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Azure reliability documentation

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Reliability consists of two principles: resiliency and availability. The goal of resiliency is to avoid failures and, if they still occur, to return your application to a fully functioning state. The goal of availability is to provide consistent access to your application or workload. It's important to plan for proactive reliability based on your application requirements.

Azure includes built-in reliability services that you can use and manage based on your business needs. Whether it’s a single hardware node failure, a rack level failure, a datacenter outage, or a large-scale regional outage, Azure provides solutions that improve reliability. For example, availability sets ensure that the virtual machines deployed on Azure are distributed across multiple isolated hardware nodes in a cluster. Availability zones protect customers’ applications and data from datacenter failures across multiple physical locations within a region. Regions and availability zones are central to your application design and resiliency strategy and are discussed in greater detail later in this article.

The Azure reliability documentation offers reliability guidance for Azure services. This guidance includes information on availability zone support, disaster recovery guidance, and availability of services.

For detailed service-specific reliability guidance, including availability zones, disaster recovery, or high availability, see Azure service-specific reliability guidance overview.

For information on reliability and reliability principles and architecture in Microsoft Azure services, see Microsoft Azure Well-Architected Framework: Reliability.

Reliability requirements

The required level of reliability for any Azure solution depends on several considerations. Availability and latency SLA and other business requirements drive the architectural choices and resiliency level and should be considered first. Availability requirements range from how much downtime is acceptable – and how much it costs your business – to the amount of money and time that you can realistically invest in making an application highly available.

Building reliability systems on Azure is a shared responsibility. Microsoft is responsible for the reliability of the cloud platform, including its global network and data centers. Azure customers and partners are responsible for the resilience of their cloud applications, using architectural best practices based on the requirements of each workload. While Azure continually strives for highest possible resiliency in SLA for the cloud platform, you must define your own target SLAs for each workload in your solution. An SLA makes it possible to evaluate whether the architecture meets the business requirements. As you strive for higher percentages of SLA guaranteed uptime, the cost and complexity to achieve that level of availability grows. An uptime of 99.99 percent translates to about five minutes of total downtime per month. Is it worth the more complexity and cost to reach that percentage? The answer depends on the individual business requirements. While deciding final SLA commitments, understand Microsoft’s supported SLAs. Each Azure service has its own SLA.

Diagram showing the shared responsibility model for Azure business continuity.

In the traditional on-premises model, the entire responsibility of managing, from the hardware for compute, storage and networking to the application, falls on you. You must plan for various types of failures and how to deal with them by creating a disaster recovery strategy. With IaaS, the cloud service provider is responsible for the core infrastructure resiliency, including storage, networking, and compute. As you move from IaaS to PaaS and then to SaaS, you’ll find that you’re responsible for less and the cloud service provider is responsible for more.  

For more information on Reliability principles, see Well-architected Framework Reliability documentation.  

Building reliability

You should define your application’s reliability requirements at the beginning of planning. If you know which applications don't need 100% high availability during certain periods of time, you can optimize costs during those non-critical periods. Identify the type of failures an application can experience, and the potential effect of each failure. A recovery plan should cover all critical services by finalizing recovery strategy at the individual component and the overall application level. Design your recovery strategy to protect against zonal, regional, and application-level failure. Perform testing of the end-to-end application environment to measure application reliability and recovery against unexpected failure.

The following checklist covers the scope of reliability planning.

Reliability planning
Define availability and recovery targets to meet business requirements.
Design the reliability features of your applications based on the availability requirements.
Align applications and data platforms to meet your reliability requirements.
Configure connection paths to promote availability.
Use availability zones and disaster recovery planning where applicable to improve reliability and optimize costs.
Ensure your application architecture is resilient to failures.
Know what happens if SLA requirements are not met.
Identify possible failure points in the system; application design should tolerate dependency failures by deploying circuit breaking.
Build applications that operate in the absence of their dependencies.

RTO and RPO  

Two important metrics to consider are the recovery time objective and recovery point objective, as they pertain to disaster recovery.  For more information on functional and non-functional design requirements, see Well-architected Framework functional and nonfunctional requirements.

  • Recovery time objective (RTO) is the maximum acceptable time that an application can be unavailable after an incident.

  • Recovery point objective (RPO) is the maximum duration of data loss that is acceptable during a disaster.

RTO and RPO are non-functional requirements of a system and should be dictated by business requirements. To derive these values, it's a good idea to conduct a risk assessment, and clearly understanding the cost of downtime or data loss.  

Regions and availability zones

Regions and availability zones are a big part of the reliability equation. Regions feature multiple, physically separate availability zones. These availability zones are connected by a high-performance network featuring less than 2ms latency between physical zones. Low latency helps your data stay synchronized and accessible when things go wrong. You can use this infrastructure strategically as you architect applications and data infrastructure that automatically replicate and deliver uninterrupted services between zones and across regions.

Microsoft Azure services support availability zones and are enabled to drive your cloud operations at optimum high availability while supporting your cross-region recovery and business continuity strategy needs.

For disaster recovery planning, regions that are paired with other regions offer cross-region replication and provide protection by asynchronously replicating data across other Azure regions. Regions without a pair follow data residency guidelines and offer high availability with availability zones and locally redundant or zone-redundant storage. Customers will need to plan for their cross-region disaster recovery based on their RTO/RPO needs.

Choose the best region for your needs based on technical and regulatory considerations—service capabilities, data residency, compliance requirements, latency—and begin advancing your reliability strategy. For more information, see Azure regions and availability zones.

Azure service dependencies

Microsoft Azure services are available globally to drive your cloud operations at an optimal level.

Azure services deployed to Azure regions are listed on the Azure global infrastructure products page. To better understand regions and Availability Zones in Azure, see Regions and Availability Zones in Azure.

Azure services are built for reliability including high availability and disaster recovery. There are no services that are dependent on a single logical data center (to avoid single points of failure). Non-regional services listed on Azure global infrastructure products are services for which there is no dependency on a specific Azure region. Non-regional services are deployed to two or more regions and if there is a regional failure, the instance of the service in another region continues servicing customers. Certain non-regional services enable customers to specify the region where the underlying virtual machine (VM) on which service runs will be deployed. For example, Azure Virtual Desktop enables customers to specify the region location where the VM resides. All Azure services that store customer data allow the customer to specify the specific regions in which their data will be stored. The exception is Microsoft Entra ID, which has geo placement (such as Europe or North America). For more information about data storage residency, see the Data residency map.

If you need to understand dependencies between Azure services to help better architect your applications and services, you can request the Azure service dependency documentation by contacting your Microsoft sales or customer representative. This document lists the dependencies for Azure services, including dependencies on any common major internal services such as control plane services. To obtain this documentation, you must be a Microsoft customer and have the appropriate non-disclosure agreement (NDA) with Microsoft.

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