High availability

High availability forms one of the core non-functional technical requirements for any business-critical service and its deployment. High availability refers to the feature of a service or application that keeps it operational on a continuous basis; it does so by meeting or surpassing its promised service level agreement (SLA). Users are promised a certain SLA based on the service type. The service should be available for consumption based on its SLA. For example, an SLA can define 99% availability for an application for the entire year. This means that it should be available for consumption by users for 361.35 days. If it fails to remain available for this period, that constitutes a breach of the SLA. Most mission-critical applications define their high-availability SLA as 99.999% for a year. This means the application should be up, running, and available throughout the year, but it can only be down and unavailable for 5.2 hours. If the downtime goes beyond that, you are eligible for credit, which will be calculated based on the total uptime percentage.

It is important to note here that high availability is defined in terms of time (yearly, monthly, weekly, or a combination of these).

A service or application is made up of multiple components and these components are deployed on separate tiers and layers. Moreover, a service or application is deployed on an operating system (OS) and hosted on a physical machine or VM. It consumes network and storage services for various purposes. It might even be dependent on external systems. For these services or applications to be highly available, it is important that networks, storage, OSes, VMs or physical machines, and each component of the application is designed with the SLA and high availability in mind. A definite application life cycle process is used to ensure that high availability should be baked in from the start of application planning until its introduction to operations. This also involves introducing redundancy. Redundant resources should be included in the overall application and deployment architecture to ensure that if one resource goes down, another takes over and serves the requests of the customer.

Some of the major factors affecting the high availability of an application are as follows:

We will be looking into each of these factors in the following sections. Let's take a closer look at how high availability is ensured for deployments in Azure.