batch: add external batching engine for bbolt operations

batch/scheduler.go

@@ -0,0 +1,103 @@
+package batch
+
+import (
+	"sync"
+	"time"
+
+	"github.com/lightningnetwork/lnd/channeldb/kvdb"
+)
+
+// TimeScheduler is a batching engine that executes requests within a fixed
+// horizon. When the first request is received, a TimeScheduler waits a
+// configurable duration for other concurrent requests to join the batch. Once
+// this time has elapsed, the batch is closed and executed. Subsequent requests
+// are then added to a new batch which undergoes the same process.
+type TimeScheduler struct {
+	db       kvdb.Backend
+	locker   sync.Locker
+	duration time.Duration
+
+	mu sync.Mutex
+	b  *batch
+}
+
+// NewTimeScheduler initializes a new TimeScheduler with a fixed duration at
+// which to schedule batches. If the operation needs to modify a higher-level
+// cache, the cache's lock should be provided to so that external consistency
+// can be maintained, as successful db operations will cause a request's
+// OnCommit method to be executed while holding this lock.
+func NewTimeScheduler(db kvdb.Backend, locker sync.Locker,
+	duration time.Duration) *TimeScheduler {
+
+	return &TimeScheduler{
+		db:       db,
+		locker:   locker,
+		duration: duration,
+	}
+}
+
+// Execute schedules the provided request for batch execution along with other
+// concurrent requests. The request will be executed within a fixed horizon,
+// parameterizeed by the duration of the scheduler. The error from the
+// underlying operation is returned to the caller.
+//
+// NOTE: Part of the Scheduler interface.
+func (s *TimeScheduler) Execute(r *Request) error {
+	req := request{
+		Request: r,
+		errChan: make(chan error, 1),
+	}
+
+	// Add the request to the current batch. If the batch has been cleared
+	// or no batch exists, create a new one.
+	s.mu.Lock()
+	if s.b == nil {
+		s.b = &batch{
+			db:     s.db,
+			clear:  s.clear,
+			locker: s.locker,
+		}
+		time.AfterFunc(s.duration, s.b.trigger)
+	}
+	s.b.reqs = append(s.b.reqs, &req)
+	s.mu.Unlock()
+
+	// Wait for the batch to process the request. If the batch didn't
+	// ask us to execute the request individually, simply return the error.
+	err := <-req.errChan
+	if err != errSolo {
+		return err
+	}
+
+	// Obtain exclusive access to the cache if this scheduler needs to
+	// modify the cache in OnCommit.
+	if s.locker != nil {
+		s.locker.Lock()
+		defer s.locker.Unlock()
+	}
+
+	// Otherwise, run the request on its own.
+	commitErr := kvdb.Update(s.db, req.Update, func() {
+		if req.Reset != nil {
+			req.Reset()
+		}
+	})
+
+	// Finally, return the commit error directly or execute the OnCommit
+	// closure with the commit error if present.
+	if req.OnCommit != nil {
+		return req.OnCommit(commitErr)
+	}
+
+	return commitErr
+}
+
+// clear resets the scheduler's batch to nil so that no more requests can be
+// added.
+func (s *TimeScheduler) clear(b *batch) {
+	s.mu.Lock()
+	if s.b == b {
+		s.b = nil
+	}
+	s.mu.Unlock()
+}