8) Cluster under pressure

Being aware of the computational limits of a VM or, in this case, a cluster of VMs is very useful to know how far we can go without breaking them. Furthermore, knowing the maximum workload supported by a device allows us to adopt one with characteristics suitable for our purposes: if a cluster with a certain configuration can manage our applications very well, even under sustained effort, it is useless to spend excessive resources.

Load, endurance and stress tests reveal how the system responds in various situations. To be more specific, these three types of analysis are defined as:

• Load test. How the system responds to a sudden increase in requests.
• Endurance test. How the system survives a constant, moderate load for longer duration of times. It can also be referred to as a soak test, referring to the long time the software spends under test.
• Stress test. How the system responds under a heavy load, with an intent to find out the point at which system is stressed and ceases functioning.

To put the VMs under pressure, this tutorial puts a lot of demand on a PHP application running in a Kubernetes cluster. The aim is for the cluster to scale horizontally, when incoming requests exceed normal usage patterns. The tests will be performed on a cluster consisting of 4 nodes (1 master and 3 worker) with the same flavor. The flavor will also be modified in turn, remaining the same between the VMs in the cluster, passing from training (2 CPUs and 4GB RAM) to large (4 CPUs and 8GB RAM) and finally xlarge (8 CPUs and 16GB RAM).