High availability is one of the major architectural concerns for any architect. It forms one of the core non-functional technical requirements for any serious service and its deployment. High availability refers to the feature of a service or application that keeps it operational on a continuous basis, meeting or surpassing its promised defined service level agreement (SLA). Users are promised a certain SLA based on service type. The service should be available for consumption based on its SLA. For example, an SLA can have 99% availability for an application for the entire year. This means it should be available for consumption by users for 361.35 days. If it becomes less than this, this constitutes a breach of the SLA. Most mission-critical applications define their high-availability SLA with five nines for a year. This means the application should be up, running...

Achieving high availability and meeting high SLA requirements is tough. Azure provides lots of features that enable high availability for applications, from the host and guest operating system to applications using its PaaS. Architects can use these features to get high availability in their applications using configuration instead of building these features from scratch or being dependent on third-party tools.

In this section, we will look at the features and capabilities provided by Azure to make applications highly available. Before we get into the architectural and configuration details, it is important to understand Azure's high-availability, related concepts.

The fundamental concepts...

Azure provides High availability to various means and at various levels. High availability could be at data center level, it could be at a region level, or across Azure. In this section, we will go through some of the architectures for high availability.

The architecture shown next shows high-availability deployment within a single Azure region. High availability is designed at the individual resource level. In this architecture, as shown in the following diagram, there are multiple VMs at each tier connected through either an application gateway or load balancer, and they are part of an availability set. Each tier is associated with...

Running applications and systems that are available to users for consumption is important for architects of any serious application. However, there is another equally important application feature that is one of the top priorities for architects, and this is the scalability of the application.

Imagine a situation in which an application is deployed and obtains great performance and availability with a few users, but both availability and performance degrades as the number of users start begins to increase. There are times when an application under normal load performs well, but degrades in performance with the increase in the number of users. This can happen if there is a sudden increase in the number of users and the environment is not built for such a large number of users.

To accommodate such spikes in the number of users, you might provision the hardware and bandwidth...

After VMSS and applications are deployed, they need to be actively maintained. Planned maintenance should be conducted periodically to ensure that both the environment and application is up to date with the latest features, and the environment is current from a security and resilience point of view.

Upgrades can be associated with applications, the guest VM instance, or the image itself. Upgrades can be quite complex because they should happen without affecting the availability, scalability, and performance of environments and applications. To ensure that updates can take place one instance at a time using rolling upgrade methods, it is important that VMSS supports and provides capabilities for these advanced scenarios.

There is a utility provided by the Azure team to manage updates for VMSS. It's a Python-based utility that can be downloaded from...

High availability and scalability are important and crucial architectural concerns. Almost every application and every architect tries to implement high availability. Azure is a mature platform that understands the need for these architectural concerns in applications and provides resources to implement them at multiple levels. These architectural concerns are not an afterthought, and should be part of the application life cycle development, starting from the planning phase itself.