aspects: optimise aspect tx Get

Cache a copy of the transaction's databag so that a new one doesn't have to be created and all the deltas applied. 

1 files changed, 23 insertions, 9 deletions

diff --git a/aspects/transaction.go b/aspects/transaction.go
index dde36b2dad..0eaa2eb717 100644
--- a/aspects/transaction.go
+++ b/aspects/transaction.go
@@ -28,9 +28,12 @@ type DatabagWrite func(JSONDataBag) error
 // Transaction performs read and writes to a databag in an atomic way.
 type Transaction struct {
	pristine JSONDataBag
-	deltas []map[string]interface{}
	schema Schema
 
+	modified JSONDataBag
+	deltas []map[string]interface{}
+	appliedDeltas int
+
	readDatabag DatabagRead
	writeDatabag DatabagWrite
	mu sync.RWMutex
@@ -65,17 +68,26 @@ func (t *Transaction) Get(path string, value interface{}) error {
	t.mu.RLock()
	defer t.mu.RUnlock()
 
-	// if there are changes, create a copy before applying (for isolation)
-	bag := t.pristine
-	if len(t.deltas) != 0 {
-	bag = t.pristine.Copy()
+	// if there aren't any changes, just use the pristine bag
+	if len(t.deltas) == 0 {
+	return t.pristine.Get(path, value)
+	}
 
-	if err := applyDeltas(bag, t.deltas); err != nil {
-	return err
-	}
+	// if there are changes, use a cached bag with modifications to do the Get
+	if t.modified == nil {
+	t.modified = t.pristine.Copy()
+	t.appliedDeltas = 0
+	}
+
+	// apply new changes since the last get
+	if err := applyDeltas(t.modified, t.deltas[t.appliedDeltas:]); err != nil {
+	t.modified = nil
+	t.appliedDeltas = 0
+	return err
	}
+	t.appliedDeltas = len(t.deltas)
 
-	return bag.Get(path, value)
+	return t.modified.Get(path, value)
 }
 
 // Commit applies the previous writes and validates the final databag. If any
@@ -113,7 +125,9 @@ func (t *Transaction) Commit() error {
	}
 
	t.pristine = pristine
+	t.modified = nil
	t.deltas = nil
+	t.appliedDeltas = 0
	return nil
 }