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Nonfunctional requirements often follow from business requirements. They include the following:

 Availability

 Reliability

 Scalability

 Durability

 Observability

Availability is a measure of the time that services are functioning correctly and accessible to users. Availability requirements are typically stated in terms of percent of time a service should be up and running, such as 99.99 percent. Fully supported Google Cloud services have SLAs for availability so that you can use them to help guide your architectural decisions. Note, alpha and beta products typically do not have SLAs.

Reliability is a closely related concept to availability. Reliability is a measure of the probability that a service will continue to function under some load for a period of time. The level of reliability that a service can achieve is highly dependent on the availability of infrastructure upon which it depends.

Scalability is the ability of a service to adapt its infrastructure to the load on the system. When load decreases, some resources may be shut down. When load increases, resources can be added. Autoscalers and managed instance groups are often used to ensure scalability when using Compute Engine. One of the advantages of services like Cloud Storage and App Engine is that scalability is managed by GCP, which reduces the operational overhead on DevOps teams.

Durability is used to measure the likelihood that a stored object will be retrievable in the future. Cloud Storage has 99.999999999 percent (eleven 9s) durability guarantees, which means it is extremely unlikely that you will lose an object stored in Cloud Storage. Because of the math, as the number of objects increases, the likelihood that one of them is lost will increase.

Observability is the ability to determine the internal state of a system by examining outputs of the system. Metrics and logs improve observability by providing information about the state of a system over time.

The Google Cloud Professional Cloud Architect exam tests your ability to understand both business requirements and technical requirements, which is reasonable since those skills are required to function as a cloud architect. Security is another common type of nonfunctional requirement, but that domain is large enough and complex enough to call for an entire chapter. See Chapter 7, “Designing for Security and Legal Compliance.”