gobreaker
=========

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[gobreaker][repo-url] implements the [Circuit Breaker pattern](https://msdn.microsoft.com/en-us/library/dn589784.aspx) in Go.

Installation
------------

```
go get github.com/sony/gobreaker
```

Usage
-----

The struct `CircuitBreaker` is a state machine to prevent sending requests that are likely to fail.
The function `NewCircuitBreaker` creates a new `CircuitBreaker`.

```go
func NewCircuitBreaker(st Settings) *CircuitBreaker
```

You can configure `CircuitBreaker` by the struct `Settings`:

```go
type Settings struct {
	Name          string
	MaxRequests   uint32
	Interval      time.Duration
	Timeout       time.Duration
	ReadyToTrip   func(counts Counts) bool
	OnStateChange func(name string, from State, to State)
}
```

- `Name` is the name of the `CircuitBreaker`.

- `MaxRequests` is the maximum number of requests allowed to pass through
  when the `CircuitBreaker` is half-open.
  If `MaxRequests` is 0, `CircuitBreaker` allows only 1 request.

- `Interval` is the cyclic period of the closed state
  for `CircuitBreaker` to clear the internal `Counts`, described later in this section.
  If `Interval` is 0, `CircuitBreaker` doesn't clear the internal `Counts` during the closed state.

- `Timeout` is the period of the open state,
  after which the state of `CircuitBreaker` becomes half-open.
  If `Timeout` is 0, the timeout value of `CircuitBreaker` is set to 60 seconds.

- `ReadyToTrip` is called with a copy of `Counts` whenever a request fails in the closed state.
  If `ReadyToTrip` returns true, `CircuitBreaker` will be placed into the open state.
  If `ReadyToTrip` is `nil`, default `ReadyToTrip` is used.
  Default `ReadyToTrip` returns true when the number of consecutive failures is more than 5.

- `OnStateChange` is called whenever the state of `CircuitBreaker` changes.

The struct `Counts` holds the numbers of requests and their successes/failures:

```go
type Counts struct {
	Requests             uint32
	TotalSuccesses       uint32
	TotalFailures        uint32
	ConsecutiveSuccesses uint32
	ConsecutiveFailures  uint32
}
```

`CircuitBreaker` clears the internal `Counts` either
on the change of the state or at the closed-state intervals.
`Counts` ignores the results of the requests sent before clearing.

`CircuitBreaker` can wrap any function to send a request:

```go
func (cb *CircuitBreaker) Execute(req func() (interface{}, error)) (interface{}, error)
```

The method `Execute` runs the given request if `CircuitBreaker` accepts it.
`Execute` returns an error instantly if `CircuitBreaker` rejects the request.
Otherwise, `Execute` returns the result of the request.
If a panic occurs in the request, `CircuitBreaker` handles it as an error
and causes the same panic again.

Example
-------

```go
var cb *breaker.CircuitBreaker

func Get(url string) ([]byte, error) {
	body, err := cb.Execute(func() (interface{}, error) {
		resp, err := http.Get(url)
		if err != nil {
			return nil, err
		}

		defer resp.Body.Close()
		body, err := ioutil.ReadAll(resp.Body)
		if err != nil {
			return nil, err
		}

		return body, nil
	})
	if err != nil {
		return nil, err
	}

	return body.([]byte), nil
}
```

See [example](https://github.com/sony/gobreaker/blob/master/example) for details.

License
-------

The MIT License (MIT)

See [LICENSE](https://github.com/sony/gobreaker/blob/master/LICENSE) for details.


[repo-url]: https://github.com/sony/gobreaker