From 74d1d3214986a3d9bc383959f35e19c59cab9fb8 Mon Sep 17 00:00:00 2001
From: Kirill Sysoev <k.sysoev@me.com>
Date: Fri, 12 Apr 2024 22:12:08 +0800
Subject: [PATCH] Adds implementation of circuit breaker middleware

---
 middleware/request/circuit_breaker.go | 87 +++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)
 create mode 100644 middleware/request/circuit_breaker.go

diff --git a/middleware/request/circuit_breaker.go b/middleware/request/circuit_breaker.go
new file mode 100644
index 0000000..581a86e
--- /dev/null
+++ b/middleware/request/circuit_breaker.go
@@ -0,0 +1,87 @@
+package request
+
+import (
+	"fmt"
+	"sync"
+	"time"
+
+	"github.com/ksysoev/wasabi"
+	"github.com/ksysoev/wasabi/dispatch"
+)
+
+type CircuitBreakerState uint8
+
+var (
+	ErrCircuitBreakerOpen = fmt.Errorf("circuit breaker is open")
+)
+
+const (
+	Closed CircuitBreakerState = iota
+	Open
+)
+
+// NewCircuitBreakerMiddleware creates a new circuit breaker middleware with the specified threshold and period.
+// The circuit breaker middleware wraps a given `wasabi.RequestHandler` and provides circuit breaking functionality.
+// The circuit breaker tracks the number of consecutive errors and opens the circuit when the error count exceeds the threshold.
+// During the open state, all requests are rejected with an `ErrCircuitBreakerOpen` error.
+// After a specified period of time, the circuit breaker transitions to the semi-open state, allowing a single request to be processed.
+// If the request succeeds, the circuit breaker resets the error count and transitions back to the closed state.
+// If the request fails, the circuit breaker remains in the open state.
+// The circuit breaker uses synchronization primitives to ensure thread safety.
+// The `treshold` parameter specifies the maximum number of consecutive errors allowed before opening the circuit.
+// The `period` parameter specifies the duration of time after which the circuit breaker transitions to the semi-open state.
+// The returned function is a middleware that can be used with the `wasabi` framework.
+func NewCircuitBreakerMiddleware(treshold int, period time.Duration) func(next wasabi.RequestHandler) wasabi.RequestHandler {
+	intervalEnds := time.Now().Add(period)
+	state := Closed
+	errorCounter := 0
+	lock := &sync.RWMutex{}
+	semiOpenLock := &sync.Mutex{}
+
+	return func(next wasabi.RequestHandler) wasabi.RequestHandler {
+		return dispatch.RequestHandlerFunc(func(conn wasabi.Connection, req wasabi.Request) error {
+			lock.RLock()
+
+			switch state {
+			case Closed:
+				err := next.Handle(conn, req)
+				if err == nil {
+					return nil
+				}
+
+				lock.RUnlock()
+				lock.Lock()
+				now := time.Now()
+				if intervalEnds.Before(time.Now()) {
+					intervalEnds = now.Add(period)
+					errorCounter = 0
+				}
+
+				errorCounter++
+				if errorCounter >= treshold {
+					state = Open
+				}
+				lock.Unlock()
+				return err
+			case Open:
+				lock.RUnlock()
+				if !semiOpenLock.TryLock() {
+					return ErrCircuitBreakerOpen
+				}
+
+				defer semiOpenLock.Unlock()
+
+				err := next.Handle(conn, req)
+				if err == nil {
+					lock.Lock()
+					state = Closed
+					errorCounter = 0
+				}
+
+				return err
+			default:
+				panic("Unknown state of circuit breaker")
+			}
+		})
+	}
+}