Availability is the proportion of time that the system is functional and working, usually measured as a percentage of uptime. It can be affected by system errors, infrastructure problems, malicious attacks and system load. Cloud applications typically provide users with a service level agreement (SLA), so applications must be designed to maximise availability.

Data management is the key element of cloud applications and influences most of their quality attributes. Data is typically hosted in different locations and across multiple servers for reasons such as performance, scalability or availability, and this can present a range of challenges. For example, data consistency must be maintained, and data will typically need to be synchronised across different locations.

Good design encompasses factors such as consistency and coherence in component design and deployment, maintainability to simplify administration and development, and reusability to allow components and subsystems to be used in other applications and scenarios. Decisions made during the design and implementation phase have a huge impact on the quality and total cost of owning cloud-hosted applications and services.

The distributed nature of cloud applications requires a messaging infrastructure that connects the components and services, ideally in a loosely coupled manner to maximise scalability. Asynchronous messaging is widely used and offers many benefits, but also brings challenges such as the ordering of messages, poison message handling, idempotency, and more.

Cloud applications run in in a remote datacentre in which you do not have full control of the infrastructure or, in some cases, the operating system. This can make management and monitoring more difficult than an on-premises deployment. Applications must expose runtime information that administrators and operators can use to manage and monitor the system, as well as supporting changing business requirements and customisation without requiring the application to be stopped or redeployed.

Performance is an indication of a system’s responsiveness in executing any action within a given timeframe, while scalability is a system’s ability either to handle increases in load with no impact on performance or to readily increase available resources. Cloud applications typically encounter variable workloads and peaks in activity. Predicting these, especially in a multi-tenant scenario, is almost impossible. Instead, applications should be able to scale out within limits to meet peaks in demand, and scale in when demand decreases. Scalability concerns not just compute instances, but other elements such as data storage, messaging infrastructure, and more.

Resiliency is a system’s ability to smoothly handle and recover from failures. The nature of cloud hosting, where applications are often multi-tenant, use shared platform services, compete for resources and bandwidth, communicate over the Internet and run on commodity hardware, means there is an increased likelihood that both transient and more permanent faults will arise. Detecting failures, and recovering quickly and efficiently, is necessary for maintaining resiliency.

Security is a system’s capacity to prevent malicious or accidental actions outside the designed usage and to prevent disclosure or loss of information. Cloud applications are exposed on the Internet outside trusted on-premises boundaries, are often open to the public, and may serve untrusted users. Applications must be designed and deployed to protect them from malicious attacks, restrict access to approved users and protect sensitive data.