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package crawl
import (
"bytes"
"encoding/binary"
"errors"
"math/rand"
"sync/atomic"
"time"
"github.com/jmhodges/levigo"
)
type queue struct {
db *gobDB
numActive int32
}
var (
queuePrefix = []byte("queue")
activePrefix = []byte("queue_active")
queueKeySep = []byte{'/'}
queueKeySepP1 = []byte{'/' + 1}
)
type queuePair struct {
key []byte
URL string
Depth int
}
// Scan the pending queue and send items on 'ch'. Returns an error
// when the queue is empty (work is done).
func (q *queue) Scan(ch chan<- queuePair) error {
snap := q.db.NewSnapshot()
defer q.db.ReleaseSnapshot(snap)
ro := levigo.NewReadOptions()
ro.SetSnapshot(snap)
defer ro.Close()
n := 0
startKey, endKey := queueScanRange()
iter := q.db.NewRangeIterator(ro, startKey, endKey)
for ; iter.Valid(); iter.Next() {
var p queuePair
if err := iter.Value(&p); err != nil {
continue
}
p.key = iter.Key()
q.acquire(p)
ch <- p
n++
}
if n == 0 && q.numActive == 0 {
return errors.New("EOF")
}
return nil
}
// Add an item to the pending work queue.
func (q *queue) Add(wb *levigo.WriteBatch, urlStr string, depth int, when time.Time) {
t := uint64(when.UnixNano())
qkey := bytes.Join([][]byte{queuePrefix, encodeUint64(t), encodeUint64(uint64(rand.Int63()))}, queueKeySep)
q.db.PutObjBatch(wb, qkey, &queuePair{URL: urlStr, Depth: depth})
}
func (q *queue) acquire(qp queuePair) {
wb := levigo.NewWriteBatch()
defer wb.Close()
q.db.PutObjBatch(wb, activeQueueKey(qp.key), qp)
wb.Delete(qp.key)
wo := levigo.NewWriteOptions()
defer wo.Close()
q.db.Write(wo, wb)
atomic.AddInt32(&q.numActive, 1)
}
// Release an item from the queue. Processing for this item is done.
func (q *queue) Release(wb *levigo.WriteBatch, qp queuePair) {
wb.Delete(activeQueueKey(qp.key))
atomic.AddInt32(&q.numActive, -1)
}
// Retry processing this item at a later time.
func (q *queue) Retry(wb *levigo.WriteBatch, qp queuePair, delay time.Duration) {
wb.Delete(activeQueueKey(qp.key))
q.Add(wb, qp.URL, qp.Depth, time.Now().Add(delay))
atomic.AddInt32(&q.numActive, -1)
}
// Recover moves all active tasks to the pending queue. To be
// called at startup to recover tasks that were active when the
// previous run terminated.
func (q *queue) Recover() {
wb := levigo.NewWriteBatch()
defer wb.Close()
ro := levigo.NewReadOptions()
defer ro.Close()
prefix := bytes.Join([][]byte{activePrefix, []byte{}}, queueKeySep)
iter := q.db.NewPrefixIterator(ro, prefix)
for ; iter.Valid(); iter.Next() {
var p queuePair
if err := iter.Value(&p); err != nil {
continue
}
p.key = iter.Key()[len(activePrefix)+1:]
q.db.PutObjBatch(wb, p.key, &p)
wb.Delete(iter.Key())
}
wo := levigo.NewWriteOptions()
defer wo.Close()
q.db.Write(wo, wb)
}
func encodeUint64(n uint64) []byte {
var b bytes.Buffer
binary.Write(&b, binary.BigEndian, n)
return b.Bytes()
}
func activeQueueKey(key []byte) []byte {
return bytes.Join([][]byte{activePrefix, key}, queueKeySep)
}
func queueScanRange() ([]byte, []byte) {
tlim := uint64(time.Now().UnixNano() + 1)
startKey := bytes.Join([][]byte{queuePrefix, []byte{}}, queueKeySep)
endKey := bytes.Join([][]byte{queuePrefix, encodeUint64(tlim)}, queueKeySep)
return startKey, endKey
}
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