// NewTask creates a new taskqueue.Task for the entity with the correct
// headers set to match those on the entity
func (l *Locker) NewTask(key *datastore.Key, entity Lockable, path string, params url.Values) *taskqueue.Task {
// prepare the lock entries
lock := entity.getLock()
lock.Timestamp = getTime()
lock.RequestID = ""
lock.Retries = 0
lock.Sequence++
json, _ := key.MarshalJSON()
// set task headers so that we can retrieve the matching entity
// and check that the executing task is the one we're expecting
task := taskqueue.NewPOSTTask(path, params)
task.Header.Set("X-Lock-Seq", strconv.Itoa(lock.Sequence))
task.Header.Set("X-Lock-Key", string(json))
if l.Host != "" {
task.Header.Set("Host", l.Host)
}
return task
}
func counterHandler(c context.Context, r *http.Request, key *datastore.Key, entity locker.Lockable) error {
counter := entity.(*Counter)
log.Debugf(c, "process: %d", counter.Sequence)
// simulate some processing work
time.Sleep(time.Duration(1) * time.Second)
if counter.Sequence == 5 {
// simulate a duplicate task execution by creating one ourselves
// needless to say, you wouldn't want to be doing this in practice
// but it should demonstrate that the locker prevents spurious
// task execution and guarantees the correct sequencing happens
json, _ := key.MarshalJSON()
t := taskqueue.NewPOSTTask("/process", nil)
t.Header.Set("X-Lock-Seq", "6")
t.Header.Set("X-Lock-Key", string(json))
taskqueue.Add(c, t, "")
}
if counter.Sequence < counter.Limit {
return l.Schedule(c, key, counter, "/process", nil)
}
return l.Complete(c, key, counter)
}