feat: added custom grpc resolver

providers/flagd/README.md

@@ -108,6 +108,7 @@ Given below are the supported configurations:
 | resolver              | FLAGD_RESOLVER                 | String - rpc, in-process | rpc       |                     |
 | host                  | FLAGD_HOST                     | String                   | localhost | rpc & in-process    |
 | port                  | FLAGD_PORT                     | int                      | 8013      | rpc & in-process    |
+| targetUri             | FLAGD_GRPC_TARGET              | string                   | null      | rpc & in-process    |
 | tls                   | FLAGD_TLS                      | boolean                  | false     | rpc & in-process    |
 | socketPath            | FLAGD_SOCKET_PATH              | String                   | null      | rpc & in-process    |
 | certPath              | FLAGD_SERVER_CERT_PATH         | String                   | null      | rpc & in-process    |
@@ -123,6 +124,7 @@ Given below are the supported configurations:
 
 > [!NOTE]  
 > Some configurations are only applicable for RPC resolver.
+>
 
 ### Unix socket support
 
@@ -239,3 +241,17 @@ FlagdProvider flagdProvider = new FlagdProvider(options);
 Please refer [OpenTelemetry example](https://opentelemetry.io/docs/instrumentation/java/manual/#example) for best practice guidelines.
 
 Provider telemetry combined with [flagd OpenTelemetry](https://flagd.dev/reference/monitoring/#opentelemetry) allows you to have distributed traces.
+
+### Target URI Support (gRPC name resolution)
+
+The `targetUri` is meant for gRPC custom name resolution (default is `dns`), this allows users to use different
+resolution method e.g. `xds`. Currently, we are supporting all [core resolver](https://grpc.io/docs/guides/custom-name-resolution/)
+and one custom resolver for `envoy` proxy resolution. For more details, please refer the 
+[RFC](https://github.com/open-feature/flagd/blob/main/docs/reference/specifications/proposal/rfc-grpc-custom-name-resolver.md) document.
+
+```java
+FlagdOptions options = FlagdOptions.builder()
+      .targetUri("envoy://localhost:9211/flag-source.service")
+      .resolverType(Config.Resolver.IN_PROCESS)
+      .build();
+```

providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/Config.java

@@ -37,6 +37,7 @@ public final class Config {
     static final String OFFLINE_SOURCE_PATH = "FLAGD_OFFLINE_FLAG_SOURCE_PATH";
     static final String KEEP_ALIVE_MS_ENV_VAR_NAME_OLD = "FLAGD_KEEP_ALIVE_TIME";
     static final String KEEP_ALIVE_MS_ENV_VAR_NAME = "FLAGD_KEEP_ALIVE_TIME_MS";
+    static final String GRPC_TARGET_ENV_VAR_NAME = "FLAGD_GRPC_TARGET";
 
     static final String RESOLVER_RPC = "rpc";
     static final String RESOLVER_IN_PROCESS = "in-process";

providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/FlagdOptions.java

@@ -123,6 +123,18 @@ public class FlagdOptions {
     @Builder.Default
     private String offlineFlagSourcePath = fallBackToEnvOrDefault(Config.OFFLINE_SOURCE_PATH, null);
 
+
+    /**
+     * gRPC custom target string.
+     *
+     * <p>Setting this will allow user to use custom gRPC name resolver at present
+     * we are supporting all core resolver along with a custom resolver for envoy proxy
+     * resolution. For more visit (https://grpc.io/docs/guides/custom-name-resolution/)
+     */
+    @Builder.Default
+    private String targetUri = fallBackToEnvOrDefault(Config.GRPC_TARGET_ENV_VAR_NAME, null);
+
+
     /**
      * Function providing an EvaluationContext to mix into every evaluations.
      * The sync-metadata response

providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/resolver/common/ChannelBuilder.java

@@ -1,8 +1,10 @@
 package dev.openfeature.contrib.providers.flagd.resolver.common;
 
 import dev.openfeature.contrib.providers.flagd.FlagdOptions;
+import dev.openfeature.contrib.providers.flagd.resolver.common.nameresolvers.EnvoyResolverProvider;
 import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
 import io.grpc.ManagedChannel;
+import io.grpc.NameResolverRegistry;
 import io.grpc.netty.GrpcSslContexts;
 import io.grpc.netty.NettyChannelBuilder;
 import io.netty.channel.epoll.Epoll;
@@ -13,6 +15,8 @@
 
 import javax.net.ssl.SSLException;
 import java.io.File;
+import java.net.URI;
+import java.net.URISyntaxException;
 import java.util.concurrent.TimeUnit;
 
 /**
@@ -50,9 +54,21 @@ public static ManagedChannel nettyChannel(final FlagdOptions options) {
 
         // build a TCP socket
         try {
+            // Register custom resolver
+            if (isEnvoyTarget(options.getTargetUri())) {
+                NameResolverRegistry.getDefaultRegistry().register(new EnvoyResolverProvider());
+            }
+
+            // default to current `dns` resolution i.e. <host>:<port>, if valid / supported
+            // target string use the user provided target uri.
+            final String defaultTarget = String.format("%s:%s", options.getHost(), options.getPort());
+            final String targetUri = isValidTargetUri(options.getTargetUri()) ? options.getTargetUri() :
+                    defaultTarget;
+
             final NettyChannelBuilder builder = NettyChannelBuilder
-                    .forAddress(options.getHost(), options.getPort())
+                    .forTarget(targetUri)
                     .keepAliveTime(keepAliveMs, TimeUnit.MILLISECONDS);
+
             if (options.isTls()) {
                 SslContextBuilder sslContext = GrpcSslContexts.forClient();
 
@@ -78,6 +94,48 @@ public static ManagedChannel nettyChannel(final FlagdOptions options) {
             SslConfigException sslConfigException = new SslConfigException("Error with SSL configuration.");
             sslConfigException.initCause(ssle);
             throw sslConfigException;
+        } catch (IllegalArgumentException argumentException) {
+            GenericConfigException genericConfigException = new GenericConfigException(
+                    "Error with gRPC target string configuration");
+            genericConfigException.initCause(argumentException);
+            throw genericConfigException;
+        }
+    }
+
+    private static  boolean isValidTargetUri(String targetUri) {
+        if (targetUri == null) {
+            return false;
+        }
+
+        try {
+            final String scheme = new URI(targetUri).getScheme();
+            if (scheme.equals(SupportedScheme.ENVOY.getScheme()) || scheme.equals(SupportedScheme.DNS.getScheme())
+                    || scheme.equals(SupportedScheme.XDS.getScheme())
+                    || scheme.equals(SupportedScheme.UDS.getScheme())) {
+                return true;
+            }
+        } catch (URISyntaxException e) {
+            throw new IllegalArgumentException("Invalid target string", e);
         }
+
+        return false;
+    }
+
+    private static boolean isEnvoyTarget(String targetUri) {
+        if (targetUri == null) {
+            return false;
+        }
+
+        try {
+            final String scheme = new URI(targetUri).getScheme();
+            if (scheme.equals(SupportedScheme.ENVOY.getScheme())) {
+                return true;
+            }
+        } catch (URISyntaxException e) {
+            throw new IllegalArgumentException("Invalid target string", e);
+        }
+
+        return false;
+
     }
 }

providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/resolver/common/GenericConfigException.java

@@ -0,0 +1,11 @@
+package dev.openfeature.contrib.providers.flagd.resolver.common;
+
+/**
+ * Custom exception for invalid gRPC configurations.
+ */
+
+public class GenericConfigException extends RuntimeException {
+    public GenericConfigException(String message) {
+        super(message);
+    }
+}

providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/resolver/common/SupportedScheme.java

@@ -0,0 +1,14 @@
+package dev.openfeature.contrib.providers.flagd.resolver.common;
+
+import lombok.Getter;
+
+@Getter
+enum SupportedScheme {
+    ENVOY("envoy"), DNS("dns"), XDS("xds"), UDS("uds");
+
+    private final String scheme;
+
+    SupportedScheme(String scheme) {
+        this.scheme = scheme;
+    }
+}

providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/resolver/common/nameresolvers/EnvoyResolver.java

@@ -0,0 +1,69 @@
+package dev.openfeature.contrib.providers.flagd.resolver.common.nameresolvers;
+
+import io.grpc.EquivalentAddressGroup;
+import io.grpc.NameResolver;
+import java.net.InetSocketAddress;
+import io.grpc.Attributes;
+import io.grpc.Status;
+import java.net.URI;
+import java.util.Collections;
+import java.util.List;
+
+/**
+ * Envoy NameResolver, will always override the authority with the specified authority and
+ * use the socketAddress to connect.
+ *
+ * <p>Custom URI Scheme:
+ *
+ * <p>envoy://[proxy-agent-host]:[proxy-agent-port]/[service-name]
+ *
+ * <p>`service-name` is used as authority instead host
+ */
+public class EnvoyResolver extends NameResolver {
+    private final URI uri;
+    private final String authority;
+    private Listener2 listener;
+
+    public EnvoyResolver(URI targetUri) {
+        this.uri = targetUri;
+        this.authority = targetUri.getPath().substring(1);
+    }
+
+    @Override
+    public String getServiceAuthority() {
+        return authority;
+    }
+
+    @Override
+    public void shutdown() {
+    }
+
+    @Override
+    public void start(Listener2 listener) {
+        this.listener = listener;
+        this.resolve();
+    }
+
+    @Override
+    public void refresh() {
+        this.resolve();
+    }
+
+    private void resolve() {
+        try {
+            InetSocketAddress address = new InetSocketAddress(this.uri.getHost(), this.uri.getPort());
+            Attributes addressGroupAttributes = Attributes.newBuilder()
+                    .set(EquivalentAddressGroup.ATTR_AUTHORITY_OVERRIDE, this.authority)
+                    .build();
+            List<EquivalentAddressGroup> equivalentAddressGroup = Collections.singletonList(
+                    new EquivalentAddressGroup(address, addressGroupAttributes)
+            );
+            ResolutionResult resolutionResult = ResolutionResult.newBuilder()
+                    .setAddresses(equivalentAddressGroup)
+                    .build();
+            this.listener.onResult(resolutionResult);
+        } catch (Exception e) {
+            this.listener.onError(Status.UNAVAILABLE.withDescription("Unable to resolve host ").withCause(e));
+        }
+    }
+}

providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/resolver/common/nameresolvers/EnvoyResolverProvider.java

@@ -0,0 +1,54 @@
+package dev.openfeature.contrib.providers.flagd.resolver.common.nameresolvers;
+
+import io.grpc.NameResolver;
+import io.grpc.NameResolverProvider;
+import java.net.URI;
+
+/**
+ * A custom NameResolver provider to resolve gRPC target uri for envoy in the
+ * format of.
+ *
+ * <p>envoy://[proxy-agent-host]:[proxy-agent-port]/[service-name]
+ */
+public class EnvoyResolverProvider extends NameResolverProvider {
+    static final String ENVOY_SCHEME = "envoy";
+
+    @Override
+    protected boolean isAvailable() {
+        return true;
+    }
+
+    // setting priority higher than the default i.e. 5
+    // could lead to issue since the resolver override the default
+    // dns provider.
+    // https://grpc.github.io/grpc-java/javadoc/io/grpc/NameResolverProvider.html?is-external=true#priority()
+    @Override
+    protected int priority() {
+        return 5;
+    }
+
+    @Override
+    public NameResolver newNameResolver(URI targetUri, NameResolver.Args args) {
+        if (!ENVOY_SCHEME.equals(targetUri.getScheme())) {
+            return null;
+        }
+
+        if (!isValidPath(targetUri.getPath()) || targetUri.getHost() == null || targetUri.getPort() == -1) {
+            throw new IllegalArgumentException("Incorrectly formatted target uri; "
+                    + "expected: '" + ENVOY_SCHEME + ":[//]<proxy-agent-host>:<proxy-agent-port>/<service-name>';"
+                    + "but was '" + targetUri + "'");
+        }
+
+        return new EnvoyResolver(targetUri);
+    }
+
+    @Override
+    public String getDefaultScheme() {
+        return ENVOY_SCHEME;
+    }
+
+    private static boolean isValidPath(String path) {
+        return !path.isEmpty() && !path.substring(1).isEmpty()
+                && !path.substring(1).contains("/");
+    }
+}

providers/flagd/src/test/java/dev/openfeature/contrib/providers/flagd/FlagdOptionsTest.java

@@ -54,6 +54,7 @@ void TestBuilderOptions() {
                 .openTelemetry(openTelemetry)
                 .customConnector(connector)
                 .resolverType(Resolver.IN_PROCESS)
+                .targetUri("dns:///localhost:8016")
                 .keepAlive(1000)
                 .build();
 
@@ -69,6 +70,7 @@ void TestBuilderOptions() {
         assertEquals(openTelemetry, flagdOptions.getOpenTelemetry());
         assertEquals(connector, flagdOptions.getCustomConnector());
         assertEquals(Resolver.IN_PROCESS, flagdOptions.getResolverType());
+        assertEquals("dns:///localhost:8016", flagdOptions.getTargetUri());
         assertEquals(1000, flagdOptions.getKeepAlive());
     }
 
@@ -187,4 +189,12 @@ void testRpcProviderFromEnv_portConfigured_usesConfiguredPort() {
         assertThat(flagdOptions.getPort()).isEqualTo(1534);
 
     }
+
+    @Test
+    @SetEnvironmentVariable(key = GRPC_TARGET_ENV_VAR_NAME, value = "envoy://localhost:1234/foo.service")
+    void testTargetOverrideFromEnv() {
+        FlagdOptions flagdOptions = FlagdOptions.builder().build();
+
+        assertThat(flagdOptions.getTargetUri()).isEqualTo("envoy://localhost:1234/foo.service");
+    }
 }