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package hystrix
import (
"context"
"fmt"
"sync"
"time"
)
type runFunc func() error
type fallbackFunc func(error) error
type runFuncC func(context.Context) error
type fallbackFuncC func(context.Context, error) error
// A CircuitError is an error which models various failure states of execution,
// such as the circuit being open or a timeout.
type CircuitError struct {
Message string
}
func (e CircuitError) Error() string {
return "hystrix: " + e.Message
}
// command models the state used for a single execution on a circuit. "hystrix command" is commonly
// used to describe the pairing of your run/fallback functions with a circuit.
type command struct {
sync.Mutex
ticket *struct{}
start time.Time
errChan chan error
finished chan bool
circuit *CircuitBreaker
run runFuncC
fallback fallbackFuncC
runDuration time.Duration
events []string
}
var (
// ErrMaxConcurrency occurs when too many of the same named command are executed at the same time.
ErrMaxConcurrency = CircuitError{Message: "max concurrency"}
// ErrCircuitOpen returns when an execution attempt "short circuits". This happens due to the circuit being measured as unhealthy.
ErrCircuitOpen = CircuitError{Message: "circuit open"}
// ErrTimeout occurs when the provided function takes too long to execute.
ErrTimeout = CircuitError{Message: "timeout"}
)
// Go runs your function while tracking the health of previous calls to it.
// If your function begins slowing down or failing repeatedly, we will block
// new calls to it for you to give the dependent service time to repair.
//
// Define a fallback function if you want to define some code to execute during outages.
func Go(name string, run runFunc, fallback fallbackFunc) chan error {
runC := func(ctx context.Context) error {
return run()
}
var fallbackC fallbackFuncC
if fallback != nil {
fallbackC = func(ctx context.Context, err error) error {
return fallback(err)
}
}
return GoC(context.Background(), name, runC, fallbackC)
}
// GoC runs your function while tracking the health of previous calls to it.
// If your function begins slowing down or failing repeatedly, we will block
// new calls to it for you to give the dependent service time to repair.
//
// Define a fallback function if you want to define some code to execute during outages.
func GoC(ctx context.Context, name string, run runFuncC, fallback fallbackFuncC) chan error {
cmd := &command{
run: run,
fallback: fallback,
start: time.Now(),
errChan: make(chan error, 1),
finished: make(chan bool, 1),
}
// dont have methods with explicit params and returns
// let data come in and out naturally, like with any closure
// explicit error return to give place for us to kill switch the operation (fallback)
circuit, _, err := GetCircuit(name)
if err != nil {
cmd.errChan <- err
return cmd.errChan
}
cmd.circuit = circuit
ticketCond := sync.NewCond(cmd)
ticketChecked := false
// When the caller extracts error from returned errChan, it's assumed that
// the ticket's been returned to executorPool. Therefore, returnTicket() can
// not run after cmd.errorWithFallback().
returnTicket := func() {
cmd.Lock()
// Avoid releasing before a ticket is acquired.
for !ticketChecked {
ticketCond.Wait()
}
cmd.circuit.executorPool.Return(cmd.ticket)
cmd.Unlock()
}
// Shared by the following two goroutines. It ensures only the faster
// goroutine runs errWithFallback() and reportAllEvent().
returnOnce := &sync.Once{}
reportAllEvent := func() {
err := cmd.circuit.ReportEvent(cmd.events, cmd.start, cmd.runDuration)
if err != nil {
log.Printf(err.Error())
}
}
go func() {
defer func() { cmd.finished <- true }()
// Circuits get opened when recent executions have shown to have a high error rate.
// Rejecting new executions allows backends to recover, and the circuit will allow
// new traffic when it feels a healthly state has returned.
if !cmd.circuit.AllowRequest() {
cmd.Lock()
// It's safe for another goroutine to go ahead releasing a nil ticket.
ticketChecked = true
ticketCond.Signal()
cmd.Unlock()
returnOnce.Do(func() {
returnTicket()
cmd.errorWithFallback(ctx, ErrCircuitOpen)
reportAllEvent()
})
return
}
// As backends falter, requests take longer but don't always fail.
//
// When requests slow down but the incoming rate of requests stays the same, you have to
// run more at a time to keep up. By controlling concurrency during these situations, you can
// shed load which accumulates due to the increasing ratio of active commands to incoming requests.
cmd.Lock()
select {
case cmd.ticket = <-circuit.executorPool.Tickets:
ticketChecked = true
ticketCond.Signal()
cmd.Unlock()
default:
ticketChecked = true
ticketCond.Signal()
cmd.Unlock()
returnOnce.Do(func() {
returnTicket()
cmd.errorWithFallback(ctx, ErrMaxConcurrency)
reportAllEvent()
})
return
}
runStart := time.Now()
runErr := run(ctx)
returnOnce.Do(func() {
defer reportAllEvent()
cmd.runDuration = time.Since(runStart)
returnTicket()
if runErr != nil {
cmd.errorWithFallback(ctx, runErr)
return
}
cmd.reportEvent("success")
})
}()
go func() {
timer := time.NewTimer(getSettings(name).Timeout)
defer timer.Stop()
select {
case <-cmd.finished:
// returnOnce has been executed in another goroutine
case <-ctx.Done():
returnOnce.Do(func() {
returnTicket()
cmd.errorWithFallback(ctx, ctx.Err())
reportAllEvent()
})
return
case <-timer.C:
returnOnce.Do(func() {
returnTicket()
cmd.errorWithFallback(ctx, ErrTimeout)
reportAllEvent()
})
return
}
}()
return cmd.errChan
}
// Do runs your function in a synchronous manner, blocking until either your function succeeds
// or an error is returned, including hystrix circuit errors
func Do(name string, run runFunc, fallback fallbackFunc) error {
runC := func(ctx context.Context) error {
return run()
}
var fallbackC fallbackFuncC
if fallback != nil {
fallbackC = func(ctx context.Context, err error) error {
return fallback(err)
}
}
return DoC(context.Background(), name, runC, fallbackC)
}
// DoC runs your function in a synchronous manner, blocking until either your function succeeds
// or an error is returned, including hystrix circuit errors
func DoC(ctx context.Context, name string, run runFuncC, fallback fallbackFuncC) error {
done := make(chan struct{}, 1)
r := func(ctx context.Context) error {
err := run(ctx)
if err != nil {
return err
}
done <- struct{}{}
return nil
}
f := func(ctx context.Context, e error) error {
err := fallback(ctx, e)
if err != nil {
return err
}
done <- struct{}{}
return nil
}
var errChan chan error
if fallback == nil {
errChan = GoC(ctx, name, r, nil)
} else {
errChan = GoC(ctx, name, r, f)
}
select {
case <-done:
return nil
case err := <-errChan:
return err
}
}
func (c *command) reportEvent(eventType string) {
c.Lock()
defer c.Unlock()
c.events = append(c.events, eventType)
}
// errorWithFallback triggers the fallback while reporting the appropriate metric events.
func (c *command) errorWithFallback(ctx context.Context, err error) {
eventType := "failure"
if err == ErrCircuitOpen {
eventType = "short-circuit"
} else if err == ErrMaxConcurrency {
eventType = "rejected"
} else if err == ErrTimeout {
eventType = "timeout"
} else if err == context.Canceled {
eventType = "context_canceled"
} else if err == context.DeadlineExceeded {
eventType = "context_deadline_exceeded"
}
c.reportEvent(eventType)
fallbackErr := c.tryFallback(ctx, err)
if fallbackErr != nil {
c.errChan <- fallbackErr
}
}
func (c *command) tryFallback(ctx context.Context, err error) error {
if c.fallback == nil {
// If we don't have a fallback return the original error.
return err
}
fallbackErr := c.fallback(ctx, err)
if fallbackErr != nil {
c.reportEvent("fallback-failure")
return fmt.Errorf("fallback failed with '%v'. run error was '%v'", fallbackErr, err)
}
c.reportEvent("fallback-success")
return nil
}