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110 lines
3.0 KiB
Markdown
110 lines
3.0 KiB
Markdown
# Circuit Breaker Pattern
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Similar to electrical fuses that prevent fires when a circuit that is connected
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to the electrical grid starts drawing a high amount of power which causes the
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wires to heat up and combust, the circuit breaker design pattern is a fail-first
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mechanism that shuts down the circuit, request/response relationship or a
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service in the case of software development, to prevent bigger failures.
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**Note:** The words "circuit" and "service" are used synonymously through this
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document.
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## Implementation
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Below is the implementation of a very simple circuit breaker to illustrate the purpose
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of the circuit breaker design pattern, only considering Open/Closed
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### Operation Counter
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`circuit.Counter` is a simple counter that records success and failure states of
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a circuit along with a timestamp and calculates the consecutive number of
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failures.
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```go
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package circuit
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import (
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"time"
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)
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type State int
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const (
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UnknownState State = iota
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FailureState
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SuccessState
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)
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type Counter interface {
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Count(State)
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ConsecutiveFailures() uint32
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LastActivity() time.Time
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Reset()
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}
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type counters struct {
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ConsecutiveFailures uint32
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ConsecutiveSuccesses uint32
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}
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```
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### Circuit Breaker
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Circuit is wrapped using the `circuit.Breaker` closure that keeps an internal operation counter.
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It returns a fast error if the circuit has failed consecutively more than the specified threshold.
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After a while it retries the request and records it.
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**Note:** Context type is used here to carry deadlines, cancellation signals, and
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other request-scoped values across API boundaries and between processes.
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```go
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package circuit
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import (
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"context"
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"time"
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)
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type Circuit func(context.Context) error
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var canRetry = func(cnt counters, failureThreshold uint32) bool {
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backoffLevel := cnt.ConsecutiveFailures - failureThreshold
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// Calculates when should the circuit breaker resume propagating requests
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// to the service
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shouldRetryAt := cnt.LastActivity().Add(time.Second << backoffLevel)
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return time.Now().After(shouldRetryAt)
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}
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func Breaker(c Circuit, failureThreshold uint32) Circuit {
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cnt := counters{}
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//ctx can be used hold parameters
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return func(ctx context.Context) error {
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if cnt.ConsecutiveFailures >= failureThreshold {
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if !canRetry(cnt, failureThreshold) {
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// Fails fast instead of propagating requests to the circuit since
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// not enough time has passed since the last failure to retry
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return ErrServiceUnavailable
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}
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}
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// Unless the failure threshold is exceeded the wrapped service mimics the
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// old behavior and the difference in behavior is seen after consecutive failures
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if err := c(ctx); err != nil {
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cnt.Count(FailureState)
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return err
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}
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cnt.Count(SuccessState)
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return nil
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}
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}
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```
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## Related Contents
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- [Circuit Breaker Pattern](https://docs.microsoft.com/en-us/previous-versions/msp-n-p/dn589784(v=pandp.10)?redirectedfrom=MSDN)
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- [sony/gobreaker](https://github.com/sony/gobreaker) is a well-tested and intuitive circuit breaker implementation for real-world use cases.
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