A network load balancing cluster on Windows or Linux – the farm cluster
High availability and scalability of web services with a network load balancing cluster
The SafeKit farm cluster implements a network load balancing cluster among several servers. It provides a simple solution to critical application scalability and high availability.
In a network load balancing cluster, the same application runs on each server, and the load is balanced by the distribution of network activity on the different servers of the farm. This type of cluster is suited to front-end applications like web services.
The SafeKit software saves the cost of hardware load balancers. It does not require specific servers above the farm for implementing the network load balancing cluster.
The network load balancing is very efficiently implemented by a network filter driver. This driver works on Windows and Linux (even on Windows editions for PCs).
SafeKit provides a generic farm module on Windows and Linux to build a network load balancing cluster. You can write your own farm module for your application starting from the generic farm module. Apache and Microsoft IIS are examples of farm modules.
Combined with the farm cluster, you can also implement a mirror cluster with real-time file replication and failover.
Principle of a virtual IP address in a network load balancing cluster
The virtual IP address is configured locally on each server in the network load balancing cluster.
The input traffic for the virtual IP address is received by all the servers and split among them by a filter inside each server's kernel.
The network load balancing algorithm inside the 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.
Note that a comparison between Microsoft NLB and SafeKit network load balancing is available here. And note that SafeKit network load balancing is working not only on Windows (including Windows editions for PCs) but also on Linux.
Stateful or stateless web services in a network load balancing cluster
With a stateful server, there is session affinity. The same client must be connected to the same server on multiple HTTP/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. This configuration is used when there are session affinities.
With a stateless server, there is no session affinity. The same client can be connected to different servers in the farm on multiple HTTP/TCP sessions; because 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.
If you are also interested by real-time replication and failover in a mirror cluster, read this article.
FAQ on Evidian SafeKit
Best use cases [+]
Customers [+]
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Video Surveillance (CCTV) [+]
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Transport [+]
Paris transport company (RATP) chose the SafeKit high availability and load balancing solution for the centralized control room of line 1 of the Paris subway.
20 SafeKit clusters are deployed on Windows and Linux.
Stéphane Guilmin, RATP, Project manager says:
“Automation of line 1 of the Paris subway is a major project for RATP, requiring a centralized command room (CCR) designed to resist IT failures. With SafeKit, we have three distinct advantages to meet this need. Firstly, SafeKit is a purely software solution that does not demand the use of shared disks on a SAN and network boxes for load balancing. It is very simple to separate our servers into separate machine rooms. Moreover, this clustering solution is homogeneous for our Windows and Unix platforms. SafeKit provides the three functions that we needed: load balancing between servers, automatic failover after an incident and real time data replication.”
And also, Philippe Marsol, Atos BU Transport, Integration Manager says:
“SafeKit is a simple and powerful product for application high availability. We have integrated SafeKit in our critical projects like the supervision of Paris metro Line 4 (the control room) or Marseille Line 1 and Line 2 (the operations center). Thanks to the simplicity of the product, we gained time for the integration and validation of the solution and we had also quick answers to our questions with a responsive Evidian team.”
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Application high availability modules [+]
SafeKit Webinar [+]
Pricing - Free trial [+]
Comparison with other solutions [+]
Demonstration of a farm cluster [+]
What are the advantages of the farm cluster [+]
- Low Complexity
- Plug&Play deployment with no specific skills
- Suitable for large deployments in many sites (very simple to deploy)
- 2 nodes or more
- No network load balancers requirement
- No proxy server requirement (above the farm cluster)
- No Domain Controller requirement
- No restriction in VMware due to multicast or unicast address
- Same solution on Windows and Linux
- Well documented API and support
- Supports multiple monitoring channels on multiple networks for server failure detection
- Supports multiple vitual IP addresses
- Offers configurable software, hardware and network checkers
- Offers the mirror cluster with synchronous real-time replication and failover
- Automatic failover with a recovery time in the order of a few seconds
- Automatic failback when a server comes back after a failure (no manual operation)
- A very simple console to deploy the solution and to maintain it afterwards for end-customer
- Supports human errors (40% of causes of unavailability) thanks to its simplicity
- Supports software failures (40% of causes of unavailability): regression on software update (version N and N+1 can coexist), Operating System frozen, software bug
- Supports hardware and environment failures (20% of causes of unavailability), including the complete failure of a computer room with 2 nodes in two remote sites
What is the recovery time (RTO) in a farm cluster [+]
RTO is the time during which the application is unavailable in case of failure. RTO of the SafeKit farm solution is in the order of a few seconds on hardware failure.
For a hardware failure, RTO = failure detection timeout through monitoring channels (default a few seconds): after the timeout the load balancing filters are reconfigured.
For a software failure or an administrator restart, RTO = time to (cleanly) stop services + time to restart them.
