autobatch: thread-safe, debounce, max delay and implement Batching

Revamp the venerable autobatch in different ways:
- make it thread safe (test -race is happy)
- deduplicate writes (avoid writing the same key multiple time), which make it act as a debounce
- introduce a maximum delay before the write happen, to avoid keeping the pending writes in memory if the count trigger is not reached
- implement the Batching interface for compatibility with more usecases

Author: MichaelMure

autobatch/autobatch.go

@@ -1,9 +1,13 @@
 // Package autobatch provides a go-datastore implementation that
 // automatically batches together writes by holding puts in memory until
-// a certain threshold is met.
+// a certain threshold is met. It also acts as a debounce.
 package autobatch
 
 import (
+	"log"
+	"sync"
+	"time"
+
 	ds "github.com/ipfs/go-datastore"
 	dsq "github.com/ipfs/go-datastore/query"
 )
@@ -12,9 +16,13 @@ import (
 type Datastore struct {
 	child ds.Batching
 
-	// TODO: discuss making ds.Batch implement the full ds.Datastore interface
-	buffer           map[ds.Key]op
-	maxBufferEntries int
+	mu     sync.RWMutex
+	buffer map[ds.Key]op
+
+	maxWrite int
+	maxDelay time.Duration
+	newWrite chan struct{}
+	exit     chan struct{}
 }
 
 type op struct {
@@ -23,28 +31,79 @@ type op struct {
 }
 
 // NewAutoBatching returns a new datastore that automatically
-// batches writes using the given Batching datastore. The size
-// of the memory pool is given by size.
-func NewAutoBatching(d ds.Batching, size int) *Datastore {
-	return &Datastore{
-		child:            d,
-		buffer:           make(map[ds.Key]op, size),
-		maxBufferEntries: size,
+// batches writes using the given Batching datastore. The maximum number of
+// write before triggering a batch is given by maxWrite. The maximum delay
+// before triggering a batch is given by maxDelay.
+func NewAutoBatching(child ds.Batching, maxWrite int, maxDelay time.Duration) *Datastore {
+	d := &Datastore{
+		child:    child,
+		buffer:   make(map[ds.Key]op, maxWrite),
+		maxWrite: maxWrite,
+		maxDelay: maxDelay,
+		newWrite: make(chan struct{}),
+		exit:     make(chan struct{}),
+	}
+	go d.runBatcher()
+	return d
+}
+
+func (d *Datastore) addOp(key ds.Key, op op) {
+	d.mu.Lock()
+	d.buffer[key] = op
+	d.mu.Unlock()
+	d.newWrite <- struct{}{}
+}
+
+func (d *Datastore) runBatcher() {
+	var timer <-chan time.Time
+
+	write := func() {
+		timer = nil
+
+		b, err := d.prepareBatch(nil)
+		if err != nil {
+			log.Println(err)
+			return
+		}
+		err = b.Commit()
+		if err != nil {
+			log.Println(err)
+			return
+		}
+	}
+
+	for {
+		select {
+		case <-d.exit:
+			return
+		case <-timer:
+			write()
+		case <-d.newWrite:
+			d.mu.RLock()
+			ready := len(d.buffer)
+			d.mu.RUnlock()
+			if ready > d.maxWrite {
+				write()
+			}
+			if timer == nil {
+				timer = time.After(d.maxDelay)
+			}
+		}
 	}
 }
 
 // Delete deletes a key/value
 func (d *Datastore) Delete(k ds.Key) error {
-	d.buffer[k] = op{delete: true}
-	if len(d.buffer) > d.maxBufferEntries {
-		return d.Flush()
-	}
+	d.addOp(k, op{delete: true})
 	return nil
 }
 
 // Get retrieves a value given a key.
 func (d *Datastore) Get(k ds.Key) ([]byte, error) {
+	d.mu.RLock()
 	o, ok := d.buffer[k]
+	d.mu.RUnlock()
+
 	if ok {
 		if o.delete {
 			return nil, ds.ErrNotFound
@@ -57,69 +116,67 @@ func (d *Datastore) Get(k ds.Key) ([]byte, error) {
 
 // Put stores a key/value.
 func (d *Datastore) Put(k ds.Key, val []byte) error {
-	d.buffer[k] = op{value: val}
-	if len(d.buffer) > d.maxBufferEntries {
-		return d.Flush()
-	}
+	d.addOp(k, op{value: val})
 	return nil
 }
 
 // Sync flushes all operations on keys at or under the prefix
 // from the current batch to the underlying datastore
 func (d *Datastore) Sync(prefix ds.Key) error {
-	b, err := d.child.Batch()
+	b, err := d.prepareBatch(&prefix)
 	if err != nil {
 		return err
 	}
-
-	for k, o := range d.buffer {
-		if !(k.Equal(prefix) || k.IsDescendantOf(prefix)) {
-			continue
-		}
-
-		var err error
-		if o.delete {
-			err = b.Delete(k)
-		} else {
-			err = b.Put(k, o.value)
-		}
-		if err != nil {
-			return err
-		}
-
-		delete(d.buffer, k)
-	}
-
 	return b.Commit()
 }
 
 // Flush flushes the current batch to the underlying datastore.
 func (d *Datastore) Flush() error {
-	b, err := d.child.Batch()
+	b, err := d.prepareBatch(nil)
 	if err != nil {
 		return err
 	}
+	return b.Commit()
+}
+
+func (d *Datastore) prepareBatch(prefix *ds.Key) (ds.Batch, error) {
+	b, err := d.child.Batch()
+	if err != nil {
+		return nil, err
+	}
+
+	d.mu.Lock()
 
 	for k, o := range d.buffer {
+		if prefix != nil && !(k.Equal(*prefix) || k.IsDescendantOf(*prefix)) {
+			continue
+		}
+
 		var err error
 		if o.delete {
 			err = b.Delete(k)
 		} else {
 			err = b.Put(k, o.value)
 		}
 		if err != nil {
-			return err
+			d.mu.Unlock()
+			return nil, err
 		}
+
+		delete(d.buffer, k)
 	}
-	// clear out buffer
-	d.buffer = make(map[ds.Key]op, d.maxBufferEntries)
 
-	return b.Commit()
+	d.mu.Unlock()
+
+	return b, nil
 }
 
 // Has checks if a key is stored.
 func (d *Datastore) Has(k ds.Key) (bool, error) {
+	d.mu.RLock()
 	o, ok := d.buffer[k]
+	d.mu.RUnlock()
+
 	if ok {
 		return !o.delete, nil
 	}
@@ -129,7 +186,10 @@ func (d *Datastore) Has(k ds.Key) (bool, error) {
 
 // GetSize implements Datastore.GetSize
 func (d *Datastore) GetSize(k ds.Key) (int, error) {
+	d.mu.RLock()
 	o, ok := d.buffer[k]
+	d.mu.RUnlock()
+
 	if ok {
 		if o.delete {
 			return -1, ds.ErrNotFound
@@ -155,6 +215,18 @@ func (d *Datastore) DiskUsage() (uint64, error) {
 	return ds.DiskUsage(d.child)
 }
 
+func (d *Datastore) Batch() (ds.Batch, error) {
+	b, err := d.child.Batch()
+	if err != nil {
+		return nil, err
+	}
+	return &batch{
+		parent:   d,
+		child:    b,
+		toDelete: make(map[ds.Key]struct{}),
+	}, nil
+}
+
 func (d *Datastore) Close() error {
 	err1 := d.Flush()
 	err2 := d.child.Close()
@@ -164,5 +236,32 @@ func (d *Datastore) Close() error {
 	if err2 != nil {
 		return err2
 	}
+	close(d.exit)
+	close(d.newWrite)
 	return nil
 }
+
+type batch struct {
+	parent   *Datastore
+	child    ds.Batch
+	toDelete map[ds.Key]struct{}
+}
+
+func (b *batch) Put(key ds.Key, value []byte) error {
+	delete(b.toDelete, key)
+	return b.child.Put(key, value)
+}
+
+func (b *batch) Delete(key ds.Key) error {
+	b.toDelete[key] = struct{}{}
+	return b.child.Delete(key)
+}
+
+func (b *batch) Commit() error {
+	b.parent.mu.Lock()
+	for key, _ := range b.toDelete {
+		delete(b.parent.buffer, key)
+	}
+	b.parent.mu.Unlock()
+	return b.child.Commit()
+}

autobatch/autobatch_test.go

@@ -4,18 +4,20 @@ import (
 	"bytes"
 	"fmt"
 	"testing"
+	"time"
 
 	ds "github.com/ipfs/go-datastore"
+	"github.com/ipfs/go-datastore/sync"
 	dstest "github.com/ipfs/go-datastore/test"
 )
 
 func TestAutobatch(t *testing.T) {
-	dstest.SubtestAll(t, NewAutoBatching(ds.NewMapDatastore(), 16))
+	dstest.SubtestAll(t, NewAutoBatching(sync.MutexWrap(ds.NewMapDatastore()), 16, time.Second))
 }
 
 func TestFlushing(t *testing.T) {
-	child := ds.NewMapDatastore()
-	d := NewAutoBatching(child, 16)
+	child := sync.MutexWrap(ds.NewMapDatastore())
+	d := NewAutoBatching(child, 16, 500*time.Millisecond)
 
 	var keys []ds.Key
 	for i := 0; i < 16; i++ {
@@ -70,6 +72,9 @@ func TestFlushing(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	// flushing is async so we can rely on having it happening immediately
+	time.Sleep(100 * time.Millisecond)
+
 	// should be flushed now, try to get keys from child datastore
 	for _, k := range keys[:14] {
 		val, err := child.Get(k)
@@ -102,11 +107,29 @@ func TestFlushing(t *testing.T) {
 	if !bytes.Equal(val, v) {
 		t.Fatal("wrong value")
 	}
+
+	// let's test the maximum delay with a single key
+	key17 := ds.NewKey("test17")
+	err = d.Put(key17, v)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	time.Sleep(600 * time.Millisecond)
+
+	val, err = child.Get(key17)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !bytes.Equal(val, v) {
+		t.Fatal("wrong value")
+	}
 }
 
 func TestSync(t *testing.T) {
 	child := ds.NewMapDatastore()
-	d := NewAutoBatching(child, 100)
+	d := NewAutoBatching(child, 100, time.Second)
 
 	put := func(key ds.Key) {
 		if err := d.Put(key, []byte(key.String())); err != nil {

mount/mount_test.go

@@ -3,6 +3,7 @@ package mount_test
 import (
 	"errors"
 	"testing"
+	"time"
 
 	datastore "github.com/ipfs/go-datastore"
 	autobatch "github.com/ipfs/go-datastore/autobatch"
@@ -730,9 +731,9 @@ func TestNestedMountSync(t *testing.T) {
 	internalDSFooBar := datastore.NewMapDatastore()
 
 	m := mount.New([]mount.Mount{
-		{Prefix: datastore.NewKey("/foo"), Datastore: autobatch.NewAutoBatching(internalDSFoo, 10)},
-		{Prefix: datastore.NewKey("/foo/bar"), Datastore: autobatch.NewAutoBatching(internalDSFooBar, 10)},
-		{Prefix: datastore.NewKey("/"), Datastore: autobatch.NewAutoBatching(internalDSRoot, 10)},
+		{Prefix: datastore.NewKey("/foo"), Datastore: autobatch.NewAutoBatching(internalDSFoo, 10, 100*time.Millisecond)},
+		{Prefix: datastore.NewKey("/foo/bar"), Datastore: autobatch.NewAutoBatching(internalDSFooBar, 10, 100*time.Millisecond)},
+		{Prefix: datastore.NewKey("/"), Datastore: autobatch.NewAutoBatching(internalDSRoot, 10, 100*time.Millisecond)},
 	})
 
 	// Testing scenarios