Talk about the execution.isolation.semaphore.maxconcurrentquests property of hystrix

  hystrix

Order

This article mainly studies the execution.isolation.semaphore.maxconcurrentquests attribute of hystrix.

AbstractCommand.applyHystrixSemantics

hystrix-core-1.5.12-sources.jar! /com/netflix/hystrix/AbstractCommand.java

    private Observable<R> applyHystrixSemantics(final AbstractCommand<R> _cmd) {
        // mark that we're starting execution on the ExecutionHook
        // if this hook throws an exception, then a fast-fail occurs with no fallback.  No state is left inconsistent
        executionHook.onStart(_cmd);

        /* determine if we're allowed to execute */
        if (circuitBreaker.attemptExecution()) {
            final TryableSemaphore executionSemaphore = getExecutionSemaphore();
            final AtomicBoolean semaphoreHasBeenReleased = new AtomicBoolean(false);
            final Action0 singleSemaphoreRelease = new Action0() {
                @Override
                public void call() {
                    if (semaphoreHasBeenReleased.compareAndSet(false, true)) {
                        executionSemaphore.release();
                    }
                }
            };

            final Action1<Throwable> markExceptionThrown = new Action1<Throwable>() {
                @Override
                public void call(Throwable t) {
                    eventNotifier.markEvent(HystrixEventType.EXCEPTION_THROWN, commandKey);
                }
            };

            if (executionSemaphore.tryAcquire()) {
                try {
                    /* used to track userThreadExecutionTime */
                    executionResult = executionResult.setInvocationStartTime(System.currentTimeMillis());
                    return executeCommandAndObserve(_cmd)
                            .doOnError(markExceptionThrown)
                            .doOnTerminate(singleSemaphoreRelease)
                            .doOnUnsubscribe(singleSemaphoreRelease);
                } catch (RuntimeException e) {
                    return Observable.error(e);
                }
            } else {
                return handleSemaphoreRejectionViaFallback();
            }
        } else {
            return handleShortCircuitViaFallback();
        }
    }

This method calls getExecutionSemaphore to get TryableSemaphore, and performs tryAcquire before execution and release after execution.

AbstractCommand.getExecutionSemaphore

hystrix-core-1.5.12-sources.jar! /com/netflix/hystrix/AbstractCommand.java

    /**
     * Get the TryableSemaphore this HystrixCommand should use for execution if not running in a separate thread.
     * 
     * @return TryableSemaphore
     */
    protected TryableSemaphore getExecutionSemaphore() {
        if (properties.executionIsolationStrategy().get() == ExecutionIsolationStrategy.SEMAPHORE) {
            if (executionSemaphoreOverride == null) {
                TryableSemaphore _s = executionSemaphorePerCircuit.get(commandKey.name());
                if (_s == null) {
                    // we didn't find one cache so setup
                    executionSemaphorePerCircuit.putIfAbsent(commandKey.name(), new TryableSemaphoreActual(properties.executionIsolationSemaphoreMaxConcurrentRequests()));
                    // assign whatever got set (this or another thread)
                    return executionSemaphorePerCircuit.get(commandKey.name());
                } else {
                    return _s;
                }
            } else {
                return executionSemaphoreOverride;
            }
        } else {
            // return NoOp implementation since we're not using SEMAPHORE isolation
            return TryableSemaphoreNoOp.DEFAULT;
        }
    }
  • Here, judgment is made on the ExecutionIsolationStrategy. If it is SEMAPHORE, the corresponding TryableSemaphore is obtained or newly created according to commandKey.
  • If created, TryableSemaphoreActual is used, and its numberOfPermits permitsparameter is the value of execution.isolation.semaphore.maxconcurrentquests.
  • If the ExecutionIsolationStrategy is THREAD, here TryableSemaphore returns TryableSemaphoreNoOp.DEFAULT, that is, it will pass without doing anything.

TryableSemaphore

    /* package */static interface TryableSemaphore {

        /**
         * Use like this:
         * <p>
         * 
         * <pre>
         * if (s.tryAcquire()) {
         * try {
         * // do work that is protected by 's'
         * } finally {
         * s.release();
         * }
         * }
         * </pre>
         * 
         * @return boolean
         */
        public abstract boolean tryAcquire();

        /**
         * ONLY call release if tryAcquire returned true.
         * <p>
         * 
         * <pre>
         * if (s.tryAcquire()) {
         * try {
         * // do work that is protected by 's'
         * } finally {
         * s.release();
         * }
         * }
         * </pre>
         */
        public abstract void release();

        public abstract int getNumberOfPermitsUsed();

    }

Three methods are defined, tryAcquire, release, getNumberOfPermitsUsed

TryableSemaphoreActual

    /**
     * Semaphore that only supports tryAcquire and never blocks and that supports a dynamic permit count.
     * <p>
     * Using AtomicInteger increment/decrement instead of java.util.concurrent.Semaphore since we don't need blocking and need a custom implementation to get the dynamic permit count and since
     * AtomicInteger achieves the same behavior and performance without the more complex implementation of the actual Semaphore class using AbstractQueueSynchronizer.
     */
    /* package */static class TryableSemaphoreActual implements TryableSemaphore {
        protected final HystrixProperty<Integer> numberOfPermits;
        private final AtomicInteger count = new AtomicInteger(0);

        public TryableSemaphoreActual(HystrixProperty<Integer> numberOfPermits) {
            this.numberOfPermits = numberOfPermits;
        }

        @Override
        public boolean tryAcquire() {
            int currentCount = count.incrementAndGet();
            if (currentCount > numberOfPermits.get()) {
                count.decrementAndGet();
                return false;
            } else {
                return true;
            }
        }

        @Override
        public void release() {
            count.decrementAndGet();
        }

        @Override
        public int getNumberOfPermitsUsed() {
            return count.get();
        }

    }

Internal is to use AtomicInteger to count, tryAcquire method, is to increase first, if more than the limit, then reduce

TryableSemaphoreNoOp.DEFAULT

    /* package */static class TryableSemaphoreNoOp implements TryableSemaphore {

        public static final TryableSemaphore DEFAULT = new TryableSemaphoreNoOp();

        @Override
        public boolean tryAcquire() {
            return true;
        }

        @Override
        public void release() {

        }

        @Override
        public int getNumberOfPermitsUsed() {
            return 0;
        }

    }

Default is a single instance of TryableSemaphoreNoOp, which is released by default.

Summary

Hystrix’s ExecutionIsolationStrategy is divided into SEMAPHORE and THREAD modes. Before and after the execution of command, the logic is built with tryAcquire and release operations for TryableSemaphore. Only when the TryableSemaphore implementation class is obtained is the restriction for the SEMAPHORE mode really limited according to the execution.isolation.semaphore.maxconcurrent trequests attribute, and if it is the THREAD mode, a TryableSemaphoreNoOp instance that is all released is returned.

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