Availability and Fault Tolerance in Distributed Systems

Availability is a measured outcome; fault tolerance is a design’s ability to preserve specified behavior under specified faults. This lesson connects those ideas through redundancy, failover and a three-zone checkout example. You will identify what must still work after a failure and calculate whether the surviving capacity is enough.

When a service like a payment gateway or a real-time messaging app goes down, the consequences can range from financial loss to a complete erosion of user trust.

Choose the failures the service should tolerate and the user-visible behavior allowed during recovery. Some requests may be delayed or fail while routing and replicas recover. A useful design bounds that disruption rather than promising that no user will ever notice a component failure.

This lesson examines the principles and patterns employed to achieve high availability and fault tolerance, ensuring that our services remain operational even when individual components fail.

Availability in distributed systems

Availability refers to the consistency with which a system or service remains operational and accessible to users when needed.