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High Availability Cluster What is a High Availability Cluster? A high availability cluster is a group of hosts that act like a single system and provide continuous uptime. High availability clusters are often used for load balancing, backup and failover purposes. To properly configure a high-availability (HA) cluster, the hosts in the cluster must all have access to the same shared storage. This allows virtual machines (VMs) on a given host to fail over to another host without any downtime in the event of a failure. HA clusters can range from two nodes to dozens of nodes, but storage administrators must be wary of the number of VMs and hosts they add to an HA cluster because too many can complicate load balancing. High Availability Clustering High-availability server clusters (aka HA Clusters) is defined as a group of servers which support applications or services that can be utilized reliably with a minimal amount of downtime. High availability software capitalizes on the redundant software installed on multiple systems by grouping or clustering together a group of servers focusing on a common goal in case components fail. Without this form of clustering, if the application or website crashes, the service will not be available until the servers are repaired. HA clustering addresses these situations by detecting the faults and quickly restarting or replacing the server or service or server with a new process that does not require human intervention. This is defined as a failover model. Uses of high availability clusters High Availability clusters are often used for mission-critical databases, data sharing, applications, and e-commerce websites spread over a network. High Availability implementations build redundancy within a cluster to remove any one single point of failure, including across multiple network connections and data storage, which can be connected redundantly via geographically diverse storage area networks. The heartbeat technique High Availability clustered servers usually use a replication methodology called Heartbeat that is used to monitor each nodes status and health within the cluster over a private network connection. One critical circumstance all clustering software must be able to address is called split-brain, which occurs when all private internal links go down simultaneously, but the nodes in the cluster continue to run. If this occurs, every node within the cluster may incorrectly determine that all the other nodes have gone down and attempt to start services that other nodes may still be running. This condition of duplicate instances running similar services, which could cause data corruption on the system. High Availability Cluster Concepts Active/Passive Cluster The idea behind service failover is that the sudden loss of any one node in a service cluster would quickly be made up by another node taking its place. For this to work, the IP address is automatically moved to the standby node in the event of a failover. Alternatively, network routing tools like load balancers can be used to redirect traffic away from failed nodes. The precise way failover happens depends on the way you have configured your nodes. Only one node will initially be configured to serve clients, and will continue to do so alone until it somehow fails. The responsibility for existing and new clients will then shift (i.e., failover) to the passiveor backupnode that until now has been kept passively in reserve. Applying the model to multiple servers or server room components (like power supplies), n+1 redundancy provides just enough resources for the current demand plus one more unit to cover for a failure. Active/Active Cluster A cluster using an active/active design will have two or more identically configured nodes independently serving clients. Should one node fail, its clients will automatically connect with the second node and, as far as resources permit, receive full resource access. Once the first node recovers or is replaced, clients will once again be split between both server nodes. The primary advantage of running active/active clusters lies in the ability to efficiently balance a workload between nodes and even networks. The load balancerwhich directs all requests from clients to available serversis configured to monitor node and network activity and use some predetermined algorithm to route traffic to those nodes best able to handle it. Routing policies might follow a round-robin pattern, where client requests are simply alternated between available nodes, or by a preset weight where one node is favored over another by some ratio. Having a passive node acting as a stand-by replacement for its partner in an active/passive cluster configuration provides significant built-in redundancy. If your operation absolutely requires uninterrupted service and seamless failover transitions, then some variation of an active/passive architecture should be your goal. Shared-Nothing vs. Shared-Disk Clusters One of the guiding principles of distributed computing is to avoid having your operation rely on any single point of failure. That is, every resource should be either actively replicated (redundant) or independently replaceable (failover), and there should be no single element whose failure could bring down your whole service. Now, imagine that youre running a few dozen nodes that all rely on a single database server for their function. Even though the failure of any number of the nodes will not affect the continued health of those nodes that remain, should the database go down, the entire cluster would become useless. Nodes in a shared-nothing cluster, however, will (usually) maintain their own databases so thatassuming theyre being properly synced and configured for ongoing transaction safetyno external failure will impact them. This will have a more significant impact on a load balanced cluster, as each load balanced node has a constant and critical need for simultaneous access to the data. The passive node on a simple failover system, however, might be able to survive some time without access. While such a setup might slow down the way the cluster responds to some requestspartly because fears of split-brain failures might require periodic fencing through stoniththe trade off can be justified for mission critical deployments where reliability is the primary consideration. High Availability Cluster Architecture The first part of a highly available system is the clearly designed utilization of clustered application servers that are engineered in advance to distribute load amongst the whole cluster, which includes the ability to failover to a secondary and possibly a tertiary system. The second part includes the need for database scalability. This entails the requirement of scaling, either horizontally or vertically, using multiple master replication, and a load balancer to improve the stability and uptime of the database. Source: liquidweb.com The third part is geographic diversity. This ensures that should a natural disaster strike a single locational, that failure will not hinder the ability to provide the service. The fourth and possibly most important component is to provide a backup replication and disaster recovery methodology. The ability to ensure a working backup guarantees that our data is safe. A strict backup strategy (3-2-3) states that you should have three copies of your data, on two different media types, in three geographically diverse offsite locations for disaster recovery.
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