Understanding Cluster of Servers and Capabilities
Server clustering refers to a group of servers working together on one system to provide users with higher availability. These clusters are used to reduce downtime and outages by allowing another server to take over in the event of an outage. Here’s how it works. A group of servers are connected to a single system. The moment one of these servers experiences a service outage, the workload is redistributed to another server before any downtime is experienced by the client. Clustered servers are generally used for applications with frequently updated data with file, print, database and messaging servers ranking as the most commonly used clusters. Overall, clustering servers offer clients a higher level of availability, reliability, and scalability than any one server could possibly offer.
In a clustered server environment, each server is responsible for the ownership and management of each of its own devices and has a copy of the operating system (along with any applications or services) being used to run the other servers in the cluster. The servers in the cluster are programmed to work together to increase the protection of data and maintain the consistency of the cluster configuration over time.
Cluster Protection Against Failures and Outages
The primary rationale for server clusters is protection against outages and downtime. As mentioned above, clustered servers offer increased protection against an entire network going black during a power failure. Clustered servers protect against three primary types of outages. These include the following.
- Application / Service Failure: An outage that affects mission-critical applications and services on the network.
- System / Hardware Failure: Outages that affect components such as CPUs, memory, adapters, drives and power supplies.
- Site Failure: Site failures that affect multiple locations are generally caused by natural disasters that result in widespread power outages.
The Three Types of Clustering Servers
There are three types of server clusters that are classified based on the way in which the cluster system (referred to as a node) is connected to the device responsible for storing configuration data. The three types include a single (or standard) quorum cluster, a majority node set cluster and a single node cluster and are reviewed in more detail below.
- Single (or Standard) Quorum Cluster: The most commonly used, this cluster is comprised of multiple nodes with one or more cluster disk arrays that utilize a single connection device (called a bus). One server manages and owns each of the individual cluster disk arrays within the cluster.
- Majority Node Set Cluster: Similar to the above cluster, this model differs in that each of the nodes owns its own copy of the cluster’s configuration data, and this data is consistent across all nodes. This model works best for clusters with individual servers that are located in different geographic locations.
- Single Node Cluster: Most often used for testing purposes, this model contains a single node.