@@ -27,9 +27,14 @@ // Tx is the interface that defines the methods for the db transaction used in
 // the merkletree storage. Examples of the interface implementation can be
 // found at db/memory and db/leveldb directories.
 type Tx interface {
+	// Get retreives the value for the given key
+	// looking first in the content of the Tx, and
+	// then into the content of the Storage
 	Get([]byte) ([]byte, error)
-	Put(k, v []byte)
-	Add(Tx)
+	// Put sets the key & value into the Tx
+	Put(k, v []byte) error
+	// Add adds the given Tx into the Tx
+	Add(Tx) error
 	Commit() error
 	Close()
 }

@@ -130,16 +130,18 @@ return value, err
 }
 
 // Put saves a key:value into the db.Storage
-func (tx *LevelDbStorageTx) Put(k, v []byte) {
+func (tx *LevelDbStorageTx) Put(k, v []byte) error {
 	tx.cache.Put(db.Concat(tx.prefix, k[:]), v)
+	return nil
 }
 
 // Add implements the method Add of the interface db.Tx
-func (tx *LevelDbStorageTx) Add(atx db.Tx) {
+func (tx *LevelDbStorageTx) Add(atx db.Tx) error {
 	ldbtx := atx.(*LevelDbStorageTx)
 	for _, v := range ldbtx.cache {
 		tx.cache.Put(v.K, v.V)
 	}
+	return nil
 }
 
 // Commit implements the method Commit of the interface db.Tx

@@ -83,8 +83,9 @@ return nil, db.ErrNotFound
 }
 
 // Put implements the method Put of the interface db.Tx
-func (tx *MemoryStorageTx) Put(k, v []byte) {
+func (tx *MemoryStorageTx) Put(k, v []byte) error {
 	tx.kv.Put(db.Concat(tx.s.prefix, k), v)
+	return nil
 }
 
 // Commit implements the method Commit of the interface db.Tx
@@ -97,11 +98,12 @@ return nil
 }
 
 // Add implements the method Add of the interface db.Tx
-func (tx *MemoryStorageTx) Add(atx db.Tx) {
+func (tx *MemoryStorageTx) Add(atx db.Tx) error {
 	mstx := atx.(*MemoryStorageTx)
 	for _, v := range mstx.kv {
 		tx.kv.Put(v.K, v.V)
 	}
+	return nil
 }
 
 // Close implements the method Close of the interface db.Tx

@@ -17,10 +17,8 @@ }
 
 // PebbleStorageTx implements the db.Tx interface
 type PebbleStorageTx struct {
-	// FUTURE currently Tx is using the same strategy than in MemoryDB and
-	// LevelDB, in next iteration can be moved to Pebble Batch strategy
 	*PebbleStorage
-	cache db.KvMap
+	batch *pebble.Batch
 }
 
 // NewPebbleStorage returns a new PebbleStorage
@@ -72,7 +70,7 @@ }
 
 // NewTx implements the method NewTx of the interface db.Storage
 func (p *PebbleStorage) NewTx() (db.Tx, error) {
-	return &PebbleStorageTx{p, make(db.KvMap)}, nil
+	return &PebbleStorageTx{p, p.pdb.NewIndexedBatch()}, nil
 }
 
 // Get retreives a value from a key in the db.Storage
@@ -124,46 +122,34 @@ var err error
 
 	fullkey := db.Concat(tx.prefix, key)
 
-	if value, ok := tx.cache.Get(fullkey); ok {
-		return value, nil
-	}
-
-	value, closer, err := tx.pdb.Get(fullkey)
+	v, closer, err := tx.batch.Get(fullkey)
 	if err == pebble.ErrNotFound {
 		return nil, db.ErrNotFound
 	}
 	closer.Close()
 
-	return value, err
+	return v, err
 }
 
 // Put saves a key:value into the db.Storage
-func (tx *PebbleStorageTx) Put(k, v []byte) {
-	tx.cache.Put(db.Concat(tx.prefix, k[:]), v)
+func (tx *PebbleStorageTx) Put(k, v []byte) error {
+	return tx.batch.Set(db.Concat(tx.prefix, k[:]), v, nil)
 }
 
 // Add implements the method Add of the interface db.Tx
-func (tx *PebbleStorageTx) Add(atx db.Tx) {
-	ldbtx := atx.(*PebbleStorageTx)
-	for _, v := range ldbtx.cache {
-		tx.cache.Put(v.K, v.V)
-	}
+func (tx *PebbleStorageTx) Add(atx db.Tx) error {
+	patx := atx.(*PebbleStorageTx)
+	return tx.batch.Apply(patx.batch, nil)
 }
 
 // Commit implements the method Commit of the interface db.Tx
 func (tx *PebbleStorageTx) Commit() error {
-	batch := tx.PebbleStorage.pdb.NewBatch()
-	for _, v := range tx.cache {
-		_ = batch.Set(v.K, v.V, nil)
-	}
-
-	tx.cache = nil
-	return batch.Commit(nil)
+	return tx.batch.Commit(nil)
 }
 
 // Close implements the method Close of the interface db.Tx
 func (tx *PebbleStorageTx) Close() {
-	tx.cache = nil
+	_ = tx.batch.Close()
 }
 
 // Close implements the method Close of the interface db.Storage

@@ -28,7 +28,8 @@ value := []byte("data")
 
 	tx, err := sto.NewTx()
 	assert.Nil(t, err)
-	tx.Put(key, value)
+	err = tx.Put(key, value)
+	assert.Nil(t, err)
 	v, err := tx.Get(key)
 	assert.Nil(t, err)
 	assert.Equal(t, value, v)
@@ -53,7 +54,8 @@ // check within tx
 
 	sto1tx, err := sto1.NewTx()
 	assert.Nil(t, err)
-	sto1tx.Put(k, []byte{4, 5, 6})
+	err = sto1tx.Put(k, []byte{4, 5, 6})
+	assert.Nil(t, err)
 	v1, err := sto1tx.Get(k)
 	assert.Nil(t, err)
 	assert.Equal(t, v1, []byte{4, 5, 6})
@@ -61,7 +63,8 @@ assert.Nil(t, sto1tx.Commit())
 
 	sto2tx, err := sto2.NewTx()
 	assert.Nil(t, err)
-	sto2tx.Put(k, []byte{8, 9})
+	err = sto2tx.Put(k, []byte{8, 9})
+	assert.Nil(t, err)
 	v2, err := sto2tx.Get(k)
 	assert.Nil(t, err)
 	assert.Equal(t, v2, []byte{8, 9})
@@ -93,16 +96,22 @@ assert.Nil(t, err)
 	assert.Equal(t, 0, len(r))
 
 	sto1tx, _ := sto1.NewTx()
-	sto1tx.Put([]byte{1}, []byte{4})
-	sto1tx.Put([]byte{2}, []byte{5})
-	sto1tx.Put([]byte{3}, []byte{6})
+	err = sto1tx.Put([]byte{1}, []byte{4})
+	assert.Nil(t, err)
+	err = sto1tx.Put([]byte{2}, []byte{5})
+	assert.Nil(t, err)
+	err = sto1tx.Put([]byte{3}, []byte{6})
+	assert.Nil(t, err)
 	assert.Nil(t, sto1tx.Commit())
 
 	sto2 := sto.WithPrefix([]byte{2})
 	sto2tx, _ := sto2.NewTx()
-	sto2tx.Put([]byte{1}, []byte{7})
-	sto2tx.Put([]byte{2}, []byte{8})
-	sto2tx.Put([]byte{3}, []byte{9})
+	err = sto2tx.Put([]byte{1}, []byte{7})
+	assert.Nil(t, err)
+	err = sto2tx.Put([]byte{2}, []byte{8})
+	assert.Nil(t, err)
+	err = sto2tx.Put([]byte{3}, []byte{9})
+	assert.Nil(t, err)
 	assert.Nil(t, sto2tx.Commit())
 
 	r = []db.KV{}
@@ -136,14 +145,17 @@ sto1tx, err := sto1.NewTx()
 	if err != nil {
 		panic(err)
 	}
-	sto1tx.Put(k, []byte{4, 5, 6})
+	err = sto1tx.Put(k, []byte{4, 5, 6})
+	assert.Nil(t, err)
 	sto2tx, err := sto2.NewTx()
 	if err != nil {
 		panic(err)
 	}
-	sto2tx.Put(k, []byte{8, 9})
+	err = sto2tx.Put(k, []byte{8, 9})
+	assert.Nil(t, err)
 
-	sto1tx.Add(sto2tx)
+	err = sto1tx.Add(sto2tx)
+	assert.Nil(t, err)
 	assert.Nil(t, sto1tx.Commit())
 
 	// check outside tx
@@ -166,16 +178,22 @@ assert.Nil(t, err)
 	assert.Equal(t, 0, len(r1))
 
 	sto1tx, _ := sto1.NewTx()
-	sto1tx.Put([]byte{1}, []byte{4})
-	sto1tx.Put([]byte{2}, []byte{5})
-	sto1tx.Put([]byte{3}, []byte{6})
+	err = sto1tx.Put([]byte{1}, []byte{4})
+	assert.Nil(t, err)
+	err = sto1tx.Put([]byte{2}, []byte{5})
+	assert.Nil(t, err)
+	err = sto1tx.Put([]byte{3}, []byte{6})
+	assert.Nil(t, err)
 	assert.Nil(t, sto1tx.Commit())
 
 	sto2 := sto.WithPrefix([]byte{2})
 	sto2tx, _ := sto2.NewTx()
-	sto2tx.Put([]byte{1}, []byte{7})
-	sto2tx.Put([]byte{2}, []byte{8})
-	sto2tx.Put([]byte{3}, []byte{9})
+	err = sto2tx.Put([]byte{1}, []byte{7})
+	assert.Nil(t, err)
+	err = sto2tx.Put([]byte{2}, []byte{8})
+	assert.Nil(t, err)
+	err = sto2tx.Put([]byte{3}, []byte{9})
+	assert.Nil(t, err)
 	assert.Nil(t, sto2tx.Commit())
 
 	r, err := sto1.List(100)

@@ -141,7 +141,10 @@ if err != nil {
 			return nil, err
 		}
 		mt.rootKey = &HashZero
-		tx.Put(rootNodeValue, mt.rootKey[:])
+		err = tx.Put(rootNodeValue, mt.rootKey[:])
+		if err != nil {
+			return nil, err
+		}
 		err = tx.Commit()
 		if err != nil {
 			return nil, err
@@ -212,7 +215,10 @@ if err != nil {
 		return err
 	}
 	mt.rootKey = newRootKey
-	mt.dbInsert(tx, rootNodeValue, DBEntryTypeRoot, mt.rootKey[:])
+	err = mt.dbInsert(tx, rootNodeValue, DBEntryTypeRoot, mt.rootKey[:])
+	if err != nil {
+		return err
+	}
 
 	if err := tx.Commit(); err != nil {
 		return err
@@ -363,8 +369,8 @@ // Check that the node key doesn't already exist
 	if _, err := tx.Get(k[:]); err == nil {
 		return nil, ErrNodeKeyAlreadyExists
 	}
-	tx.Put(k[:], v)
-	return k, nil
+	err = tx.Put(k[:], v)
+	return k, err
 }
 
 // updateNode updates an existing node in the MT.  Empty nodes are not stored
@@ -382,8 +388,8 @@ if err != nil {
 		return nil, err
 	}
 	v := n.Value()
-	tx.Put(k[:], v)
-	return k, nil
+	err = tx.Put(k[:], v)
+	return k, err
 }
 
 // Get returns the value of the leaf for the given key
@@ -487,7 +493,10 @@ if err != nil {
 					return nil, err
 				}
 				mt.rootKey = newRootKey
-				mt.dbInsert(tx, rootNodeValue, DBEntryTypeRoot, mt.rootKey[:])
+				err = mt.dbInsert(tx, rootNodeValue, DBEntryTypeRoot, mt.rootKey[:])
+				if err != nil {
+					return nil, err
+				}
 				cp.NewRoot = newRootKey
 				if err := tx.Commit(); err != nil {
 					return nil, err
@@ -580,14 +589,20 @@ // rmAndUpload removes the key, and goes up until the root updating all the nodes with the new values.
 func (mt *MerkleTree) rmAndUpload(tx db.Tx, path []bool, kHash *Hash, siblings []*Hash) error {
 	if len(siblings) == 0 {
 		mt.rootKey = &HashZero
-		mt.dbInsert(tx, rootNodeValue, DBEntryTypeRoot, mt.rootKey[:])
+		err := mt.dbInsert(tx, rootNodeValue, DBEntryTypeRoot, mt.rootKey[:])
+		if err != nil {
+			return err
+		}
 		return tx.Commit()
 	}
 
 	toUpload := siblings[len(siblings)-1]
 	if len(siblings) < 2 { //nolint:gomnd
 		mt.rootKey = siblings[0]
-		mt.dbInsert(tx, rootNodeValue, DBEntryTypeRoot, mt.rootKey[:])
+		err := mt.dbInsert(tx, rootNodeValue, DBEntryTypeRoot, mt.rootKey[:])
+		if err != nil {
+			return err
+		}
 		return tx.Commit()
 	}
 	for i := len(siblings) - 2; i >= 0; i-- { //nolint:gomnd
@@ -608,13 +623,19 @@ if err != nil {
 				return err
 			}
 			mt.rootKey = newRootKey
-			mt.dbInsert(tx, rootNodeValue, DBEntryTypeRoot, mt.rootKey[:])
+			err = mt.dbInsert(tx, rootNodeValue, DBEntryTypeRoot, mt.rootKey[:])
+			if err != nil {
+				return err
+			}
 			break
 		}
 		// if i==0 (root position), stop and store the sibling of the deleted leaf as root
 		if i == 0 {
 			mt.rootKey = toUpload
-			mt.dbInsert(tx, rootNodeValue, DBEntryTypeRoot, mt.rootKey[:])
+			err := mt.dbInsert(tx, rootNodeValue, DBEntryTypeRoot, mt.rootKey[:])
+			if err != nil {
+				return err
+			}
 			break
 		}
 	}
@@ -650,9 +671,9 @@ }
 
 // dbInsert is a helper function to insert a node into a key in an open db
 // transaction.
-func (mt *MerkleTree) dbInsert(tx db.Tx, k []byte, t NodeType, data []byte) {
+func (mt *MerkleTree) dbInsert(tx db.Tx, k []byte, t NodeType, data []byte) error {
 	v := append([]byte{byte(t)}, data...)
-	tx.Put(k, v)
+	return tx.Put(k, v)
 }
 
 // GetNode gets a node by key from the MT.  Empty nodes are not stored in the