## 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>3.6.3</version>
    </dependency>

- Gradle (in your `build.gradle` file):

    compile 'io.vertx:vertx-circuit-breaker:3.6.3'

## Using the circuit breaker

To use the circuit breaker you need to:

1. Create a circuit breaker, with the configuration you want (timeout, number of failure before opening the circuit)

2. 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(future -> {
      // some code executing with the breaker
      // the code reports failures or success on the given future.
      // if this future is marked as failed, the breaker increased the
      // number of failures
    }).setHandler(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(future -> {
      vertx.createHttpClient().getNow(8080, "localhost", "/", response -> {
        if (response.statusCode() != 200) {
          future.fail("HTTP error");
        } else {
          response
              .exceptionHandler(future::fail)
              .bodyHandler(buffer -> {
                future.complete(buffer.toString());
              });
        }
      });
    }).setHandler(ar -> {
      // Do something with the result
    });

The result of the operation is provided using the:

- returned `Future` when calling `execute` methods

- provided `Future` when calling the `executeAndReport` methods

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(
        future -> {
          vertx.createHttpClient().getNow(8080, "localhost", "/", response -> {
            if (response.statusCode() != 200) {
              future.fail("HTTP error");
            } else {
              response
                  .exceptionHandler(future::fail)
                  .bodyHandler(buffer -> {
                    future.complete(buffer.toString());
                  });
            }
          });
        }, v -> {
          // Executed when the circuit is opened
          return "Hello";
        })
        .setHandler(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.execute(
        future -> {
          vertx.createHttpClient().getNow(8080, "localhost", "/", response -> {
            if (response.statusCode() != 200) {
              future.fail("HTTP error");
            } else {
              response
                  .exceptionHandler(future::fail)
                  .bodyHandler(buffer -> {
                    future.complete(buffer.toString());
                  });
            }
          });
        });

## 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.execute(
      future -> {
        vertx.createHttpClient().getNow(8080, "localhost", "/", response -> {
          if (response.statusCode() != 200) {
            future.fail("HTTP error");
          } else {
            // Do something with the response
            future.complete();
          }
        });
      });

## 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.execute(
        future -> {
          vertx.createHttpClient().getNow(8080, "localhost", "/", response -> {
            if (response.statusCode() != 200) {
              future.fail("HTTP error");
            } else {
              // Do something with the response
              future.complete();
            }
          });
        });

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::accept)
      .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;
    });
    });
    });