Circuit breaker: what it does in a system design

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Circuit breaker

Watches how the one component behind it answers. When too many calls fail or run slow it opens and turns requests away at once, then lets a few probes through after a cooldown to see if it has recovered.

Settings

Trip at failure rate (%): The share of recent calls that must fail to trip the breaker, in percent. Once tripped, calls are refused instantly instead of hitting a struggling component. Lower trips sooner.
Minimum calls before tripping: How many calls must be seen in the window before the breaker may trip. Prevents a couple of early failures from tripping it. Higher is calmer but slower to react.
Window (ms): How far back the breaker looks when counting failures. A short window reacts fast but jumps around; a long one is steadier but slow to notice trouble.
Cooldown while open (ms): How long the breaker stays open, refusing everything, before it lets a few test calls through. Longer gives the failing component more time to recover but fails more requests.
Probe requests when half-open: How many test calls go through after the cooldown. If all succeed the breaker closes; if one fails it opens again for another cooldown.
Count calls slower than (ms) as failures: Calls slower than this count as failures, so a slow but working dependency can also trip the breaker. Leave blank to count only real errors.