Talk about DiscoveryPlugin of Elasticsearch.

  elasticsearch

Order

This article mainly studies DiscoveryPlugin of Elasticsearch.

DiscoveryPlugin

elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/plugins/DiscoveryPlugin.java

public interface DiscoveryPlugin {

    /**
     * Override to add additional {@link NetworkService.CustomNameResolver}s.
     * This can be handy if you want to provide your own Network interface name like _mycard_
     * and implement by yourself the logic to get an actual IP address/hostname based on this
     * name.
     *
     * For example: you could call a third party service (an API) to resolve _mycard_.
     * Then you could define in elasticsearch.yml settings like:
     *
     * <pre>{@code
     * network.host: _mycard_
     * }</pre>
     */
    default NetworkService.CustomNameResolver getCustomNameResolver(Settings settings) {
        return null;
    }

    /**
     * Returns providers of seed hosts for discovery.
     *
     * The key of the returned map is the name of the host provider
     * (see {@link org.elasticsearch.discovery.DiscoveryModule#DISCOVERY_SEED_PROVIDERS_SETTING}), and
     * the value is a supplier to construct the host provider when it is selected for use.
     *
     * @param transportService Use to form the {@link org.elasticsearch.common.transport.TransportAddress} portion
     *                         of a {@link org.elasticsearch.cluster.node.DiscoveryNode}
     * @param networkService Use to find the publish host address of the current node
     */
    default Map<String, Supplier<SeedHostsProvider>> getSeedHostProviders(TransportService transportService,
                                                                          NetworkService networkService) {
        return Collections.emptyMap();
    }

    /**
     * Returns a consumer that validate the initial join cluster state. The validator, unless <code>null</code> is called exactly once per
     * join attempt but might be called multiple times during the lifetime of a node. Validators are expected to throw a
     * {@link IllegalStateException} if the node and the cluster-state are incompatible.
     */
    default BiConsumer<DiscoveryNode,ClusterState> getJoinValidator() { return null; }
}
  • DiscoveryPlugin defines three default methods: getCustomNameResolver, getSeedHostProviders, and getJoinValidator

GceDiscoveryPlugin

elasticsearch-7.0.1/plugins/discovery-gce/src/main/java/org/elasticsearch/plugin/discovery/gce/GceDiscoveryPlugin.java

public class GceDiscoveryPlugin extends Plugin implements DiscoveryPlugin, Closeable {

    /** Determines whether settings those reroutes GCE call should be allowed (for testing purposes only). */
    private static final boolean ALLOW_REROUTE_GCE_SETTINGS =
        Booleans.parseBoolean(System.getProperty("es.allow_reroute_gce_settings", "false"));

    public static final String GCE = "gce";
    protected final Settings settings;
    private static final Logger logger = LogManager.getLogger(GceDiscoveryPlugin.class);
    // stashed when created in order to properly close
    private final SetOnce<GceInstancesService> gceInstancesService = new SetOnce<>();

    static {
        /*
         * GCE's http client changes access levels because its silly and we
         * can't allow that on any old stack so we pull it here, up front,
         * so we can cleanly check the permissions for it. Without this changing
         * the permission can fail if any part of core is on the stack because
         * our plugin permissions don't allow core to "reach through" plugins to
         * change the permission. Because that'd be silly.
         */
        Access.doPrivilegedVoid( () -> ClassInfo.of(HttpHeaders.class, true));
    }

    public GceDiscoveryPlugin(Settings settings) {
        this.settings = settings;
        logger.trace("starting gce discovery plugin...");
    }

    // overrideable for tests
    protected GceInstancesService createGceInstancesService() {
        return new GceInstancesServiceImpl(settings);
    }

    @Override
    public Map<String, Supplier<SeedHostsProvider>> getSeedHostProviders(TransportService transportService,
                                                                         NetworkService networkService) {
        return Collections.singletonMap(GCE, () -> {
            gceInstancesService.set(createGceInstancesService());
            return new GceSeedHostsProvider(settings, gceInstancesService.get(), transportService, networkService);
        });
    }

    @Override
    public NetworkService.CustomNameResolver getCustomNameResolver(Settings settings) {
        logger.debug("Register _gce_, _gce:xxx network names");
        return new GceNameResolver(new GceMetadataService(settings));
    }

    @Override
    public List<Setting<?>> getSettings() {
        List<Setting<?>> settings = new ArrayList<>(
            Arrays.asList(
                // Register GCE settings
                GceInstancesService.PROJECT_SETTING,
                GceInstancesService.ZONE_SETTING,
                GceSeedHostsProvider.TAGS_SETTING,
                GceInstancesService.REFRESH_SETTING,
                GceInstancesService.RETRY_SETTING,
                GceInstancesService.MAX_WAIT_SETTING)
        );

        if (ALLOW_REROUTE_GCE_SETTINGS) {
            settings.add(GceMetadataService.GCE_HOST);
            settings.add(GceInstancesServiceImpl.GCE_ROOT_URL);
        }
        return Collections.unmodifiableList(settings);
    }



    @Override
    public void close() throws IOException {
        IOUtils.close(gceInstancesService.get());
    }
}
  • GceDiscoveryPlugin implements the DiscoveryPlugin interface. Its getCustomNameResolver method returns GceNameResolver, and its getSeedHostProviders method returns GceSeedHostsProvider

DiscoveryModule

elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/discovery/DiscoveryModule.java

public class DiscoveryModule {
    private static final Logger logger = LogManager.getLogger(DiscoveryModule.class);

    public static final String ZEN_DISCOVERY_TYPE = "legacy-zen";
    public static final String ZEN2_DISCOVERY_TYPE = "zen";

    public static final String SINGLE_NODE_DISCOVERY_TYPE = "single-node";

    public static final Setting<String> DISCOVERY_TYPE_SETTING =
        new Setting<>("discovery.type", ZEN2_DISCOVERY_TYPE, Function.identity(), Property.NodeScope);
    public static final Setting<List<String>> LEGACY_DISCOVERY_HOSTS_PROVIDER_SETTING =
        Setting.listSetting("discovery.zen.hosts_provider", Collections.emptyList(), Function.identity(),
            Property.NodeScope, Property.Deprecated);
    public static final Setting<List<String>> DISCOVERY_SEED_PROVIDERS_SETTING =
        Setting.listSetting("discovery.seed_providers", Collections.emptyList(), Function.identity(),
            Property.NodeScope);

    private final Discovery discovery;

    public DiscoveryModule(Settings settings, ThreadPool threadPool, TransportService transportService,
                           NamedWriteableRegistry namedWriteableRegistry, NetworkService networkService, MasterService masterService,
                           ClusterApplier clusterApplier, ClusterSettings clusterSettings, List<DiscoveryPlugin> plugins,
                           AllocationService allocationService, Path configFile, GatewayMetaState gatewayMetaState) {
        final Collection<BiConsumer<DiscoveryNode, ClusterState>> joinValidators = new ArrayList<>();
        final Map<String, Supplier<SeedHostsProvider>> hostProviders = new HashMap<>();
        hostProviders.put("settings", () -> new SettingsBasedSeedHostsProvider(settings, transportService));
        hostProviders.put("file", () -> new FileBasedSeedHostsProvider(configFile));
        for (DiscoveryPlugin plugin : plugins) {
            plugin.getSeedHostProviders(transportService, networkService).forEach((key, value) -> {
                if (hostProviders.put(key, value) != null) {
                    throw new IllegalArgumentException("Cannot register seed provider [" + key + "] twice");
                }
            });
            BiConsumer<DiscoveryNode, ClusterState> joinValidator = plugin.getJoinValidator();
            if (joinValidator != null) {
                joinValidators.add(joinValidator);
            }
        }

        List<String> seedProviderNames = getSeedProviderNames(settings);
        // for bwc purposes, add settings provider even if not explicitly specified
        if (seedProviderNames.contains("settings") == false) {
            List<String> extendedSeedProviderNames = new ArrayList<>();
            extendedSeedProviderNames.add("settings");
            extendedSeedProviderNames.addAll(seedProviderNames);
            seedProviderNames = extendedSeedProviderNames;
        }

        final Set<String> missingProviderNames = new HashSet<>(seedProviderNames);
        missingProviderNames.removeAll(hostProviders.keySet());
        if (missingProviderNames.isEmpty() == false) {
            throw new IllegalArgumentException("Unknown seed providers " + missingProviderNames);
        }

        List<SeedHostsProvider> filteredSeedProviders = seedProviderNames.stream()
            .map(hostProviders::get).map(Supplier::get).collect(Collectors.toList());

        String discoveryType = DISCOVERY_TYPE_SETTING.get(settings);

        final SeedHostsProvider seedHostsProvider = hostsResolver -> {
            final List<TransportAddress> addresses = new ArrayList<>();
            for (SeedHostsProvider provider : filteredSeedProviders) {
                addresses.addAll(provider.getSeedAddresses(hostsResolver));
            }
            return Collections.unmodifiableList(addresses);
        };

        if (ZEN2_DISCOVERY_TYPE.equals(discoveryType) || SINGLE_NODE_DISCOVERY_TYPE.equals(discoveryType)) {
            discovery = new Coordinator(NODE_NAME_SETTING.get(settings),
                settings, clusterSettings,
                transportService, namedWriteableRegistry, allocationService, masterService,
                () -> gatewayMetaState.getPersistedState(settings, (ClusterApplierService) clusterApplier), seedHostsProvider,
                clusterApplier, joinValidators, new Random(Randomness.get().nextLong()));
        } else if (ZEN_DISCOVERY_TYPE.equals(discoveryType)) {
            discovery = new ZenDiscovery(settings, threadPool, transportService, namedWriteableRegistry, masterService, clusterApplier,
                clusterSettings, seedHostsProvider, allocationService, joinValidators, gatewayMetaState);
        } else {
            throw new IllegalArgumentException("Unknown discovery type [" + discoveryType + "]");
        }

        logger.info("using discovery type [{}] and seed hosts providers {}", discoveryType, seedProviderNames);
    }

    private List<String> getSeedProviderNames(Settings settings) {
        if (LEGACY_DISCOVERY_HOSTS_PROVIDER_SETTING.exists(settings)) {
            if (DISCOVERY_SEED_PROVIDERS_SETTING.exists(settings)) {
                throw new IllegalArgumentException("it is forbidden to set both [" + DISCOVERY_SEED_PROVIDERS_SETTING.getKey() + "] and ["
                    + LEGACY_DISCOVERY_HOSTS_PROVIDER_SETTING.getKey() + "]");
            }
            return LEGACY_DISCOVERY_HOSTS_PROVIDER_SETTING.get(settings);
        }
        return DISCOVERY_SEED_PROVIDERS_SETTING.get(settings);
    }

    public Discovery getDiscovery() {
        return discovery;
    }
}
  • The constructor of the DiscoveryModule receives a list of DiscoveryPlugin type, then traverses the list to add its SeedHostsProvider to hostProviders, and adds the joinValidator to joinValidators

Summary

  • DiscoveryPlugin defines three default methods: getCustomNameResolver, getSeedHostProviders, and getJoinValidator
  • GceDiscoveryPlugin implements the DiscoveryPlugin interface. Its getCustomNameResolver method returns GceNameResolver, and its getSeedHostProviders method returns GceSeedHostsProvider
  • The constructor of the DiscoveryModule receives a list of DiscoveryPlugin type, then traverses the list to add its SeedHostsProvider to hostProviders, and adds the joinValidator to joinValidators

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