- * Template for circuit breakers, which are components designed to protect - * clients from broken services by preventing faults from propagating across - * integration points. + * Template for circuit breakers, which are components designed to protect clients from broken services by preventing + * faults from propagating across integration points. + *
+ *+ * For example, suppose that we have a web application that calls a web service. If the web service is having a problem + * (e.g., severe latency or perhaps complete unavailability), we don't want this to create problems for the app. Instead + * we want to isolate the problem. *
*- * For example, suppose that we have a web application that calls a web service. - * If the web service is having a problem (e.g., severe latency or perhaps - * complete unavailability), we don't want this to create problems for the app. - * Instead we want to isolate the problem. + * The circuit breaker allows us to do just this. We associate a circuit breaker with each integration point, and the + * breaker mediates calls from the client to the service. Under normal circumstances the breaker is in the closed state, + * and calls pass through to the service. If however there is a problem, then the breaker goes into the open state for + * some period of time. While the breaker is open, all attempts to call the service fail with a {@link + * CircuitOpenException}. Once the problem is resolved, the breaker returns to the closed state and normal operations + * resume. *
*
- * The circuit breaker allows us to do just this. We associate a circuit breaker
- * with each integration point, and calls from the client to the service are
- * mediated by the breaker. Under normal circumstances the breaker is in the
- * closed state, and calls pass through to the service. If however there is a
- * problem, then the breaker goes into the open state for some period of time.
- * While the breaker is open, all attempts to call the service fail with a
- * {@link CircuitOpenException}. Once the problem is resolved, the breaker
- * returns to the closed state and normal operations resume.
+ * 
*
- * The circuit breaker pattern is described in detail in Michael Nygard's book, - * Release It! - * (Pragmatic). + * The circuit breaker pattern is described in detail in Michael Nygard's book, Release It! (Pragmatic). + *
+ *+ * We expose this guard as a JMX MBean so it can be queried and manipulated in management contexts. You can, for + * example, use JMX to trip and reset breakers manually. *
* * @author Willie Wheeler (willie.wheeler@gmail.com) @@ -77,9 +81,7 @@ public enum State { CLOSED, OPEN, HALF_OPEN }; private final AtomicInteger exceptionCount = new AtomicInteger(); private volatile long retryTime = NO_SCHEDULED_RETRY; - public CircuitBreakerTemplate() { - handledExceptions.add(Exception.class); - } + public CircuitBreakerTemplate() { handledExceptions.add(Exception.class); } /** *@@ -90,9 +92,7 @@ public CircuitBreakerTemplate() { * @return exception threshold for this breaker */ @ManagedAttribute(description = "Breaker trips when threshold is reached") - public int getExceptionThreshold() { - return exceptionThreshold; - } + public int getExceptionThreshold() { return exceptionThreshold; } /** *
@@ -112,7 +112,7 @@ public int getExceptionThreshold() {
description = "Breaker trips when threshold is reached",
defaultValue = "5")
public void setExceptionThreshold(int threshold) {
- Assert.isTrue(threshold >= 1, "threshold must be >= 1");
+ isTrue(threshold >= 1, "threshold must be >= 1");
this.exceptionThreshold = threshold;
}
@@ -141,7 +141,7 @@ public void setExceptionThreshold(int threshold) {
description = "Delay in ms before open breaker goes half-open",
defaultValue = "30000")
public void setTimeout(long timeout) {
- Assert.isTrue(timeout >= 0L, "timeout must be >= 0");
+ isTrue(timeout >= 0L, "timeout must be >= 0");
this.timeout = timeout;
}
@@ -150,7 +150,7 @@ public List
- * Template component that fails with an exception when a concurrency threshold is exceeded.
+ * Guard that fails with an exception when a concurrency threshold is exceeded.
*
* Implementation is based on a counting semaphore.
*
+ * We expose this guard as a JMX MBean so it can be queried and manipulated in management contexts.
+ *
- * Note that all counts reset on the hour, as measured by the wall clock. + * Note that all counts reset on the hour, as measured by the wall clock. This allows us to avoid tracking individual + * client requests and their timestamps. + *
+ *+ * We expose this guard as a JMX MBean so it can be queried and manipulated in management contexts. *
* * @author Willie Wheeler (willie.wheeler@gmail.com)