Add ServiceInstanceTopology & ServiceInstanceNode (#20)

* add ServiceInstanceNode
* Update topology.graphqls
* Fix typo

topology.graphqls

@@ -20,6 +20,12 @@ type Topology {
     calls: [Call!]!
 }
 
+# The instance topology based on the given serviceIds
+type ServiceInstanceTopology {
+    nodes: [ServiceInstanceNode!]!
+    calls: [Call!]!
+}
+
 # Node in Topology
 type Node {
     # The global id of each node,
@@ -36,6 +42,24 @@ type Node {
     isReal: Boolean!
 }
 
+# Node in ServiceInstanceTopology
+type ServiceInstanceNode {
+    # The instance id of each node,
+    id: ID!
+    # The literal name of the #id.
+    name: String!
+    # Service id
+    serviceId: ID!
+    # The literal name of the #serviceId.
+    serviceName: String!
+    # The type name may be
+    # 1. The service provider/middleware tech, such as: Tomcat, SpringMVC
+    # 2. Conjectural Service, e.g. MySQL, Redis, Kafka
+    type: String
+    # It is a conjuecture node or real node, to represent an instance.
+    isReal: Boolean!
+}
+
 # The Call represents a directed distributed call,
 # from the `source` to the `target`.
 type Call {
@@ -63,7 +87,7 @@ extend type Query {
     # Query the topology, based on the given service
     getServiceTopology(serviceId: ID!, duration: Duration!): Topology
     # Query the instance topology, based on the given clientServiceId and serverServiceId
-    getServiceInstanceTopology(clientServiceId: ID!, serverServiceId: ID!, duration: Duration!): Topology
+    getServiceInstanceTopology(clientServiceId: ID!, serverServiceId: ID!, duration: Duration!): ServiceInstanceTopology
     # Query the topology, based on the given endpoint
     getEndpointTopology(endpointId: ID!, duration: Duration!): Topology
 }