Reliability
A system's ability to recover from failures and continue to function.
Reliability describes a cloud system’s capacity to recover from failures and continue operating as expected. In Azure’s context it encompasses two distinct properties: resiliency, the ability to detect, respond to, and recover from failures, and availability, the percentage of time a service remains accessible. Azure supports reliability through features such as Availability Zones, geo-redundant replication, and automatic failover in services like Azure SQL Database and Azure Storage. On the AZ-900 exam, reliability is one of the five pillars of the Azure Well-Architected Framework. A common confusion is equating reliability solely with uptime: availability is only one dimension, and a system that recovers quickly from an outage is still reliable. Watch for questions that distinguish reliability (recovery-focused design) from fault tolerance (designing so failures cause zero impact).
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