Circuit breaker
- Kotlin course
- Advanced
- Lesson 71/83
- 5 min
Learn the circuit breaker pattern.
Circuit breaker
A circuit breaker temporarily stops a call that fails repeatedly. It has three states: closed (normal), open (calls rejected) and half-open (trial).
Example: class CircuitBreaker(val threshold: Int). When failures reach threshold, the circuit goes open; calls are rejected. This pattern prevents a failing dependency from dragging the whole system down. After a period, half-open tries again.
Threshold and the open state
When failures reach threshold, the circuit goes open. In the open state, calls are rejected immediately; no real work is done. This reduces load when the external service is failing.
Example: threshold = 2. After two failed calls, the circuit goes open. The third call does no work; it returns "open". This pattern prevents error accumulation and gives the system recovery time.
Practice Write the CircuitBreaker class
Define class CircuitBreaker(val threshold: Int) with var failures = 0 and var open = false. Write fun execute(action: () -> String): String: if open, return "open"; otherwise call action and return its result on success, or on exception increment failures, set open = true when reaching threshold and return "error". In main create CircuitBreaker(2) and print three execute { throw RuntimeException() } results.
Starter code
// CircuitBreaker class: threshold, failures, open
// execute: open returns open, action success returns result, exception returns error
// in main print three failing call resultsSample solution
class CircuitBreaker(val threshold: Int) {
var failures = 0
var open = false
fun execute(action: () -> String): String {
if (open) return "open"
return try {
action()
} catch (e: Exception) {
failures++
if (failures >= threshold) open = true
"error"
}
}
}
fun main() {
val cb = CircuitBreaker(2)
println(cb.execute { throw RuntimeException() })
println(cb.execute { throw RuntimeException() })
println(cb.execute { "ok" })
}Expected output
error error open
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the CircuitBreaker definition
____ CircuitBreaker(val threshold: Int)class CircuitBreaker(val threshold: Int)Explanation: class CircuitBreaker declares a class.
2 Match the circuit-breaker parts
| Term | Meaning |
|---|---|
threshold |
Failure threshold |
closed |
Normal state |
open |
Call rejection |
half-open |
Trial |
3 What happens when a circuit breaker is in the open state?
- All calls succeed
- Calls are rejected Correct answer
- System restarts
- Threshold resets
Explanation: In the open state, calls are rejected immediately.