Add drop_data field to api to drop data except schema and types. (#53)

This change adds API support for a new operation called drop_data which
will drop all the data but not the schema nor the types. This is useful
when the schema will be resued after dropping the data and the schema is
large enough that recreating it might take more than a couple seconds.

examples_test.go

@@ -21,6 +21,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"log"
+	"strings"
 	"time"
 
 	"github.com/dgraph-io/dgo"
@@ -37,7 +38,22 @@ func getDgraphClient() (*dgo.Dgraph, CancelFunc) {
 	}
 
 	dc := api.NewDgraphClient(conn)
-	return dgo.NewDgraphClient(dc), func() {
+	dg := dgo.NewDgraphClient(dc)
+	ctx := context.Background()
+
+	for {
+        // keep retrying until we succeed or receive a non-retriable error
+        err = dg.Login(ctx, "groot", "password")
+        if err == nil || !strings.Contains(err.Error(), "Please retry") {
+            break
+        }
+        time.Sleep(time.Second)
+    }
+	if err != nil {
+		log.Fatalf("While trying to login %v", err.Error())
+	}
+
+	return dg, func() {
 		if err := conn.Close(); err != nil {
 			log.Printf("Error while closing connection:%v", err)
 		}
@@ -47,9 +63,7 @@ func getDgraphClient() (*dgo.Dgraph, CancelFunc) {
 func ExampleDgraph_Alter_dropAll() {
 	dg, cancel := getDgraphClient()
 	defer cancel()
-	op := api.Operation{
-		DropAll: true,
-	}
+	op := api.Operation{DropAll: true}
 	ctx := context.Background()
 	if err := dg.Alter(ctx, &op); err != nil {
 		log.Fatal(err)
@@ -450,9 +464,7 @@ func ExampleTxn_Mutate_facets() {
 	defer cancel()
 	// Doing a dropAll isn't required by the user. We do it here so that we can verify that the
 	// example runs as expected.
-	op := api.Operation{
-		DropAll: true,
-	}
+	op := api.Operation{DropAll: true}
 	ctx := context.Background()
 	if err := dg.Alter(ctx, &op); err != nil {
 		log.Fatal(err)
@@ -957,15 +969,13 @@ func ExampleTxn_Mutate_deletePredicate() {
 		log.Fatal(err)
 	}
 
-	op = &api.Operation{
-		DropAttr: "friend",
-	}
+	op = &api.Operation{DropAttr: "friend"}
 	err = dg.Alter(ctx, op)
 	if err != nil {
 		log.Fatal(err)
 	}
 
-	op.DropAttr = "married"
+	op = &api.Operation{DropAttr: "married"}
 	err = dg.Alter(ctx, op)
 	if err != nil {
 		log.Fatal(err)

protos/api.proto

@@ -79,11 +79,17 @@ message Mutation {
 	bool ignore_index_conflict = 15; // this field is not parsed and used by the server anymore.
 }
 
-
 message Operation {
 	string schema = 1;
 	string drop_attr = 2;
 	bool drop_all = 3;
+
+	enum DropOp {
+		NONE = 0;
+		ALL = 1;
+		DATA = 2;
+	}
+	DropOp drop_op = 4;
 }
 
 // Worker services.