Sentinel 源码分析- 熔断降级原理分析
直接从Sentinel 源码demo ExceptionRatioCircuitBreakerDemo看起
直接看他的main函数
public static void main(String[] args) throws Exception {
initDegradeRule();
......
final int concurrency = 8;
for (int i = 0; i < concurrency; i++) {
Thread entryThread = new Thread(() -> {
while (true) {
Entry entry = null;
try {
entry = SphU.entry(KEY);
sleep(ThreadLocalRandom.current().nextInt(5, 10));
pass.addAndGet(1);
// Error probability is 45%
if (ThreadLocalRandom.current().nextInt(0, 100) > 55) {
// biz code raise an exception.
throw new RuntimeException("oops");
}
} catch (BlockException e) {
block.addAndGet(1);
sleep(ThreadLocalRandom.current().nextInt(5, 10));
} catch (Throwable t) {
bizException.incrementAndGet();
// It's required to record exception here manually.
Tracer.traceEntry(t, entry);
} finally {
total.addAndGet(1);
if (entry != null) {
entry.exit();
}
}
}
});
entryThread.setName("sentinel-simulate-traffic-task-" + i);
entryThread.start();
}
}
从源码中可以看出，比如限流那一套多了一个在catch Throwable的操作 Tracer.traceEntry(t, entry);
看一下其中的源码
public static void traceEntry(Throwable e, Entry entry) {
if (!shouldTrace(e)) {
return;
}
traceEntryInternal(e, entry);
}
private static void traceEntryInternal(/*@NeedToTrace*/ Throwable e, Entry entry) {
if (entry == null) {
return;
}
entry.setError(e);
}
// Entry.java
public void setError(Throwable error) {
this.error = error;
}
从源码可以看出 这里仅仅是将error赋给entry的成员error。这里是为了后面的finally逻辑做铺垫
异常被触发并执行完catch逻辑后，会执行finally的逻辑，会调用entry.exit();方法。
//CtEntry.java
protected void exitForContext(Context context, int count, Object... args) throws ErrorEntryFreeException {
if (context != null) {
// Null context should exit without clean-up.
if (context instanceof NullContext) {
return;
}
if (context.getCurEntry() != this) {
String curEntryNameInContext = context.getCurEntry() == null ? null
: context.getCurEntry().getResourceWrapper().getName();
// Clean previous call stack.
CtEntry e = (CtEntry) context.getCurEntry();
while (e != null) {
e.exit(count, args);
e = (CtEntry) e.parent;
}
String errorMessage = String.format("The order of entry exit can't be paired with the order of entry"
+ ", current entry in context: <%s>, but expected: <%s>", curEntryNameInContext,
resourceWrapper.getName());
throw new ErrorEntryFreeException(errorMessage);
} else {
// Go through the onExit hook of all slots.
if (chain != null) {
//最终会调用到chain的exit
chain.exit(context, resourceWrapper, count, args);
}
// Go through the existing terminate handlers (associated to this invocation).
callExitHandlersAndCleanUp(context);
// Restore the call stack.
context.setCurEntry(parent);
if (parent != null) {
((CtEntry) parent).child = null;
}
if (parent == null) {
// Default context (auto entered) will be exited automatically.
if (ContextUtil.isDefaultContext(context)) {
ContextUtil.exit();
}
}
// Clean the reference of context in current entry to avoid duplicate exit.
clearEntryContext();
}
}
}
最终会执行exitForContext,并调用到chain.exit(context, resourceWrapper, count, args);, 于是会调用到DegradeSlot的 exit
@Override
public void exit(Context context, ResourceWrapper r, int count, Object... args) {
Entry curEntry = context.getCurEntry();
if (curEntry.getBlockError() != null) {
fireExit(context, r, count, args);
return;
}
List<CircuitBreaker> circuitBreakers = DegradeRuleManager.getCircuitBreakers(r.getName());
if (circuitBreakers == null || circuitBreakers.isEmpty()) {
fireExit(context, r, count, args);
return;
}
if (curEntry.getBlockError() == null) {
// passed request
for (CircuitBreaker circuitBreaker : circuitBreakers) {
//这里是最关键的一步
circuitBreaker.onRequestComplete(context);
}
}
fireExit(context, r, count, args);
}
这里的 circleBreaker 是 ExceptionCircuitBreaker 对象，最终会调用
@Override
public void onRequestComplete(Context context) {
Entry entry = context.getCurEntry();
if (entry == null) {
return;
}
// 这里和上面的entry缓存error就关联上了，发现错误并计数
Throwable error = entry.getError();
SimpleErrorCounter counter = stat.currentWindow().value();
if (error != null) {
counter.getErrorCount().add(1);
}
counter.getTotalCount().add(1);
//核心函数，根据统计，继续状态切换 close -> open (--> open -> half-open)
handleStateChangeWhenThresholdExceeded(error);
}
private void handleStateChangeWhenThresholdExceeded(Throwable error) {
......
List<SimpleErrorCounter> counters = stat.values();
long errCount = 0;
long totalCount = 0;
for (SimpleErrorCounter counter : counters) {
errCount += counter.errorCount.sum();
totalCount += counter.totalCount.sum();
}
if (totalCount < minRequestAmount) {
return;
}
double curCount = errCount;
if (strategy == DEGRADE_GRADE_EXCEPTION_RATIO) {
// Use errorRatio
curCount = errCount * 1.0d / totalCount;
}
if (curCount > threshold) {
transformToOpen(curCount);
}
}
通过counter计算出单位统计时间的错误量，判断是否超过锚点，调用transformToOpen -> fromCloseToOpen
protected boolean fromCloseToOpen(double snapshotValue) {
State prev = State.CLOSED;
if (currentState.compareAndSet(prev, State.OPEN)) {
updateNextRetryTimestamp();
notifyObservers(prev, State.OPEN, snapshotValue);
return true;
}
return false;
}
保证线程安全，通过CAS (compareAndSet) 将状态改为OPEN。
这是本资源对应的熔断器状态变为OPEN，之后流量进来时
//DegradeSlot.java
void performChecking(Context context, ResourceWrapper r) throws BlockException {
List<CircuitBreaker> circuitBreakers = DegradeRuleManager.getCircuitBreakers(r.getName());
if (circuitBreakers == null || circuitBreakers.isEmpty()) {
return;
}
for (CircuitBreaker cb : circuitBreakers) {
//调用 breaker的tryPass方法
if (!cb.tryPass(context)) {
throw new DegradeException(cb.getRule().getLimitApp(), cb.getRule());
}
}
}
//AbstractCircuitBreaker.java
@Override
public boolean tryPass(Context context) {
// Template implementation.
if (currentState.get() == State.CLOSED) {
return true;
}
if (currentState.get() == State.OPEN) {
// For half-open state we allow a request for probing.
//状态为Open，会进来这里
return retryTimeoutArrived() && fromOpenToHalfOpen(context);
}
return false;
}
