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package document
import (
"container/list"
"fmt"
"io"
"sync"
"sync/atomic"
"time"
)
const maxAttempts = 10
// Dispatcher dispatches documents from a queue to a Feeder.
type Dispatcher struct {
feeder Feeder
throttler Throttler
circuitBreaker CircuitBreaker
stats Stats
started bool
ready chan Id
results chan Result
inflight map[string]*documentGroup
inflightCount int64
errWriter io.Writer
mu sync.RWMutex
wg sync.WaitGroup
resultWg sync.WaitGroup
}
// documentOp represents a document operation and the number of times it has been attempted.
type documentOp struct {
document Document
attempts int
}
// documentGroup holds document operations which share an ID, and must be dispatched in order.
type documentGroup struct {
ops *list.List
mu sync.Mutex
}
func (g *documentGroup) add(op documentOp, first bool) {
g.mu.Lock()
defer g.mu.Unlock()
if g.ops == nil {
g.ops = list.New()
}
if first {
g.ops.PushFront(op)
} else {
g.ops.PushBack(op)
}
}
func NewDispatcher(feeder Feeder, throttler Throttler, breaker CircuitBreaker, errWriter io.Writer) *Dispatcher {
d := &Dispatcher{
feeder: feeder,
throttler: throttler,
circuitBreaker: breaker,
inflight: make(map[string]*documentGroup),
errWriter: errWriter,
}
d.start()
return d
}
func (d *Dispatcher) sendDocumentIn(group *documentGroup) {
group.mu.Lock()
defer group.mu.Unlock()
defer d.releaseSlot()
first := group.ops.Front()
if first == nil {
panic("sending from empty document group, this should not happen")
}
op := group.ops.Remove(first).(documentOp)
op.attempts++
result := d.feeder.Send(op.document)
d.results <- result
if d.shouldRetry(op, result) {
d.enqueue(op)
}
}
func (d *Dispatcher) shouldRetry(op documentOp, result Result) bool {
if result.HTTPStatus/100 == 2 || result.HTTPStatus == 404 || result.HTTPStatus == 412 {
d.throttler.Success()
d.circuitBreaker.Success()
return false
}
if result.HTTPStatus == 429 || result.HTTPStatus == 503 {
fmt.Fprintf(d.errWriter, "feed: %s was throttled with status %d: retrying\n", op.document, result.HTTPStatus)
d.throttler.Throttled(atomic.LoadInt64(&d.inflightCount))
return true
}
if result.Err != nil || result.HTTPStatus == 500 || result.HTTPStatus == 502 || result.HTTPStatus == 504 {
retry := op.attempts <= maxAttempts
msg := "feed: " + op.document.String() + " failed with "
if result.Err != nil {
msg += "error " + result.Err.Error()
} else {
msg += fmt.Sprintf("status %d", result.HTTPStatus)
}
if retry {
msg += ": retrying"
} else {
msg += fmt.Sprintf(": giving up after %d attempts", maxAttempts)
}
fmt.Fprintln(d.errWriter, msg)
d.circuitBreaker.Error(fmt.Errorf("request failed with status %d", result.HTTPStatus))
if retry {
return true
}
}
return false
}
func (d *Dispatcher) start() {
d.mu.Lock()
defer d.mu.Unlock()
if d.started {
return
}
d.ready = make(chan Id, 4096)
d.results = make(chan Result, 4096)
d.started = true
d.wg.Add(1)
go func() {
defer d.wg.Done()
d.readDocuments()
}()
d.resultWg.Add(1)
go func() {
defer d.resultWg.Done()
d.readResults()
}()
}
func (d *Dispatcher) readDocuments() {
for id := range d.ready {
d.mu.RLock()
group := d.inflight[id.String()]
d.mu.RUnlock()
d.wg.Add(1)
go func() {
defer d.wg.Done()
d.sendDocumentIn(group)
}()
}
}
func (d *Dispatcher) readResults() {
for result := range d.results {
d.stats.Add(result.Stats)
}
}
func (d *Dispatcher) enqueue(op documentOp) error {
d.mu.Lock()
if !d.started {
return fmt.Errorf("dispatcher is closed")
}
group, ok := d.inflight[op.document.Id.String()]
if !ok {
group = &documentGroup{}
d.inflight[op.document.Id.String()] = group
}
d.mu.Unlock()
group.add(op, op.attempts > 0)
d.enqueueWithSlot(op.document.Id)
return nil
}
func (d *Dispatcher) enqueueWithSlot(id Id) {
d.acquireSlot()
d.ready <- id
d.throttler.Sent()
}
func (d *Dispatcher) acquireSlot() {
for atomic.LoadInt64(&d.inflightCount) >= d.throttler.TargetInflight() {
time.Sleep(time.Millisecond)
}
atomic.AddInt64(&d.inflightCount, 1)
}
func (d *Dispatcher) releaseSlot() { atomic.AddInt64(&d.inflightCount, -1) }
func closeAndWait[T any](ch chan T, wg *sync.WaitGroup, d *Dispatcher, markClosed bool) {
d.mu.Lock()
if d.started {
close(ch)
if markClosed {
d.started = false
}
}
d.mu.Unlock()
wg.Wait()
}
func (d *Dispatcher) Enqueue(doc Document) error { return d.enqueue(documentOp{document: doc}) }
func (d *Dispatcher) Stats() Stats { return d.stats }
// Close closes the dispatcher and waits for all inflight operations to complete.
func (d *Dispatcher) Close() error {
closeAndWait(d.ready, &d.wg, d, false)
closeAndWait(d.results, &d.resultWg, d, true)
return nil
}
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