Vert.x Circuit Breaker
Vert.x Circuit Breaker is an implementation of the Circuit Breaker pattern for Vert.x. It keeps track of the number of failures and opens the circuit when a threshold is reached. Optionally, a fallback is executed.
Supported failures are:
-
failures reported by your code in a
Future
-
exception thrown by your code
-
uncompleted futures (timeout)
Operations guarded by a circuit breaker are intended to be non-blocking and asynchronous in order to benefit from the Vert.x execution model.
Using the vert.x circuit breaker
To use the Vert.x Circuit Breaker, add the following dependency to the dependencies section of your build descriptor:
-
Maven (in your
pom.xml
):
<dependency>
<groupId>io.vertx</groupId>
<artifactId>vertx-circuit-breaker</artifactId>
<version>4.2.7</version>
</dependency>
-
Gradle (in your
build.gradle
file):
compile 'io.vertx:vertx-circuit-breaker:4.2.7'
Using the circuit breaker
To use the circuit breaker you need to:
-
Create a circuit breaker, with the configuration you want (timeout, number of failure before opening the circuit)
-
Execute some code using the breaker
Important: Don’t recreate a circuit breaker on every call. A circuit breaker is a staful entity. It is recommended to store the circuit breaker instance in a field.
Here is an example:
CircuitBreaker breaker = CircuitBreaker.create("my-circuit-breaker", vertx,
new CircuitBreakerOptions()
.setMaxFailures(5) // number of failure before opening the circuit
.setTimeout(2000) // consider a failure if the operation does not succeed in time
.setFallbackOnFailure(true) // do we call the fallback on failure
.setResetTimeout(10000) // time spent in open state before attempting to re-try
);
// ---
// Store the circuit breaker in a field and access it as follows
// ---
breaker.execute(promise -> {
// some code executing with the breaker
// the code reports failures or success on the given promise.
// if this promise is marked as failed, the breaker increased the
// number of failures
}).onComplete(ar -> {
// Get the operation result.
});
The executed block receives a Future
object as parameter, to denote the
success or failure of the operation as well as the result. For example in the following example, the result is the
output of a REST endpoint invocation:
CircuitBreaker breaker = CircuitBreaker.create("my-circuit-breaker", vertx,
new CircuitBreakerOptions().setMaxFailures(5).setTimeout(2000)
);
// ---
// Store the circuit breaker in a field and access it as follows
// ---
breaker.<String>execute(promise -> {
vertx.createHttpClient().request(HttpMethod.GET, 8080, "localhost", "/")
.compose(req -> req
.send()
.compose(resp -> {
if (resp.statusCode() != 200) {
return Future.failedFuture("HTTP error");
} else {
return resp.body().map(Buffer::toString);
}
})).onComplete(promise);
}).onComplete(ar -> {
// Do something with the result
});
The result of the operation is provided using the:
Optionally, you can provide a fallback which is executed when the circuit is open:
CircuitBreaker breaker = CircuitBreaker.create("my-circuit-breaker", vertx,
new CircuitBreakerOptions().setMaxFailures(5).setTimeout(2000)
);
// ---
// Store the circuit breaker in a field and access it as follows
// ---
breaker.executeWithFallback(
promise -> {
vertx.createHttpClient().request(HttpMethod.GET, 8080, "localhost", "/")
.compose(req -> req
.send()
.compose(resp -> {
if (resp.statusCode() != 200) {
return Future.failedFuture("HTTP error");
} else {
return resp.body().map(Buffer::toString);
}
})).onComplete(promise);
}, v -> {
// Executed when the circuit is opened
return "Hello";
})
.onComplete(ar -> {
// Do something with the result
});
The fallback is called whenever the circuit is open, or if the
isFallbackOnFailure
is enabled. When a fallback is
set, the result is using the output of the fallback function. The fallback function takes as parameter a
Throwable
object and returns an object of the expected type.
The fallback can also be set on the CircuitBreaker
object directly:
CircuitBreaker breaker = CircuitBreaker.create("my-circuit-breaker", vertx,
new CircuitBreakerOptions().setMaxFailures(5).setTimeout(2000)
).fallback(v -> {
// Executed when the circuit is opened.
return "hello";
});
breaker.<String>execute(
promise -> {
vertx.createHttpClient().request(HttpMethod.GET, 8080, "localhost", "/")
.compose(req -> req
.send()
.compose(resp -> {
if (resp.statusCode() != 200) {
return Future.failedFuture("HTTP error");
} else {
return resp.body().map(Buffer::toString);
}
})).onComplete(promise);
});
Retries
You can also specify how often the circuit breaker should try your code before failing with
setMaxRetries
.
If you set this to something higher than 0 your code gets executed several times before finally failing
in the last execution. If the code succeeded in one of the retries your handler gets notified and any
retries left are skipped. Retries are only supported when the circuit is closed.
Notice that if you set maxRetries
to 2 for instance, your operation may be called 3 times: the initial attempt
and 2 retries.
By default the timeout between retries is set to 0 which means that retries will be executed one after another without
any delay. This, however, will result in increased load on the called service and may delay it’s recovery.
In order to mitigate this problem it is recommended to execute retries with delays. retryPolicy
method can be used to specify retry policy. Retry policy is a function which receives retry count as single argument
and returns timeout in milliseconds before retry is executed and allows to implement a more complex policy, e.g.
exponential backoff with jitter. Below is an example of retry timeout which grows linearly with each retry count:
CircuitBreaker breaker = CircuitBreaker.create("my-circuit-breaker", vertx,
new CircuitBreakerOptions().setMaxFailures(5).setMaxRetries(5).setTimeout(2000)
).openHandler(v -> {
System.out.println("Circuit opened");
}).closeHandler(v -> {
System.out.println("Circuit closed");
}).retryPolicy(retryCount -> retryCount * 100L);
breaker.<String>execute(
promise -> {
vertx.createHttpClient().request(HttpMethod.GET, 8080, "localhost", "/")
.compose(req -> req
.send()
.compose(resp -> {
if (resp.statusCode() != 200) {
return Future.failedFuture("HTTP error");
} else {
return resp.body().map(Buffer::toString);
}
})).onComplete(promise);
});
Callbacks
You can also configures callbacks invoked when the circuit is opened or closed:
CircuitBreaker breaker = CircuitBreaker.create("my-circuit-breaker", vertx,
new CircuitBreakerOptions().setMaxFailures(5).setTimeout(2000)
).openHandler(v -> {
System.out.println("Circuit opened");
}).closeHandler(v -> {
System.out.println("Circuit closed");
});
breaker.<String>execute(
promise -> {
vertx.createHttpClient().request(HttpMethod.GET, 8080, "localhost", "/")
.compose(req -> req
.send()
.compose(resp -> {
if (resp.statusCode() != 200) {
return Future.failedFuture("HTTP error");
} else {
return resp.body().map(Buffer::toString);
}
})).onComplete(promise);
});
You can also be notified when the circuit breaker decides to attempt to reset (half-open state). You can register
such a callback with halfOpenHandler
.
Event bus notification
Every time the circuit state changes, an event is published on the event bus. The address on which the events are
sent is configurable with
setNotificationAddress
. If null
is
passed to this method, the notifications are disabled. By default, the used address is vertx.circuit-breaker
.
Each event contains a Json Object with:
-
state
: the new circuit breaker state (OPEN
,CLOSED
,HALF_OPEN
) -
name
: the name of the circuit breaker -
failures
: the number of failures -
node
: the identifier of the node (local
if Vert.x is not running in cluster mode)
The half-open state
When the circuit is "open", calls to the circuit breaker fail immediately, without any attempt to execute the real
operation. After a suitable amount of time (configured from
setResetTimeout
, the circuit breaker decides that the
operation has a chance of succeeding, so it goes into the half-open
state. In this state, the next call to the
circuit breaker is allowed to execute the dangerous operation. Should the call succeed, the circuit breaker resets
and returns to the closed
state, ready for more routine operation. If this trial call fails, however, the circuit
breaker returns to the open
state until another timeout elapses.
Reported exceptions
The fallback receives a:
-
OpenCircuitException
when the circuit breaker is opened -
TimeoutException
when the operation timed out
Pushing circuit breaker metrics to the Hystrix Dashboard
Netflix Hystrix comes with a dashboard to present the current state of the circuit breakers. The Vert.x circuit
breakers can publish their metrics in order to be consumed by this Hystrix Dashboard. The Hystrix dashboard requires
a SSE stream sending the metrics. This stream is provided by the
HystrixMetricHandler
Vert.x Web Handler:
CircuitBreaker breaker = CircuitBreaker.create("my-circuit-breaker", vertx);
CircuitBreaker breaker2 = CircuitBreaker.create("my-second-circuit-breaker", vertx);
// Create a Vert.x Web router
Router router = Router.router(vertx);
// Register the metric handler
router.get("/hystrix-metrics").handler(HystrixMetricHandler.create(vertx));
// Create the HTTP server using the router to dispatch the requests
vertx.createHttpServer()
.requestHandler(router)
.listen(8080);
In the Hystrix Dashboard, configure the stream url like: http://localhost:8080/metrics
. The dashboard now consumes
the metrics from the Vert.x circuit breakers.
Notice that the metrics are collected by the Vert.x Web handler using the event bus notifications. If you don’t use the default notification address, you need to pass it when creating the metrics handler.
Using Netflix Hystrix
Hystrix provides an implementation of the circuit breaker pattern. You can use Hystrix with Vert.x instead of this circuit breaker or in combination of. This section describes the tricks to use Hystrix in a vert.x application.
First you would need to add the Hystrix dependency to your classpath or build descriptor. Refer to the Hystrix
page for details. Then, you need to isolate the "protected" call in a Command
. Once you have your command, you
can execute it:
HystrixCommand<String> someCommand = getSomeCommandInstance();
String result = someCommand.execute();
However, the command execution is blocking, so have to call the command execution either in an executeBlocking
block or in a worker verticle:
HystrixCommand<String> someCommand = getSomeCommandInstance();
vertx.<String>executeBlocking(
future -> future.complete(someCommand.execute()),
ar -> {
// back on the event loop
String result = ar.result();
}
);
If you use the async support of Hystrix, be careful that callbacks are not called in a vert.x thread and you have
to keep a reference on the context before the execution (with getOrCreateContext
,
and in the callback, switch back to the event loop using
runOnContext
. Without this, you are loosing the Vert.x
concurrency model and have to manage the synchronization and ordering yourself:
vertx.runOnContext(v -> {
Context context = vertx.getOrCreateContext();
HystrixCommand<String> command = getSomeCommandInstance();
command.observe().subscribe(result -> {
context.runOnContext(v2 -> {
// Back on context (event loop or worker)
String r = result;
});
});
});