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java code breaker

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Code Breaker 'In Review'

Bill Dudney's Weblog (27 reads)

I got the next chapter in Gala Factory pushed out to the app store, just waiting on apple review. In the mean time please check out the demo video on youtube, and then line up to buy a thousand copies. Thanks!

Circuit Breaker in Java

It's 2008, and if you are not writing some sort of automated tests (unit, functional, whatever), then IMHO you are committing an act of professional negligence.  Now I have a second one: if you write mission-critical software and you haven't read Michael Nygard's Release It, you are also missing the boat.

In talking about patterns to alleviate failures due to integration points, Nygard points out that sometime the remote systems that our app depends on might go down because of us: the day-after-Thanksgiving rush might cause our e-commerce app to pound the inventory system we integrate with into submission.  He introduces a pattern, the Circuit Breaker, to help with this.  The Circuit Breaker basically watches a "circuit" (the integration to the remote system), and when things are looking bad (the remote system starts throwing errors) we back off and stop pounding the poor guy to give him time to recover.

 I implemented the Circuit Breaker in Java using the GoF State Pattern.  Nygard's book includes the statechart diagram so it's practically a matter of simply typing in the code.  The main breaker is a Spring AOP aspect, and looks like this:

     public Object invoke(MethodInvocation invocation) throws Throwable
    {
        Object result = null;
        try
        {
            getState().preInvoke(this);
            result = invocation.proceed();
            getState().postInvoke(this);
        }
        catch(Throwable t)
        {
            getState().onError(this, t);
            throw t;
        }
        return result;
    }

The call to getState() returns the reference to the current CircuitBreakerState object (which is held in an java.util.concurrent.atomic.AtomicReference, but that's another story).

Normally, the breaker is Closed, so let's look at the ClosedState.

    public void preInvoke(CircuitBreakerAspect circuitBreakerAspect) throws Throwable
    {
        // NO OP
    }

    public void postInvoke(CircuitBreakerAspect circuitBreakerAspect) throws Throwable
    {
        resetFailureCount();
    }

    public void onError(CircuitBreakerAspect circuitBreakerAspect, Throwable t) throws Throwable
    {
        int currentCount = failureCount.incrementAndGet();
        int threshold = failureThreshold.get();
        if(currentCount >= threshold)
            circuitBreakerAspect.tripBreaker();
    }

The logic here is that if the calls fail a certain number (failureThreshold) times in a row, we trip (open) the breaker.  If a call succeeds, all is forgiven and we reset the failureCount. FailureCount and failureThreshold are AtomicIntegers (and probably should have been encapsulated with getters, pardon my mess).  The tripBreaker() method on the main circuit breaker sets the state to OpenState, and also calls trip() on the OpenState.  Let's take a look at OpenState:

    public void preInvoke(CircuitBreakerAspect circuitBreakerAspect) throws Throwable
    {
        long now = System.currentTimeMillis();
        long elapsed = now - tripTime.get();
        if(elapsed > timeout.get())
        {
            circuitBreakerAspect.attemptReset();
        }
        else
        {
            throw new CircuitBreakerException("Circuit Breaker is open; calls are failing fast");
        }
    }

    public void postInvoke(CircuitBreakerAspect circuitBreakerAspect) throws Throwable
    {
        // NO OP
    }

    public void onError(CircuitBreakerAspect circuitBreakerAspect, Throwable t) throws Throwable
    {
        // NO OP
    }

    void trip()
    {
        long now = System.currentTimeMillis();
        tripTime.set(now);
    }

In the above OpenState, we fail fast for a preset amount of time, throwing a runtime exception (CircuitBreakerException) on each call, thus giving the remote system time to recover.  After the specified time period has passed, we gingerly try calling the remote system again by calling attemptReset() on the main breaker.  AttemptReset() simply puts the breaker into the HalfOpenState.  Again, in the state pattern, the implementation of this class is trivial:

    public void preInvoke(CircuitBreakerAspect circuitBreakerAspect) throws Throwable
    {
        // NO OP
    }

    public void postInvoke(CircuitBreakerAspect circuitBreakerAspect) throws Throwable
    {
        circuitBreakerAspect.reset();
    }

    public void onError(CircuitBreakerAspect circuitBreakerAspect, Throwable e) throws Throwable
    {
        circuitBreakerAspect.tripBreaker();
        throw new CircuitBreakerException(e);
    }

If the call succeeds and we make it to postInvoke(), reset the circuit breaker (back to ClosedState, where we started).  If the call fails, give the remote system more time to recover by tripping the breaker again, going back to OpenState.

Our actual implementation is more complex, with the breaker itself exposed in JMX and exporting all kinds of interesting information: the total number of trips, the current error count, time until next reset, etc.  I love this because now I can check not only on the current health of the system, but get a summarized history too ("the breaker to system X tripped 10 times overnight").

Incidentally, this implementation also shows how cool the State Pattern is.  It's one of my favorite GoF patterns, even though I don't get to use it that often. 

Circuit Breaker is just one of the goodies in Nygard's book.  As the chief architect of a fairly big consumer-facing website, I found the book to be quite a page turner, and our systems are unquestionably more stable because of it.

 

Breaker

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