Kora Grpc Server

Kora gRPC Server

Kora sub-skill — obey the kora-v1 meta rules on every task: R0 ensure .kora-agent/ docs+examples are cloned · R1 read this sub-skill before writing code · R2 Kora APIs only — no Spring/Micronaut/Quarkus, no invented annotations or config keys · R3 journal any incorrect Kora usage. Add comments/Javadoc only if asked.

Serve gRPC RPCs from a Kora application. The .proto contract is the source of truth: the protobuf Gradle plugin generates message classes and a *Grpc.*ImplBase base type, and you implement a Kora @Component that extends that base. GrpcServerModule discovers every @Component handler and ServerInterceptor and starts the server — no reflection-based wiring, everything is resolved through the compile-time graph.

When to use vs NOT

Use this skill when:

• implementing gRPC handlers that extend the generated *GrpcImplBase,
• wiring GrpcServerModule into a @KoraApp,
• configuring grpcServer (port, message size, telemetry, keepalive, reflection),
• adding io.grpc.ServerInterceptor beans for auth/logging/metrics,
• enabling gRPC Server Reflection for grpcurl.

Do NOT use this skill for:

• consuming gRPC services (declarative stubs) — that is the kora-grpc-client skill,
• HTTP/JSON endpoints — use kora-http-server.

Quick Start

1. Dependencies (BOM pins all ru.tinkoff.kora:* versions)

dependencies {
    koraBom platform("ru.tinkoff.kora:kora-parent:1.2.19")
    annotationProcessor "ru.tinkoff.kora:annotation-processors"

    implementation "ru.tinkoff.kora:grpc-server"
    implementation "ru.tinkoff.kora:config-hocon"
    implementation "ru.tinkoff.kora:logging-logback"
    implementation "io.grpc:grpc-protobuf:1.74.0"
    implementation "javax.annotation:javax.annotation-api:1.3.2"

    // Optional: gRPC Server Reflection (grpcurl / Postman gRPC)
    implementation "io.grpc:grpc-services:1.74.0"
}

Kotlin: replace the processor with ksp "ru.tinkoff.kora:symbol-processors". Never put a version on a ru.tinkoff.kora:* artifact — the BOM controls them.

2. Protobuf Gradle plugin

plugins {
    id "com.google.protobuf" version "0.10.0"
}

protobuf {
    protoc { artifact = "com.google.protobuf:protoc:3.25.3" }
    plugins {
        grpc { artifact = "io.grpc:protoc-gen-grpc-java:1.74.0" }
    }
    generateProtoTasks {
        all()*.plugins { grpc {} }
    }
}

sourceSets {
    main.java {
        srcDirs "build/generated/source/proto/main/grpc"
        srcDirs "build/generated/source/proto/main/java"
    }
}

3. .proto contract (src/main/proto/user_service.proto)

syntax = "proto3";

package ru.tinkoff.kora.example.grpc;
option java_multiple_files = true;

import "google/protobuf/timestamp.proto";

service UserService {
  rpc CreateUser(CreateUserRequest) returns (UserResponse) {}
  rpc GetUser(GetUserRequest) returns (UserResponse) {}
}

message CreateUserRequest { string name = 1; string email = 2; }
message GetUserRequest { string user_id = 1; }
message UserResponse {
  string id = 1;
  string name = 2;
  string email = 3;
  google.protobuf.Timestamp created_at = 4;
}

4. Application module

@KoraApp
public interface Application extends
        HoconConfigModule,
        LogbackModule,
        GrpcServerModule {

    static void main(String[] args) {
        KoraApplication.run(ApplicationGraph::graph);
    }
}

5. Handler — @Component extending the generated *ImplBase

import io.grpc.Status;
import io.grpc.stub.StreamObserver;
import ru.tinkoff.kora.common.Component;

@Component
public final class UserServiceGrpcHandler extends UserServiceGrpc.UserServiceImplBase {

    private final UserService userService;

    public UserServiceGrpcHandler(UserService userService) {
        this.userService = userService;
    }

    @Override
    public void getUser(GetUserRequest request, StreamObserver<UserResponse> responseObserver) {
        var user = userService.getUser(request.getUserId())
            .orElseThrow(() -> Status.NOT_FOUND
                .withDescription("User not found: " + request.getUserId())
                .asRuntimeException());
        responseObserver.onNext(toGrpcUser(user));
        responseObserver.onCompleted();
    }
}

The handler is a plain Kora component: constructor injection, business logic delegated to UserService. The generated protobuf types are transport DTOs — keep domain logic out of the handler.

6. Configuration (application.conf)

grpcServer {
  port = ${GRPC_PORT}
  telemetry.logging.enabled = true
}

7. Run and probe

./gradlew clean classes   # generate proto + build the compile-time graph
./gradlew run
grpcurl -plaintext -d '{"user_id":"42"}' \
  localhost:8090 ru.tinkoff.kora.example.grpc.UserService/GetUser

The gRPC method's full name is <proto package>.<service>/<Method>.

References

Assets (templates): see assets/.

Core patterns

RPC method signatures

Server streaming: emit each item with onNext, then a single onCompleted.

@Override
public void getAllUsers(Empty request, StreamObserver<UserResponse> responseObserver) {
    for (var user : userService.getAllUsers()) {
        responseObserver.onNext(toGrpcUser(user));
    }
    responseObserver.onCompleted();
}

Client/bidirectional streaming: return a StreamObserver<Req> that accumulates onNext values and replies on onCompleted.

@Override
public StreamObserver<CreateUserRequest> createUsers(StreamObserver<CreateUsersResponse> responseObserver) {
    return new StreamObserver<>() {
        private final List<UserRequest> requests = new ArrayList<>();
        public void onNext(CreateUserRequest v) { requests.add(new UserRequest(v.getName(), v.getEmail())); }
        public void onError(Throwable t) { responseObserver.onError(t); }
        public void onCompleted() {
            var created = userService.createUsers(requests);
            responseObserver.onNext(CreateUsersResponse.newBuilder()
                .setCreatedCount(created.size()).build());
            responseObserver.onCompleted();
        }
    };
}

See references/grpc-service-reference.md for full examples including timestamp/ByteString conversion.

Errors via io.grpc.Status

Map domain failures to gRPC status codes. Common: NOT_FOUND, INVALID_ARGUMENT, ALREADY_EXISTS, PERMISSION_DENIED, UNAUTHENTICATED, INTERNAL, UNAVAILABLE.

throw Status.NOT_FOUND.withDescription("User not found: " + id).asRuntimeException();

Inside a handler use either throw ...asRuntimeException() (when the call propagates) or responseObserver.onError(...). Send exactly one terminal signal per call — never onNext after onError/onCompleted. Details: references/grpc-error-handling-reference.md.

Interceptors

Register a cross-cutting interceptor by implementing io.grpc.ServerInterceptor and annotating it @Component; GrpcServerModule adds it automatically.

import io.grpc.*;
import ru.tinkoff.kora.common.Component;

@Component
public final class MyServerInterceptor implements ServerInterceptor {
    private final Logger logger = LoggerFactory.getLogger(MyServerInterceptor.class);

    @Override
    public <ReqT, RespT> ServerCall.Listener<ReqT> interceptCall(
            ServerCall<ReqT, RespT> call, Metadata headers, ServerCallHandler<ReqT, RespT> next) {
        logger.info("gRPC call: {}", call.getMethodDescriptor().getFullMethodName());
        return next.startCall(call, headers);
    }
}

Default interceptors registered by the module: ContextServerInterceptor, CoroutineContextInjectInterceptor, MetricCollectorServerInterceptor, LoggingServerInterceptor. To replace the default list, override the serverBuilder method from GrpcModule. Auth and exception-mapping examples: references/grpc-interceptors-reference.md.

Reflection

Add io.grpc:grpc-services and set reflectionEnabled = true (default is false) for grpcurl/Postman discovery.

grpcServer { reflectionEnabled = ${?GRPC_REFLECTION_ENABLED} }

grpcurl -plaintext localhost:8090 list

Keep it disabled in production unless the endpoint is internal-only. See references/grpc-reflection-reference.md.

Telemetry

The module emits metrics (rpc.server.duration, rpc.server.requests_per_rpc, rpc.server.responses_per_rpc), tracing, and logging — all toggled under grpcServer.telemetry. Metrics default on, tracing default on, logging default off. Add custom metric tags / trace attributes under telemetry.metrics.tags / telemetry.tracing.attributes (references/grpc-config-reference.md).

Common pitfalls