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Health check

A load balancer's regular probe of each target; failing ones are taken out of rotation.

Taught in Chapter 23: Availability.

More from the same chapter

• Availability: The share of time or requests a system serves correctly, usually counted in nines.
• CRDT: A data type built so concurrent changes always merge the same way, such as a counter that adds every region's increments.
• Failover: Promoting a replica to take over when the primary dies, then sending writes to it.
• Fencing: Cutting the old primary off before promoting a replica, so two nodes can't both accept writes.
• Last writer wins: Settling conflicting writes by keeping the one with the newest timestamp. The other is dropped without an error.
• Multi-leader replication: Each region accepts writes for the same data and copies them to the others, so two regions can take conflicting writes that must be settled.
• Redundancy: Running spares of each part, so one can fail while another serves.
• Single point of failure: Any part whose loss takes the whole system down.
• SLI, SLO and SLA: What you measure, the goal for it, and a promise to customers with a penalty attached.
• Split brain: Two nodes both acting as primary and accepting writes; prevented by fencing the old one.