Circuit Breaker Pattern

A SCION example of the circuit breaker resilience pattern.

This example models the circuit breaker resilience pattern. The statechart keeps calls flowing while closed, fails fast while open, and uses a half-open probe to decide whether the dependency has recovered.

SCXML source

<scxml datamodel="ecmascript"
       initial="closed"
       name="CircuitBreaker"
       version="1.0"
       xmlns="http://www.w3.org/2005/07/scxml">
  <datamodel>
    <data id="failureCount" expr="0"/>
    <data id="failureThreshold" expr="3"/>
    <data id="lastResult" expr="'none'"/>
  </datamodel>

  <state id="closed">
    <onentry>
      <send event="trace">
        <content expr="'Circuit is closed: requests flow to the dependency.'"/>
      </send>
    </onentry>
    <transition event="call.success">
      <assign location="failureCount" expr="0"/>
      <assign location="lastResult" expr="'success'"/>
      <send event="trace">
        <content expr="'Call succeeded; failure count reset.'"/>
      </send>
    </transition>
    <transition event="call.failure" cond="failureCount + 1 &gt;= failureThreshold" target="open">
      <assign location="failureCount" expr="failureCount + 1"/>
      <assign location="lastResult" expr="'failure'"/>
      <send event="trace">
        <content expr="'Failure threshold reached; opening circuit.'"/>
      </send>
    </transition>
    <transition event="call.failure">
      <assign location="failureCount" expr="failureCount + 1"/>
      <assign location="lastResult" expr="'failure'"/>
      <send event="trace">
        <content expr="'Call failed; failure count increased.'"/>
      </send>
    </transition>
  </state>

  <state id="open">
    <onentry>
      <send event="trace">
        <content expr="'Circuit is open: calls fail fast.'"/>
      </send>
      <send event="cooldown.elapsed" delay="3000ms" id="cooldown"/>
    </onentry>
    <onexit>
      <cancel sendid="cooldown"/>
    </onexit>
    <transition event="cooldown.elapsed" target="halfOpen"/>
    <transition event="reset" target="closed">
      <assign location="failureCount" expr="0"/>
      <assign location="lastResult" expr="'manual reset'"/>
    </transition>
  </state>

  <state id="halfOpen">
    <onentry>
      <send event="trace">
        <content expr="'Circuit is half-open: allow one probe request.'"/>
      </send>
    </onentry>
    <transition event="call.success" target="closed">
      <assign location="failureCount" expr="0"/>
      <assign location="lastResult" expr="'probe success'"/>
      <send event="trace">
        <content expr="'Probe succeeded; closing circuit.'"/>
      </send>
    </transition>
    <transition event="call.failure" target="open">
      <assign location="failureCount" expr="failureThreshold"/>
      <assign location="lastResult" expr="'probe failure'"/>
      <send event="trace">
        <content expr="'Probe failed; reopening circuit.'"/>
      </send>
    </transition>
    <transition event="reset" target="closed">
      <assign location="failureCount" expr="0"/>
      <assign location="lastResult" expr="'manual reset'"/>
    </transition>
  </state>
</scxml>