BayesianCircuitBreaker

index.js

class BayesianCircuitBreaker {
  constructor(windowSize, thresholdHealthy, thresholdStruggling, credibleLevel, gcInterval = 100) {
    this.windowSize = windowSize
    this.thresholdHealthy = thresholdHealthy
    this.thresholdStruggling = thresholdStruggling
    this.credibleLevel = credibleLevel
    this.successCount = 0
    this.failureCount = 0
    this.samples = []
    this.gcTimer = setInterval((self) => {
      const t = Date.now()
      // Remove events that are outside the window
      while (self.samples.length > 0) {
        const oldestEventTime = self.samples[0].timestamp
        if (t - oldestEventTime <= gcInterval) {
          break
        }

        // Remove the oldest event
        self.shift()
      }
    }, gcInterval, this)
  }

  // Method to record the result of a request (success: true, failure: false)
  push(success) {
    if (success) {
      this.successCount++
    } else {
      this.failureCount++
    }

    this.samples.push(success)
    if (this.samples.length > this.windowSize) {
       // Remove the oldest event
      this.shift()
    }
  }
  
  shift() {
    const removedEvent = this.samples.shift()
    if (removedEvent) {
      this.successCount--
    } else {
      this.failureCount--
    }
    return removedEvent
  }

  // Method to estimate the Highest Posterior Density (HPD) interval of the posteriors (Beta distribution)
  estimateHPDInterval(alpha, beta) {
    alpha += 5
    beta += 1

    // Calculate the quantiles for the Beta distribution using percentiles approach
    const lowerQuantile = jStat.beta.inv(this.credibleLevel / 2, alpha, beta)
    const upperQuantile = jStat.beta.inv(1 - this.credibleLevel / 2, alpha, beta)

    return [lowerQuantile, upperQuantile]
  }

  // Method to get the current circuit status ('open', 'closed', or 'half-open')
  getStatus() {
    const [lowerBoundHealthy, upperBoundHealthy] = this.estimateHPDInterval(this.successCount, this.failureCount)
    const [lowerBoundStruggling, upperBoundStruggling] = this.estimateHPDInterval(this.failureCount, this.successCount)

    if (upperBoundHealthy < this.thresholdHealthy && upperBoundStruggling < this.thresholdStruggling) {
      return 'half-open' // Both posteriors are below the HPD interval, HALF-OPEN state
    } else if (lowerBoundHealthy >= this.thresholdHealthy) {
      return 'closed' // Server is healthy, CLOSE the circuit
    } else if (upperBoundStruggling >= this.thresholdStruggling) {
      return 'open' // Server is struggling, OPEN the circuit
    } else {
      return 'half-open' // Otherwise, HALF-OPEN the circuit to explore more data
    }
  }
}

// Example usage:
const cb = new BayesianCircuitBreaker(100, 0.85, 0.4, 0.95)
const range = (n) => Array.from(Array(n), (_, k) => k + 1)
console.log('Circuit Status 0:', cb.getStatus())

// Simulate some requests and record results (~1% error rate)
range(10).forEach((n) => {
  const event =  Math.random() >= 0.01
  cb.push(event)
  // Get the current circuit status
  console.log(`Circuit Status ${n} (${event}): ${cb.getStatus()}`)
})

range(10).forEach((n) => {
  const event = true
  cb.push(event)
  // Get the current circuit status
  console.log(`Circuit Status ${10 + n} (${event}): ${cb.getStatus()}`)
})

range(10).forEach((n) => {
  const event = false
  cb.push(event)
  // Get the current circuit status
  console.log(`Circuit Status ${20 + n} (${event}): ${cb.getStatus()}`)
})

setTimeout(() => {
  // Get the current circuit status
  console.log(`Circuit Status (1 sec later):`, cb.getStatus())
}, 1000)