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+// Copyright 2015 The TensorFlow Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the 'License');
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an 'AS IS' BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// =============================================================================
+
+// GraphExplorer is a tool that supports interactive, hierarchical visualization
+// of graphs. GraphExplorer renders graphs generated by TensorFlow represented
+// as GraphDef messages defined in tensorflow/core/framework/graph.proto. The
+// GraphDef proto does not allow for explicitly specifying visual attributes of
+// the graph such as color, line thickness, fonts, etc. This file introduces a
+// new proto for representing graphs and specifying visual attributes of graphs.
+//
+// The structure of the Graph proto is given by the EBNF grammar below. Consult
+// the message definitions below for details.
+//
+//  graph ::= node* edge* node_attribute* metanode_attribute* edge_attribute*
+//            graph_attribute*
+//  node  ::= node_id node_attribute* metanode_attribute* node_data*
+//  edge  ::= source_id target_id edge_attribute* edge_data*
+//
+// A graph consists of a list of nodes and a list of edges and attributes for
+// nodes, edges and the graph. Attributes have a name and a value and are
+// represented as key-value pairs, with {"color", "blue"} being an example.
+// Attributes have a scope, where the broadest scope is the graph and the
+// narrowest is a node that has no internal structure.
+syntax = "proto3";
+
+package graph_explorer;
+
+// There are two types of nodes. A 'metanode' contains other
+// nodes and a 'leaf node' has no internal structure.  The metanode containment
+// relationship is acyclic, meaning that if a metanode 'A' contains the metanode
+// 'B', then 'B' cannot contain 'A'.
+message Node {
+  // The identifier of a node is a sequence of strings separated by '/'. The
+  // identifier provides a unique name for a node and defines its hierarchical
+  // relation to other nodes.  If no label is provided  the last part of the
+  // identifier is used as a label.
+  //
+  // Example: In the graph below, metanodes are written with square brackets and
+  // leaf nodes with parentheses. The metanode 'node1' contains the leaf node
+  // 'node4' and the metanode 'node2', which contains the leaf node 'node3'.
+  //
+  //   [node1 [node2 (node3)] (node4)]
+  //
+  // The identifiers for these nodes are: "node1", "node1/node2",
+  // "node1/node2/node3", and "node1/node4".
+  string name = 1;
+
+  // A node attribute is information used by Graph Explorer to style a node.
+  map<string, string> node_attr = 2;
+
+  // A metanode attribute is one that is inherited by all nodes inside the
+  // current metanode. If an attribute applies only to the current node and
+  // should not be inherited, it should be specified as a node attribute.
+  map<string, string> metanode_attr = 3;
+};
+
+// An edge consists of a source and a target node, specified by their
+// identifiers. An edge has attributes and data that are similar to node
+// attributes and node data. Edges do not form a hierarchy so there are no
+// metanode attributes.
+message Edge {
+  // The source and target fields must have the format of a Node name.
+  string source = 1;
+  string target = 2;
+
+  // Edge attributes.
+  map<string, string> edge_attr = 3;
+}
+
+message Graph {
+  // List of nodes in the graph.
+  repeated Node node = 1;
+
+  // List of edges in the graph.
+  repeated Edge edge = 2;
+
+  // Default values of node, metanode and edge attributes.
+  map<string, string> node_attr = 3;
+  map<string, string> metanode_attr = 4;
+  map<string, string> edge_attr = 5;
+
+  // Graph attributes.
+  map<string, string> graph_attr = 6;
+};