Add bi-directional bindings support (#350)

* add bi-directional bindings

* added invoke response

* added explicit types

* change operationtype to operationkind

bindings/Readme.md

@@ -24,10 +24,14 @@ type InputBinding interface {
 
 Output binding:
 
+An output binding can be used to invoke an external system and also to return data from it.
+Each output binding can decide which operations it supports. This information is communicated to the caller via the `Operations()` method.
+
 ```go
 type OutputBinding interface {
 	Init(metadata Metadata) error
-	Write(req *WriteRequest) error
+	Invoke(req *InvokeRequest) error
+	Operations() []OperationKind
 }
 ```
 A spec is also needed in [Dapr docs](https://github.com/dapr/docs/tree/master/reference/specs/bindings).

bindings/alicloud/oss/oss.go

@@ -50,7 +50,11 @@ func (s *AliCloudOSS) Init(metadata bindings.Metadata) error {
 	return nil
 }
 
-func (s *AliCloudOSS) Write(req *bindings.WriteRequest) error {
+func (s *AliCloudOSS) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (s *AliCloudOSS) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	key := ""
 	if val, ok := req.Metadata["key"]; ok && val != "" {
 		key = val
@@ -63,16 +67,16 @@ func (s *AliCloudOSS) Write(req *bindings.WriteRequest) error {
 	bucket, err := s.client.Bucket(s.metadata.Bucket)
 
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	// Upload a byte array.
 	err = bucket.PutObject(key, bytes.NewReader(req.Data))
 	if err != nil {
-		return err
+		return nil, err
 	}
 
-	return err
+	return nil, err
 }
 
 func (s *AliCloudOSS) parseMetadata(metadata bindings.Metadata) (*ossMetadata, error) {

bindings/aws/dynamodb/dynamodb.go

@@ -55,16 +55,20 @@ func (d *DynamoDB) Init(metadata bindings.Metadata) error {
 	return nil
 }
 
-func (d *DynamoDB) Write(req *bindings.WriteRequest) error {
+func (d *DynamoDB) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (d *DynamoDB) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	var obj interface{}
 	err := json.Unmarshal(req.Data, &obj)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	item, err := dynamodbattribute.MarshalMap(obj)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	input := &dynamodb.PutItemInput{
@@ -74,10 +78,10 @@ func (d *DynamoDB) Write(req *bindings.WriteRequest) error {
 
 	_, err = d.client.PutItem(input)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
-	return nil
+	return nil, nil
 }
 
 func (d *DynamoDB) getDynamoDBMetadata(spec bindings.Metadata) (*dynamoDBMetadata, error) {

bindings/aws/kinesis/kinesis.go

@@ -122,7 +122,11 @@ func (a *AWSKinesis) Init(metadata bindings.Metadata) error {
 	return nil
 }
 
-func (a *AWSKinesis) Write(req *bindings.WriteRequest) error {
+func (a *AWSKinesis) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (a *AWSKinesis) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	partitionKey := req.Metadata[partitionKeyName]
 	if partitionKey == "" {
 		partitionKey = uuid.New().String()
@@ -132,7 +136,7 @@ func (a *AWSKinesis) Write(req *bindings.WriteRequest) error {
 		Data:         req.Data,
 		PartitionKey: &partitionKey,
 	})
-	return err
+	return nil, err
 }
 
 func (a *AWSKinesis) Read(handler func(*bindings.ReadResponse) error) error {

bindings/aws/s3/s3.go

@@ -54,7 +54,11 @@ func (s *AWSS3) Init(metadata bindings.Metadata) error {
 	return nil
 }
 
-func (s *AWSS3) Write(req *bindings.WriteRequest) error {
+func (s *AWSS3) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (s *AWSS3) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	key := ""
 	if val, ok := req.Metadata["key"]; ok && val != "" {
 		key = val
@@ -69,7 +73,7 @@ func (s *AWSS3) Write(req *bindings.WriteRequest) error {
 		Key:    aws.String(key),
 		Body:   r,
 	})
-	return err
+	return nil, err
 }
 
 func (s *AWSS3) parseMetadata(metadata bindings.Metadata) (*s3Metadata, error) {

bindings/aws/sns/sns.go

@@ -81,11 +81,15 @@ func (a *AWSSNS) getClient(metadata *snsMetadata) (*sns.SNS, error) {
 	return c, nil
 }
 
-func (a *AWSSNS) Write(req *bindings.WriteRequest) error {
+func (a *AWSSNS) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (a *AWSSNS) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	var payload dataPayload
 	err := json.Unmarshal(req.Data, &payload)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	msg := fmt.Sprintf("%v", payload.Message)
@@ -99,7 +103,7 @@ func (a *AWSSNS) Write(req *bindings.WriteRequest) error {
 
 	_, err = a.client.Publish(input)
 	if err != nil {
-		return err
+		return nil, err
 	}
-	return nil
+	return nil, nil
 }

bindings/aws/sqs/sqs.go

@@ -62,13 +62,17 @@ func (a *AWSSQS) Init(metadata bindings.Metadata) error {
 	return nil
 }
 
-func (a *AWSSQS) Write(req *bindings.WriteRequest) error {
+func (a *AWSSQS) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (a *AWSSQS) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	msgBody := string(req.Data)
 	_, err := a.Client.SendMessage(&sqs.SendMessageInput{
 		MessageBody: &msgBody,
 		QueueUrl:    a.QueueURL,
 	})
-	return err
+	return nil, err
 }
 
 func (a *AWSSQS) Read(handler func(*bindings.ReadResponse) error) error {

bindings/azure/blobstorage/blobstorage.go

@@ -90,7 +90,11 @@ func (a *AzureBlobStorage) parseMetadata(metadata bindings.Metadata) (*blobStora
 	return &m, nil
 }
 
-func (a *AzureBlobStorage) Write(req *bindings.WriteRequest) error {
+func (a *AzureBlobStorage) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (a *AzureBlobStorage) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	name := ""
 	if val, ok := req.Metadata[blobName]; ok && val != "" {
 		name = val
@@ -108,7 +112,7 @@ func (a *AzureBlobStorage) Write(req *bindings.WriteRequest) error {
 	if val, ok := req.Metadata[contentMD5]; ok && val != "" {
 		sDec, err := b64.StdEncoding.DecodeString(val)
 		if err != nil || len(sDec) != 16 {
-			return fmt.Errorf("the MD5 value specified in Content MD5 is invalid, MD5 value must be 128 bits and base64 encoded")
+			return nil, fmt.Errorf("the MD5 value specified in Content MD5 is invalid, MD5 value must be 128 bits and base64 encoded")
 		}
 		blobHTTPHeaders.ContentMD5 = sDec
 		delete(req.Metadata, contentMD5)
@@ -138,5 +142,5 @@ func (a *AzureBlobStorage) Write(req *bindings.WriteRequest) error {
 		Metadata:        req.Metadata,
 		BlobHTTPHeaders: blobHTTPHeaders,
 	})
-	return err
+	return nil, err
 }

bindings/azure/cosmosdb/cosmosdb.go

@@ -97,24 +97,28 @@ func (c *CosmosDB) parseMetadata(metadata bindings.Metadata) (*cosmosDBCredentia
 	return &creds, nil
 }
 
-func (c *CosmosDB) Write(req *bindings.WriteRequest) error {
+func (c *CosmosDB) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (c *CosmosDB) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	var obj interface{}
 	err := json.Unmarshal(req.Data, &obj)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	val, err := c.getPartitionKeyValue(c.partitionKey, obj)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	_, err = c.client.CreateDocument(c.collection.Self, obj, documentdb.PartitionKey(val))
 	if err != nil {
-		return err
+		return nil, err
 	}
 
-	return nil
+	return nil, nil
 }
 
 func (c *CosmosDB) getPartitionKeyValue(key string, obj interface{}) (interface{}, error) {

bindings/azure/eventgrid/eventgrid.go

@@ -107,11 +107,15 @@ func (a *AzureEventGrid) Read(handler func(*bindings.ReadResponse) error) error
 	return nil
 }
 
-func (a *AzureEventGrid) Write(req *bindings.WriteRequest) error {
+func (a *AzureEventGrid) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (a *AzureEventGrid) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	err := a.ensureOutputBindingMetadata()
 	if err != nil {
 		a.logger.Error(err.Error())
-		return err
+		return nil, err
 	}
 
 	request := fasthttp.AcquireRequest()
@@ -129,16 +133,16 @@ func (a *AzureEventGrid) Write(req *bindings.WriteRequest) error {
 	err = client.Do(request, response)
 	if err != nil {
 		a.logger.Error(err.Error())
-		return err
+		return nil, err
 	}
 
 	if response.StatusCode() != fasthttp.StatusOK {
 		body := response.Body()
 		a.logger.Error(string(body))
-		return errors.New(string(body))
+		return nil, errors.New(string(body))
 	}
 
-	return nil
+	return nil, nil
 }
 
 func (a *AzureEventGrid) ensureInputBindingMetadata() error {

bindings/azure/eventhubs/eventhubs.go

@@ -138,8 +138,12 @@ func parseMetadata(meta bindings.Metadata) (*azureEventHubsMetadata, error) {
 	return m, nil
 }
 
+func (a *AzureEventHubs) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
 // Write posts an event hubs message
-func (a *AzureEventHubs) Write(req *bindings.WriteRequest) error {
+func (a *AzureEventHubs) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	event := &eventhub.Event{
 		Data: req.Data,
 	}
@@ -156,10 +160,10 @@ func (a *AzureEventHubs) Write(req *bindings.WriteRequest) error {
 
 	err := a.hub.Send(context.Background(), event)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
-	return nil
+	return nil, nil
 }
 
 // Read gets messages from eventhubs in a non-blocking fashion

bindings/azure/servicebusqueues/servicebusqueues.go

@@ -127,7 +127,11 @@ func (a *AzureServiceBusQueues) parseMetadata(metadata bindings.Metadata) (*serv
 	return &m, nil
 }
 
-func (a *AzureServiceBusQueues) Write(req *bindings.WriteRequest) error {
+func (a *AzureServiceBusQueues) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (a *AzureServiceBusQueues) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	ctx, cancel := context.WithTimeout(context.Background(), time.Second*5)
 	defer cancel()
 
@@ -141,14 +145,14 @@ func (a *AzureServiceBusQueues) Write(req *bindings.WriteRequest) error {
 
 	ttl, ok, err := bindings.TryGetTTL(req.Metadata)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	if ok {
 		msg.TTL = &ttl
 	}
 
-	return a.client.Send(ctx, msg)
+	return nil, a.client.Send(ctx, msg)
 }
 
 func (a *AzureServiceBusQueues) Read(handler func(*bindings.ReadResponse) error) error {

bindings/azure/servicebusqueues/servicebusqueues_integration_test.go

@@ -83,7 +83,7 @@ func TestQueueWithTTL(t *testing.T) {
 
 	// Assert that if waited too long, we won't see any message
 	const tooLateMsgContent = "too_late_msg"
-	err = a.Write(&bindings.WriteRequest{Data: []byte(tooLateMsgContent)})
+	err = a.Write(&bindings.InvokeRequest{Data: []byte(tooLateMsgContent)})
 	assert.Nil(t, err)
 
 	time.Sleep(time.Second * 2)
@@ -97,7 +97,7 @@ func TestQueueWithTTL(t *testing.T) {
 
 	// Getting before it is expired, should return it
 	const testMsgContent = "test_msg"
-	err = a.Write(&bindings.WriteRequest{Data: []byte(testMsgContent)})
+	err = a.Write(&bindings.InvokeRequest{Data: []byte(testMsgContent)})
 	assert.Nil(t, err)
 
 	msg, ok, err := getMessageWithRetries(queue, maxGetDuration)
@@ -136,7 +136,7 @@ func TestPublishingWithTTL(t *testing.T) {
 	assert.Equal(t, defaultAzureServiceBusMessageTimeToLive, *queueEntity.DefaultMessageTimeToLive)
 
 	const tooLateMsgContent = "too_late_msg"
-	writeRequest := bindings.WriteRequest{
+	writeRequest := bindings.InvokeRequest{
 		Data: []byte(tooLateMsgContent),
 		Metadata: map[string]string{
 			bindings.TTLMetadataKey: "1",
@@ -160,7 +160,7 @@ func TestPublishingWithTTL(t *testing.T) {
 	assert.Nil(t, err)
 
 	const testMsgContent = "test_msg"
-	writeRequest = bindings.WriteRequest{
+	writeRequest = bindings.InvokeRequest{
 		Data: []byte(testMsgContent),
 		Metadata: map[string]string{
 			bindings.TTLMetadataKey: "1",

bindings/azure/signalr/signalr.go

@@ -95,7 +95,7 @@ func (s *SignalR) Init(metadata bindings.Metadata) error {
 	return nil
 }
 
-func (s *SignalR) resolveAPIURL(req *bindings.WriteRequest) (string, error) {
+func (s *SignalR) resolveAPIURL(req *bindings.InvokeRequest) (string, error) {
 	hub := s.hub
 	if hub == "" {
 		hubFromRequest, ok := req.Metadata[hubKey]
@@ -148,23 +148,27 @@ func (s *SignalR) sendMessageToSignalR(url string, token string, data []byte) er
 	return nil
 }
 
-func (s *SignalR) Write(req *bindings.WriteRequest) error {
+func (s *SignalR) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (s *SignalR) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	url, err := s.resolveAPIURL(req)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	token, err := s.ensureValidToken(url)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	err = s.sendMessageToSignalR(url, token, req.Data)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
-	return nil
+	return nil, nil
 }
 
 func (s *SignalR) ensureValidToken(url string) (string, error) {

bindings/azure/signalr/signalr_test.go

@@ -177,7 +177,7 @@ func TestWriteShouldFail(t *testing.T) {
 
 	t.Run("Missing hub should fail", func(t *testing.T) {
 		httpTransport.reset()
-		err := s.Write(&bindings.WriteRequest{
+		_, err := s.Invoke(&bindings.InvokeRequest{
 			Data:     []byte("hello world"),
 			Metadata: map[string]string{},
 		})
@@ -189,7 +189,7 @@ func TestWriteShouldFail(t *testing.T) {
 		httpTransport.reset()
 		httpErr := errors.New("fake error")
 		httpTransport.errToReturn = httpErr
-		err := s.Write(&bindings.WriteRequest{
+		_, err := s.Invoke(&bindings.InvokeRequest{
 			Data: []byte("hello world"),
 			Metadata: map[string]string{
 				hubKey: "testHub",
@@ -203,7 +203,7 @@ func TestWriteShouldFail(t *testing.T) {
 	t.Run("SignalR call returns status != [200, 202]", func(t *testing.T) {
 		httpTransport.reset()
 		httpTransport.response.StatusCode = 401
-		err := s.Write(&bindings.WriteRequest{
+		_, err := s.Invoke(&bindings.InvokeRequest{
 			Data: []byte("hello world"),
 			Metadata: map[string]string{
 				hubKey: "testHub",
@@ -228,7 +228,7 @@ func TestWriteShouldSucceed(t *testing.T) {
 
 	t.Run("Has authorization", func(t *testing.T) {
 		httpTransport.reset()
-		err := s.Write(&bindings.WriteRequest{
+		_, err := s.Invoke(&bindings.InvokeRequest{
 			Data: []byte("hello world"),
 			Metadata: map[string]string{
 				hubKey: "testHub",
@@ -262,7 +262,7 @@ func TestWriteShouldSucceed(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			httpTransport.reset()
 			s.hub = tt.hubInMetadata
-			err := s.Write(&bindings.WriteRequest{
+			_, err := s.Invoke(&bindings.InvokeRequest{
 				Data: []byte("hello world"),
 				Metadata: map[string]string{
 					hubKey:   tt.hubInWriteRequest,

bindings/azure/storagequeues/storagequeues.go

@@ -191,11 +191,15 @@ func (a *AzureStorageQueues) parseMetadata(metadata bindings.Metadata) (*storage
 	return &m, nil
 }
 
-func (a *AzureStorageQueues) Write(req *bindings.WriteRequest) error {
+func (a *AzureStorageQueues) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (a *AzureStorageQueues) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	ttlToUse := a.metadata.ttl
 	ttl, ok, err := bindings.TryGetTTL(req.Metadata)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	if ok {
@@ -204,9 +208,9 @@ func (a *AzureStorageQueues) Write(req *bindings.WriteRequest) error {
 
 	err = a.helper.Write(req.Data, ttlToUse)
 	if err != nil {
-		return err
+		return nil, err
 	}
-	return nil
+	return nil, nil
 }
 
 func (a *AzureStorageQueues) Read(handler func(*bindings.ReadResponse) error) error {

bindings/azure/storagequeues/storagequeues_test.go

@@ -51,9 +51,9 @@ func TestWriteQueue(t *testing.T) {
 	err := a.Init(m)
 	assert.Nil(t, err)
 
-	r := bindings.WriteRequest{Data: []byte("This is my message")}
+	r := bindings.InvokeRequest{Data: []byte("This is my message")}
 
-	err = a.Write(&r)
+	_, err = a.Invoke(&r)
 
 	assert.Nil(t, err)
 }
@@ -73,9 +73,9 @@ func TestWriteWithTTLInQueue(t *testing.T) {
 	err := a.Init(m)
 	assert.Nil(t, err)
 
-	r := bindings.WriteRequest{Data: []byte("This is my message")}
+	r := bindings.InvokeRequest{Data: []byte("This is my message")}
 
-	err = a.Write(&r)
+	_, err = a.Invoke(&r)
 
 	assert.Nil(t, err)
 }
@@ -95,12 +95,12 @@ func TestWriteWithTTLInWrite(t *testing.T) {
 	err := a.Init(m)
 	assert.Nil(t, err)
 
-	r := bindings.WriteRequest{
+	r := bindings.InvokeRequest{
 		Data:     []byte("This is my message"),
 		Metadata: map[string]string{bindings.TTLMetadataKey: "1"},
 	}
 
-	err = a.Write(&r)
+	_, err = a.Invoke(&r)
 
 	assert.Nil(t, err)
 }
@@ -116,7 +116,7 @@ func TestWriteWithTTLInWrite(t *testing.T) {
 	err := a.Init(m)
 	assert.Nil(t, err)
 
-	r := bindings.WriteRequest{Data: []byte("This is my message")}
+	r := bindings.InvokeRequest{Data: []byte("This is my message")}
 
 	err = a.Write(&r)
 
@@ -135,9 +135,9 @@ func TestReadQueue(t *testing.T) {
 	err := a.Init(m)
 	assert.Nil(t, err)
 
-	r := bindings.WriteRequest{Data: []byte("This is my message")}
+	r := bindings.InvokeRequest{Data: []byte("This is my message")}
 
-	err = a.Write(&r)
+	_, err = a.Invoke(&r)
 
 	assert.Nil(t, err)
 
@@ -169,9 +169,9 @@ func TestReadQueueDecode(t *testing.T) {
 	err := a.Init(m)
 	assert.Nil(t, err)
 
-	r := bindings.WriteRequest{Data: []byte("VGhpcyBpcyBteSBtZXNzYWdl")}
+	r := bindings.InvokeRequest{Data: []byte("VGhpcyBpcyBteSBtZXNzYWdl")}
 
-	err = a.Write(&r)
+	_, err = a.Invoke(&r)
 
 	assert.Nil(t, err)
 
@@ -200,7 +200,7 @@ func TestReadQueueDecode(t *testing.T) {
 	err := a.Init(m)
 	assert.Nil(t, err)
 
-	r := bindings.WriteRequest{Data: []byte("This is my message")}
+	r := bindings.InvokeRequest{Data: []byte("This is my message")}
 
 	err = a.Write(&r)
 

bindings/gcp/bucket/bucket.go

@@ -74,7 +74,11 @@ func (g *GCPStorage) parseMetadata(metadata bindings.Metadata) ([]byte, error) {
 	return b, nil
 }
 
-func (g *GCPStorage) Write(req *bindings.WriteRequest) error {
+func (g *GCPStorage) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (g *GCPStorage) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	name := ""
 	if val, ok := req.Metadata["name"]; ok && val != "" {
 		name = val
@@ -84,7 +88,7 @@ func (g *GCPStorage) Write(req *bindings.WriteRequest) error {
 	h := g.client.Bucket(g.metadata.Bucket).Object(name).NewWriter(context.Background())
 	defer h.Close()
 	if _, err := h.Write(req.Data); err != nil {
-		return err
+		return nil, err
 	}
-	return nil
+	return nil, nil
 }

bindings/gcp/pubsub/pubsub.go

@@ -92,7 +92,11 @@ func (g *GCPPubSub) Read(handler func(*bindings.ReadResponse) error) error {
 	return err
 }
 
-func (g *GCPPubSub) Write(req *bindings.WriteRequest) error {
+func (g *GCPPubSub) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (g *GCPPubSub) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	topicName := g.metadata.Topic
 	if val, ok := req.Metadata[topic]; ok && val != "" {
 		topicName = val
@@ -103,5 +107,5 @@ func (g *GCPPubSub) Write(req *bindings.WriteRequest) error {
 	_, err := t.Publish(ctx, &pubsub.Message{
 		Data: req.Data,
 	}).Get(ctx)
-	return err
+	return nil, err
 }

bindings/http/http.go

@@ -81,14 +81,18 @@ func (h *HTTPSource) Read(handler func(*bindings.ReadResponse) error) error {
 	return nil
 }
 
-func (h *HTTPSource) Write(req *bindings.WriteRequest) error {
+func (h *HTTPSource) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (h *HTTPSource) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	client := http.Client{Timeout: time.Second * 5}
 	resp, err := client.Post(h.metadata.URL, "application/json; charset=utf-8", bytes.NewBuffer(req.Data))
 	if err != nil {
-		return err
+		return nil, err
 	}
 	if resp != nil && resp.Body != nil {
 		resp.Body.Close()
 	}
-	return nil
+	return nil, nil
 }

bindings/kafka/kafka.go

@@ -104,7 +104,11 @@ func (k *Kafka) Init(metadata bindings.Metadata) error {
 	return nil
 }
 
-func (k *Kafka) Write(req *bindings.WriteRequest) error {
+func (k *Kafka) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (k *Kafka) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	msg := &sarama.ProducerMessage{
 		Topic: k.publishTopic,
 		Value: sarama.ByteEncoder(req.Data),
@@ -115,10 +119,10 @@ func (k *Kafka) Write(req *bindings.WriteRequest) error {
 
 	_, _, err := k.producer.SendMessage(msg)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
-	return nil
+	return nil, nil
 }
 
 // GetKafkaMetadata returns new Kafka metadata

bindings/mqtt/mqtt.go

@@ -83,10 +83,14 @@ func (m *MQTT) getMQTTMetadata(metadata bindings.Metadata) (*mqttMetadata, error
 	return &mMetadata, nil
 }
 
-func (m *MQTT) Write(req *bindings.WriteRequest) error {
+func (m *MQTT) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (m *MQTT) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	m.client.Publish(m.metadata.Topic, 0, false, string(req.Data))
 	m.client.Disconnect(0)
-	return nil
+	return nil, nil
 }
 
 func (m *MQTT) Read(handler func(*bindings.ReadResponse) error) error {

bindings/output_binding.go

@@ -8,5 +8,6 @@ package bindings
 // OutputBinding is the interface for an output binding, allowing users to invoke remote systems with optional payloads
 type OutputBinding interface {
 	Init(metadata Metadata) error
-	Write(req *WriteRequest) error
+	Invoke(req *InvokeRequest) (*InvokeResponse, error)
+	Operations() []OperationKind
 }

bindings/rabbitmq/rabbitmq.go

@@ -72,7 +72,11 @@ func (r *RabbitMQ) Init(metadata bindings.Metadata) error {
 	return nil
 }
 
-func (r *RabbitMQ) Write(req *bindings.WriteRequest) error {
+func (r *RabbitMQ) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (r *RabbitMQ) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	pub := amqp.Publishing{
 		DeliveryMode: amqp.Persistent,
 		ContentType:  "text/plain",
@@ -81,7 +85,7 @@ func (r *RabbitMQ) Write(req *bindings.WriteRequest) error {
 
 	ttl, ok, err := bindings.TryGetTTL(req.Metadata)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	// The default time to live has been set in the queue
@@ -94,10 +98,10 @@ func (r *RabbitMQ) Write(req *bindings.WriteRequest) error {
 	err = r.channel.Publish("", r.metadata.QueueName, false, false, pub)
 
 	if err != nil {
-		return err
+		return nil, err
 	}
 
-	return nil
+	return nil, nil
 }
 
 func (r *RabbitMQ) parseMetadata(metadata bindings.Metadata) error {

bindings/rabbitmq/rabbitmq_integration_test.go

@@ -83,7 +83,7 @@ func TestQueuesWithTTL(t *testing.T) {
 	defer ch.Close()
 
 	const tooLateMsgContent = "too_late_msg"
-	err = r.Write(&bindings.WriteRequest{Data: []byte(tooLateMsgContent)})
+	err = r.Write(&bindings.InvokeRequest{Data: []byte(tooLateMsgContent)})
 	assert.Nil(t, err)
 
 	time.Sleep(time.Second + (ttlInSeconds * time.Second))
@@ -94,7 +94,7 @@ func TestQueuesWithTTL(t *testing.T) {
 
 	// Getting before it is expired, should return it
 	const testMsgContent = "test_msg"
-	err = r.Write(&bindings.WriteRequest{Data: []byte(testMsgContent)})
+	err = r.Write(&bindings.InvokeRequest{Data: []byte(testMsgContent)})
 	assert.Nil(t, err)
 
 	msg, ok, err := getMessageWithRetries(ch, queueName, maxGetDuration)
@@ -140,7 +140,7 @@ func TestPublishingWithTTL(t *testing.T) {
 	defer ch.Close()
 
 	const tooLateMsgContent = "too_late_msg"
-	writeRequest := bindings.WriteRequest{
+	writeRequest := bindings.InvokeRequest{
 		Data: []byte(tooLateMsgContent),
 		Metadata: map[string]string{
 			bindings.TTLMetadataKey: strconv.Itoa(ttlInSeconds),
@@ -162,7 +162,7 @@ func TestPublishingWithTTL(t *testing.T) {
 	assert.Nil(t, err)
 
 	const testMsgContent = "test_msg"
-	writeRequest = bindings.WriteRequest{
+	writeRequest = bindings.InvokeRequest{
 		Data: []byte(testMsgContent),
 		Metadata: map[string]string{
 			bindings.TTLMetadataKey: strconv.Itoa(ttlInSeconds * 1000),

bindings/redis/redis.go

@@ -118,14 +118,18 @@ func (r *Redis) parseMetadata(meta bindings.Metadata) (metadata, error) {
 	return m, nil
 }
 
-func (r *Redis) Write(req *bindings.WriteRequest) error {
+func (r *Redis) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (r *Redis) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	if val, ok := req.Metadata["key"]; ok && val != "" {
 		key := val
 		_, err := r.client.DoContext(context.Background(), "SET", key, req.Data).Result()
 		if err != nil {
-			return err
+			return nil, err
 		}
-		return nil
+		return nil, nil
 	}
-	return errors.New("redis binding: missing key on write request metadata")
+	return nil, errors.New("redis binding: missing key on write request metadata")
 }

bindings/requests.go

@@ -5,8 +5,20 @@
 
 package bindings
 
-// WriteRequest is the object given to an dapr output binding
+// InvokeRequest is the object given to a dapr output binding
-type WriteRequest struct {
+type InvokeRequest struct {
 	Data      []byte            `json:"data"`
 	Metadata  map[string]string `json:"metadata"`
+	Operation OperationKind     `json:"operation"`
 }
+
+// OperationKind defines an output binding operation
+type OperationKind string
+
+// Non exhaustive list of operations. A binding can add operations that are not in this list.
+const (
+	GetOperation    OperationKind = "get"
+	CreateOperation OperationKind = "create"
+	DeleteOperation OperationKind = "delete"
+	ListOperation   OperationKind = "list"
+)

bindings/responses.go

@@ -23,3 +23,9 @@ type AppResponse struct {
 	State       []state.SetRequest `json:"state"`
 	Concurrency string             `json:"concurrency"`
 }
+
+// InvokeResponse is the response object returned from an output binding
+type InvokeResponse struct {
+	Data     []byte            `json:"data"`
+	Metadata map[string]string `json:"metadata"`
+}

bindings/twilio/sendgrid/sendgrid.go

@@ -82,8 +82,12 @@ func (sg *SendGrid) Init(metadata bindings.Metadata) error {
 	return nil
 }
 
+func (sg *SendGrid) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
 // Write does the work of sending message to SendGrid API
-func (sg *SendGrid) Write(req *bindings.WriteRequest) error {
+func (sg *SendGrid) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	// We allow two possible sources of the properties we need,
 	// the component metadata or request metadata, request takes priority if present
 
@@ -96,7 +100,7 @@ func (sg *SendGrid) Write(req *bindings.WriteRequest) error {
 		fromAddress = mail.NewEmail("", req.Metadata["emailFrom"])
 	}
 	if fromAddress == nil {
-		return fmt.Errorf("error SendGrid from email not supplied")
+		return nil, fmt.Errorf("error SendGrid from email not supplied")
 	}
 
 	// Build email to address, this is required
@@ -108,7 +112,7 @@ func (sg *SendGrid) Write(req *bindings.WriteRequest) error {
 		toAddress = mail.NewEmail("", req.Metadata["emailTo"])
 	}
 	if toAddress == nil {
-		return fmt.Errorf("error SendGrid to email not supplied")
+		return nil, fmt.Errorf("error SendGrid to email not supplied")
 	}
 
 	// Build email subject, this is required
@@ -120,7 +124,7 @@ func (sg *SendGrid) Write(req *bindings.WriteRequest) error {
 		subject = req.Metadata["subject"]
 	}
 	if subject == "" {
-		return fmt.Errorf("error SendGrid subject not supplied")
+		return nil, fmt.Errorf("error SendGrid subject not supplied")
 	}
 
 	// Build email cc address, this is optional
@@ -165,7 +169,7 @@ func (sg *SendGrid) Write(req *bindings.WriteRequest) error {
 	client := sendgrid.NewSendClient(sg.metadata.APIKey)
 	resp, err := client.Send(email)
 	if err != nil {
-		return fmt.Errorf("error from SendGrid, sending email failed: %+v", err)
+		return nil, fmt.Errorf("error from SendGrid, sending email failed: %+v", err)
 	}
 
 	// Check SendGrid response is OK
@@ -174,9 +178,9 @@ func (sg *SendGrid) Write(req *bindings.WriteRequest) error {
 		sendGridError := sendGridRestError{}
 		json.NewDecoder(strings.NewReader(resp.Body)).Decode(&sendGridError)
 		// Pass it back to the caller, so they have some idea what went wrong
-		return fmt.Errorf("error from SendGrid, sending email failed: %d %+v", resp.StatusCode, sendGridError)
+		return nil, fmt.Errorf("error from SendGrid, sending email failed: %d %+v", resp.StatusCode, sendGridError)
 	}
 
 	sg.logger.Info("sent email with SendGrid")
-	return nil
+	return nil, nil
 }

bindings/twilio/sms/sms.go

@@ -75,12 +75,16 @@ func (t *SMS) Init(metadata bindings.Metadata) error {
 	return nil
 }
 
-func (t *SMS) Write(req *bindings.WriteRequest) error {
+func (t *SMS) Operations() []bindings.OperationKind {
+	return []bindings.OperationKind{bindings.CreateOperation}
+}
+
+func (t *SMS) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, error) {
 	toNumberValue := t.metadata.toNumber
 	if toNumberValue == "" {
 		toNumberFromRequest, ok := req.Metadata[toNumber]
 		if !ok || toNumberFromRequest == "" {
-			return errors.New("twilio missing \"toNumber\" field")
+			return nil, errors.New("twilio missing \"toNumber\" field")
 		}
 		toNumberValue = toNumberFromRequest
 	}
@@ -94,7 +98,7 @@ func (t *SMS) Write(req *bindings.WriteRequest) error {
 	twilioURL := fmt.Sprintf("%s%s/Messages.json", twilioURLBase, t.metadata.accountSid)
 	httpReq, err := http.NewRequest("POST", twilioURL, &vDr)
 	if err != nil {
-		return err
+		return nil, err
 	}
 	httpReq.SetBasicAuth(t.metadata.accountSid, t.metadata.authToken)
 	httpReq.Header.Add("Accept", "application/json")
@@ -102,11 +106,11 @@ func (t *SMS) Write(req *bindings.WriteRequest) error {
 
 	resp, err := t.httpClient.Do(httpReq)
 	if err != nil {
-		return err
+		return nil, err
 	}
 	defer resp.Body.Close()
 	if !(resp.StatusCode >= 200 && resp.StatusCode < 300) {
-		return fmt.Errorf("error from Twilio: %s", resp.Status)
+		return nil, fmt.Errorf("error from Twilio: %s", resp.Status)
 	}
-	return nil
+	return nil, nil
 }

bindings/twilio/sms/sms_test.go

@@ -69,7 +69,7 @@ func TestWriteShouldSucceed(t *testing.T) {
 
 	t.Run("Should succeed with expected url and headers", func(t *testing.T) {
 		httpTransport.reset()
-		err := tw.Write(&bindings.WriteRequest{
+		_, err := tw.Invoke(&bindings.InvokeRequest{
 			Data: []byte("hello world"),
 			Metadata: map[string]string{
 				toNumber: "toNumber",
@@ -104,7 +104,7 @@ func TestWriteShouldFail(t *testing.T) {
 
 	t.Run("Missing 'to' should fail", func(t *testing.T) {
 		httpTransport.reset()
-		err := tw.Write(&bindings.WriteRequest{
+		_, err := tw.Invoke(&bindings.InvokeRequest{
 			Data:     []byte("hello world"),
 			Metadata: map[string]string{},
 		})
@@ -116,7 +116,7 @@ func TestWriteShouldFail(t *testing.T) {
 		httpTransport.reset()
 		httpErr := errors.New("twilio fake error")
 		httpTransport.errToReturn = httpErr
-		err := tw.Write(&bindings.WriteRequest{
+		_, err := tw.Invoke(&bindings.InvokeRequest{
 			Data: []byte("hello world"),
 			Metadata: map[string]string{
 				toNumber: "toNumber",
@@ -130,7 +130,7 @@ func TestWriteShouldFail(t *testing.T) {
 	t.Run("Twilio call returns status not >=200 and <300", func(t *testing.T) {
 		httpTransport.reset()
 		httpTransport.response.StatusCode = 401
-		err := tw.Write(&bindings.WriteRequest{
+		_, err := tw.Invoke(&bindings.InvokeRequest{
 			Data: []byte("hello world"),
 			Metadata: map[string]string{
 				toNumber: "toNumber",