Know the Difference : Active-Active vs Active-Passive High Availability Cluster
Know the Difference:
Active-Active High Availability Cluster vs Active-Passive High Availability Cluster
Having cluster designs for servers is an expensive solution, especially for small medium enterprises. But the benefits of clustering surpasses its complexity and technical challenges it presents. Among all the challenges faced from incompatible server and applications or complicated cluster designs, the one thing that all server configurations must decide is whether to use Active-Active High Availability Cluster or Active-Passive High Availability Cluster.
What are the difference between the two? Let’s find out.
Active-Active high availability cluster
Figure 1 : Active-Active high availability cluster
An active-active cluster is made up of at least two nodes, both running the same kind of service simultaneously. The reason to have an active-active cluster is to achieve load balancing. Load balancing distributes workloads across all nodes in order to prevent each node from getting crowded with client’s requests. Because there are more than one node serving, response times and performance are optimal for all nodes.
The setup above, which consists of a load balancer and two (2) HTTP servers (i.e. 2 nodes), is an example of this type of High Availability (HA) cluster configuration. Instead of connecting directly to an HTTP server, web clients go through the load balancer, which in turn connects each client to any amount of the HTTP servers behind it.
Assigning of clients to the nodes in the cluster isn’t an arbitrary process. Rather, it’s based on what ever load balancing algorithm is set on the load balancer. So, for example in a “Round Robin” algorithm, the first client to connect is sent to the Server 1, while the second client to the Server 2, the 3rd client back to the Server 1, the 4th client goes to Server 2 and it continues on endlessly till for all client’s requests.
In order for the high availability cluster to operate seamlessly, it’s recommended that the two nodes be configured for redundancy, which means that the server’s individual configurations or settings must be virtually identical.
Active-Passive high availability cluster
Figure 2 : Active-Passive high availability cluster
Much like the active-active configuration, active-passive also have at least two nodes. However, not all nodes are going to be active. In the case of two nodes, for example, if the first node is already active, the second node is on passive or on standby.
The objective of the passive (a.k.a. failover) server serves as a backup that’s ready to take over as soon as the active (a.k.a. primary) server should it gets disconnected or is unable to serve.
When clients connect to a 2-node cluster in active-passive configuration as seen in the setup in Figure 2, do take note that all clients will connect to either HTTP Server 1 or HTTP Server 2. This means that clients will only connect to one server at any given time, never both at the same time. Just the same as the active-active configuration, it is vital that the two servers have exactly the same settings (i.e., redundant).
If changes are made on the settings of the primary server, those changes must be cascaded to the failover server. So when the failover does take over, the clients won’t be able to tell the difference.
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