any type of data
More info >
 |
|
File replication vs disk replication
More info >
 |
|
File replication vs shared disk
More info >
 |
|
Remote sites and virtual IP address
More info >
 |
|
Quorum and split brain
More info >
 |
|
Active/active cluster
More info >
 |
|
Uniform high availability solution
More info >
 |
|
RTO / RPO
More info >
 |
|
Evidian SafeKit farm cluster with load balancing and failover |
|
No load balancer or dedicated proxy servers or special multicast Ethernet address
More info >
 |
|
|
All clustering features
More info >
|
|
|
Remote sites and virtual IP address
More info >
|
|
|
Uniform high availability solution
More info >
|
|
Software clustering vs hardware clustering More info > |
|
|
|
Shared nothing vs a shared disk cluster More info > |
|
|
|
Application High Availability vs Full Virtual Machine High Availability More info > |
|
|
|
High availability vs fault tolerance More info > |
|
|
|
Synchronous replication vs asynchronous replication More info > |
|
|
|
Byte-level file replication vs block-level disk replication More info > |
|
|
|
Heartbeat, failover and quorum to avoid 2 master nodes More info > |
|
|
|
Virtual IP address primary/secondary, network load balancing, failover More info > |
|
|
|