Update after review

Author: dadoonet

docs/plugins/discovery-ec2.asciidoc

@@ -176,11 +176,13 @@ The following are a list of settings (prefixed with `discovery.ec2`) that can fu
 --
 The type of host type to use to communicate with other instances. Can be
 one of `private_ip`, `public_ip`, `private_dns`, `public_dns` or `tag:TAGNAME` where
-`TAGNAME` is the tag field you defined for your ec2 instance.
+`TAGNAME` refers to a name of a tag configured for all EC2 instances. Instances which don't
+have this tag set will be ignored by the discovery process.
 
 For example if you defined a tag `my-elasticsearch-host` in ec2 and set it to `myhostname1.mydomain.com`, then
 setting `host_type: tag:my-elasticsearch-host` will tell Discovery Ec2 plugin to read the host name from the
 `my-elasticsearch-host` tag. In this case, it will be resolved to `myhostname1.mydomain.com`.
+http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/Using_Tags.html[Read more about EC2 Tags].
 
 Defaults to `private_ip`.
 --

plugins/discovery-ec2/src/main/java/org/elasticsearch/cloud/aws/AwsEc2Service.java

@@ -174,8 +174,9 @@ class HostType {
 
         /**
          * discovery.ec2.host_type: The type of host type to use to communicate with other instances.
-         * Can be one of private_ip, public_ip, private_dns, public_dns or meta:XXXX where
-         * XXXX is the metadata field name we will read the address from. Defaults to private_ip.
+         * Can be one of private_ip, public_ip, private_dns, public_dns or tag:XXXX where
+         * XXXX refers to a name of a tag configured for all EC2 instances. Instances which don't
+         * have this tag set will be ignored by the discovery process. Defaults to private_ip.
          */
         Setting<String> HOST_TYPE_SETTING =
             new Setting<>("discovery.ec2.host_type", HostType.PRIVATE_IP, Function.identity(), Property.NodeScope);

plugins/discovery-ec2/src/main/java/org/elasticsearch/discovery/ec2/AwsEc2UnicastHostsProvider.java

@@ -181,7 +181,7 @@ && disjoint(securityGroupIds, groups)) {
                         TransportAddress[] addresses = transportService.addressesFromString(address, 1);
                         for (int i = 0; i < addresses.length; i++) {
                             logger.trace("adding {}, address {}, transport_address {}", instance.getInstanceId(), address, addresses[i]);
-                            discoNodes.add(new DiscoveryNode("#cloud-" + instance.getInstanceId() + "-" + i, addresses[i],
+                            discoNodes.add(new DiscoveryNode(instance.getInstanceId(), "#cloud-" + instance.getInstanceId() + "-" + i, addresses[i],
                                     emptyMap(), emptySet(), Version.CURRENT.minimumCompatibilityVersion()));
                         }
                     } catch (Exception e) {

plugins/discovery-ec2/src/test/java/org/elasticsearch/discovery/ec2/Ec2DiscoveryTests.java

@@ -268,7 +268,7 @@ public void testReadHostFromTag() throws InterruptedException, UnknownHostExcept
 
         for (int node = 0; node < nodes; node++) {
             addresses[node] = "192.168.0." + (node + 1);
-            poorMansDNS.put("bar_" + node, new TransportAddress(InetAddress.getByName(addresses[node]), 9300));
+            poorMansDNS.put("node" + (node + 1), new TransportAddress(InetAddress.getByName(addresses[node]), 9300));
         }
 
         Settings nodeSettings = Settings.builder()
@@ -279,15 +279,17 @@ public void testReadHostFromTag() throws InterruptedException, UnknownHostExcept
 
         for (int node = 0; node < nodes; node++) {
             List<Tag> tags = new ArrayList<>();
-            tags.add(new Tag("foo", "bar_" + node));
+            tags.add(new Tag("foo", "node" + (node + 1)));
             tagsList.add(tags);
         }
 
         logger.info("started [{}] instances", nodes);
         List<DiscoveryNode> discoveryNodes = buildDynamicNodes(nodeSettings, nodes, tagsList);
         assertThat(discoveryNodes, hasSize(nodes));
         for (DiscoveryNode discoveryNode : discoveryNodes) {
-            assertThat(discoveryNode.getHostName(), isOneOf(addresses));
+            TransportAddress address = discoveryNode.getAddress();
+            TransportAddress expected = poorMansDNS.get(discoveryNode.getName());
+            assertEquals(address, expected);
         }
     }