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circuit

A highly-tunable circuit breaker

🚫 WARNING: This project is still a work in progress and is therefore unversioned and unlicensed. It's public only due to the limited number of collaborators imposed by GitHub on private repos. Please don't use this in production, but do feel free to file issues and PRs

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Circuit implements the circuit breaker design pattern in Go.

Installation

go get -u github.com/schigh/circuit

Usage

Create new circuit breakers with NewBreaker

You can create new circuit breakers with NewBreaker. NewBreaker takes a single BreakerOptions parameter:

// BreakerOptions contains configuration options for a circuit breaker
type BreakerOptions struct {
    // Name is the circuit breaker name. If Name is not provided,
    // a unique Name will be created based on the caller to NewBreaker
    Name string

    // Timeout is the maximum duration that the Run func
    // can execute before timing out.  The default Timeout
    // is 3 seconds.
    Timeout time.Duration

    // BaudRate is the duration between error calculations.
    // The default BaudRate is 250ms.  The minimum BaudRate
    // is 10ms.
    BaudRate time.Duration

    // Backoff is the duration that a circuit breaker is
    // throttled.  The default Backoff is 1 minute.
    // The minimum Backoff is 1 second.
    BackOff time.Duration

    // Window is the length of time checked for error
    // calculation. The default Window is 5 minutes.
    // The minimum Window is 5 seconds.
    Window time.Duration

    // Threshold is the maximum number of errors that
    // can occur within the window before the circuit
    // breaker opens. By default, one error will open
    // the circuit breaker.
    Threshold uint32

    // LockOut is the length of time that a circuit breaker
    // is forced open before attempting to throttle.
    // If no lockout is provided, the circuit breaker will
    // transition to a throttled state only after its error
    // count is at or below the threshold.  While a circuit
    // breaker is open, all requests are rejected and no
    // new errors are recorded.
    LockOut time.Duration

    // OpeningWillResetErrors will cause the error count to reset
    // when the circuit breaker opens.  If this is set true, all
    // blocked calls will come from the throttled backoff, unless
    // the circuit breaker has a lockout duration.
    OpeningWillResetErrors bool

    // IgnoreContext will prevent context cancellation to
    // propagate to any in-flight Run functions.
    IgnoreContext bool

    // EstimationFunc is the function used to determine
    // the chance of a request being throttled during the
    // backoff period.  By default, Linear estimation is used.
    EstimationFunc EstimationFunc

    // PreProcessors are functions that execute in order before
    // the Runner function is executed.  If a preprocessor
    // returns an error, the execution is canceled and the error
    // is returned from Run.
    PreProcessors []PreProcessor

    // PostProcessors are functions that execute in order after
    // the Runner has executed.  The return values from the
    // Runner are passed along to the first postprocessor.
    // If there are subsequent postprocessors, each will take
    // the return value of its predecessor.
    PostProcessors []PostProcessor
}
breaker := circuit.NewBreaker(circuit.BreakerOptions{
    Name:              "expensive_api_call",
    Timeout:           5 * time.Second,
    BackOff:           time.Minute,
    Window:            time.Minute,
    LockOut:           5 * time.Second,
    EstimationFunc: circuit.Exponential,
})

⚠️ You must use NewBreaker to create circuit breakers.
Instantiating the Breaker struct directly will cause all calls to Run(...) to fail immediately

Running the circuit breaker

You can run the circuit breaker by calling its Run(...) function. Run takes two parameters: a context.Context instance, and a function matching this signature:

func(context.Context) (interface{}, error)
breaker := circuit.NewBreaker(circuit.BreakerOptions{
    Name:              "expensive_api_call",
    Timeout:           5 * time.Second,
    BackOff:           time.Minute,
    Window:            time.Minute,
    LockOut:           5 * time.Second,
    EstimationFunc: circuit.Exponential,
})

// ...

resultIface, err := breaker.Run(context.Background, func(ctx context.Context) (interface{}, error) {
    // DoSomethingExpensive returns *ResultType, error
    return dependency.DoSomethingExpensive()
})

if err != nil {
    // handle error
}
result, _ := resultIface.(*ResultType)
// do something with result

The error returned from Run can take the following forms:

Error Kind Description Affects Error Count
An error returned from your function An error from your implementation Yes
circuit.TimeoutError Returned when Run times out before completion Yes
circuit.StateOpenError Returned when the circuit breaker is open and/or locked out No
circuit.StateThrottledError Returned when the circuit breaker is throttled No

If the circuit breaker's Timeout setting is eclipsed while Run is executing, the context will cancel and Run will return nil, circuit.TimeoutError. The canceled context will propagate downstream to any logic that is listening for context cancellation. If the IgnoreContext flag of BreakerOptions is set true, the context cancellation will not propagate downstream.

Preprocessing and Postprocessing

You can intercept the parameters before invoking the Runner via preprocessors. Likewise, you can intercept the Runner's output before returning from Run via postprocessors. Take the following example:

type str string

breaker := circuit.NewBreaker(circuit.BreakerOptions{
    PreProcessors: []circuit.PreProcessor{
        func(ctx context.Context, runner circuit.Runner) (context.Context, circuit.Runner, error) {
            return context.WithValue(ctx, str("start"), time.Now().Unix()), runner, nil
        },
    },
    PostProcessors: []circuit.PostProcessor{
        func(ctx context.Context, i interface{}, err error) (interface{}, error) {
            end := time.Now().Unix()
            start, _ := ctx.Value(str("start")).(int64)
            fmt.Printf("Run took %d seconds", end-start)

            return i, err
        },
    }
})

The preprocessor intercepts the context and replaces it with a context with an added value. The Runner executes, and then the postprocessor intercepts the context and extracts the value added in the preprocessor, which in this case is a time stamp.

The function signatures are as follows:

type PreProcessor func(context.Context, Runner) (context.Context, Runner, error)
type PostProcessor func(context.Context, interface{}, error) (interface{}, error)

Preprocessors run before the circuit breaker's state or the context is checked for fitness. Preprocessors run in order, where the output context.Context and circuit.Runner are input to each subscquent preprocessor.

ℹ️ If any preprocessor returns a non-nil error, that error is returned from Run.

⚠️ The Run function will always return an error if a canceled or deadlined context is passed to the Runner. If you want to override this behavior, you would use a preprocessor.

Postprocessors are executed in order after the Runner has returned a value, including if the circuit breaker has timed out. Like preprocessors, postprocessors pass their output to the next postprocessor if there is one. The final outputs are then returned from Run.

Transition States

The state transitions for circuit breakers are as follows:

From To Description
New Closed The circuit breaker was just created
Closed Open The number of errors returned from Run exceeded the threshold for the measurement window
Open Throttled The circuit breaker's lockout has just expired (if set), and the number of errors is at or below the threshold
Throttled Open The number of errors returned from Run exceeded the threshold for the measurement window
Throttled Closed The backoff period has expired, and the number of errors is at or below the threshold

LockOut

When a circuit breaker opens, it can lock out for a specified duration. During that time, all invocations of Run immediately return a circuit.StateOpenError, even if the error count has gone below the threshold. By default, the lockout duration is 0. It can be set with the LockOut property of the BreakerOptions struct.

If there is no lockout set, the circuit breaker will transition from open to throttled as soon as the error count is at or below the threshold. If you want to throttle immediately, set the OpeningWillResetErrors property of the BreakerOptions struct to true. If OpeningWillResetErrors is used, the error count drops to zero immediately when the circuit breaker opens.

⚠️ Take care when using the OpeningWillResetErrors flag. If used in conjunction with a short or no LockOut, the circuit breaker will cycle from open to throttled with high frequency

Backoff Strategy

When the circuit breaker enters the throttled state, it uses an estimation function to determine how frequently it will allow subsequent calls to Run to be executed. By default, the circuit breaker uses the circuit.Linear backoff strategy.

The backoff strategy works by dividing the backoff duration into 100 units. For each of those units i in [1...100], the estimation function returns a probability score for i such that any call to Runwill return a circuit.StateThrottledError. If the estimation function returns 100, all calls to Run return a throttling error. If the estimation function returns 0, no calls to Run return a throttling error.

Linear estimation (default)

Linear estimation

Linear estimation returns a probability inversely proportional to i. A circuit breaker will default to linear estimation if an estimation function is not provided in BreakerOptions.

Logarithmic estimation

Logarithmic estimation

Logarithmic estimation initially returns a high probability, and quickly decreases after half of the backoff duration has elapsed.

Exponential estimation

Exponential estimation

Exponential estimation will initially decrease probability rapidly and eases up after half of the backoff duration has elapsed.

Ease-In-Out estimation

Ease-In-Out estimation

Ease-In-Out estimation will initially return a higher probability and quickly decrease as the backoff duration approaches its midpoint. The rate of decrease will slow down shortly after the backoff midpoint has elapsed.

⚠️ If a throttled circuit breaker reopens, the current backoff period is discarded and will start over if the circuit breaker becomes throttled again.

Capturing circuit breaker state changes

All circuit breaker state changes are exposed via a read-only channel. You can access this channel by calling StateChange() on the circuit breaker:

breaker := circuit.NewBreaker(circuit.BreakerOptions{})
changeChan := breaker.StateChange()

go func(changeChan chan circuit.BreakerState) {
    for {
        select {
        case state := <-changeChan:
            doSomethingWith(state)
        }
    }
}(changeChan)

ℹ️ The breaker will not block if there's no receiver for the channel returned from StateChange(). However, this is the only way to detect state changes from a circuit breaker.

BreakerState

The circuit breaker uses the BreakerState struct to communicate state changes:

type BreakerState struct {
    Name        string     `json:"string"`
    State       State      `json:"state"`
    ClosedSince *time.Time `json:"closed_since,omitempty"`
    Opened      *time.Time `json:"opened,omitempty"`
    LockoutEnds *time.Time `json:"lockout_ends,omitempty"`
    Throttled   *time.Time `json:"throttled,omitempty"`
    BackOffEnds *time.Time `json:"backoff_ends,omitempty"`
}

The properties of BreakerState will vary depending on the current state of the circuit breaker:

  • Name and State are always set
  • ClosedSince is nil unless the state is Closed. This value indicates when the circuit breaker was closed.
  • Opened is nil unless the state is Open. This value indicates when the circuit breaker was opened.
  • LockoutEnds is nil unless the state is Open and the circuit breaker has a lockout duration set. This value indicates when the current lockout period will end.
  • Throttled is nil unless the state is Throttled. This value indicates when the circuit breaker entered the throttled state.
  • BackOffEnds is nil unless the state is Throttled. This value indicates when the current backoff period will end.

ℹ️ The State property of BreakerState is an integer. However, when the circuit breaker state is serialized, its string value is used.

Reading circuit breaker state

There are a few ways you can examine a circuit breaker to see how it's getting along:

State()

If you only want the circuit breaker's current state, this is the function you want to use, as it's the least disruptive to the internal workings of the circuit breaker. State() returns a circuit.State value:

breaker := circuit.NewBreaker(circuit.BreakerOptions{})
state := breaker.State()
// do something with state

Size()

Size() returns the number of errors counted in the current window. The primary usefullness of this function is to observe the decay rate for circuit breaker tuning:

breaker := circuit.NewBreaker(circuit.BreakerOptions{})

...

t := time.NewTicker(100 * time.Millisecond)
go func(breaker *circuit.Breaker, t *time.Ticker){
    for {
        select {
        case <-t.C:
            _,_ = fmt.Fprintf(os.StdErr, "size: %d\n", breaker.Size())
        }
    }
}(breaker, t)

Snapshot()

Snapshot() will give you the same information that is made available during a state change event (a BreakerState struct).

Managing circuit breakers with BreakerBox

Managing an individual circuit breaker is fairly simple, but it can quickly become tedious and verbose when you need to manage several circuit breakers at once.

The BreakerBox allows you to easily manage multiple circuit breakers within your application.

Create a breaker box and listen for state changes

You should always create a new breaker box with NewBreakerBox():

box := circuit.NewBreakerBox()

⚠️ Breaker boxes must be created with NewBreakerBox. Creating a breaker box any other way will cause your program to panic.

A breaker box exposes a read-only channel of BreakerState structs, just like a Breaker does. If you're using the breaker box, you don't want to listen for state changes on individual circuit breakers. All circuit breaker state changes are funneled to the box state change channel, accessible via StateChange():

changeChan := box.StateChange()

go func(changeChan chan circuit.BreakerState) {
    for {
        select {
        case state := <-changeChan:
            doSomethingWith(state)
        }
    }
}(changeChan)

Getting and setting circuit breakers

The most efficient way to get a circuit breaker is to use LoadOrCreate:

box := circuit.NewBreakerBox()

// ...

breaker, err := box.LoadOrCreate(circuit.BreakerOptions{
    Name: "more_cowbell_cb",
})
if err != nil {
    // handle error
}

// ...

result, err := breaker.Run(context.Background(), func(ctx context.Context) (interface{}, error) {
    // needMoreCowbell returns interface{}, error
    return needMoreCowbell()
})

// ...

If the circuit breaker already exists, it is returned, and the error will be nil. Otherwise, LoadOrCreate will make a new circuit breaker using the supplied options.

⚠️ Circuit breakers created by the breaker box must have an explicitly-created name, unlike a standalone circuit breaker. If no name is provided, UnnamedBreakerError is returned

Other box functions

Load(name string)

Load will return a circuit breaker by name if it exists, and nil otherwise

Create(opts BreakerOptions)

Create will create a new circuit breaker with the supplied options and return it. If a circuit breaker with the same name already exists, it will be overwritten. Just as in LoadOrCreate, the Name option must be set for the new circuit breaker.

AddBYO(b *Breaker)

AddBYO allows you to create circuit breakers outside of the breaker box and then use the box for storage and retrieval. However, circuit breakers added via AddBYO will not report their state changes to the breaker box, so you will need to manage those state changes yourself.

Tuning circuit breakers

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