# Quarkus Virtual Thread support for gRPC services

This guide explains how to benefit from Java virtual threads when implementing a gRPC service.

| This guide focuses on using virtual threads with the gRPC extensions. Please refer to Writing simpler reactive REST services with Quarkus Virtual Thread support to read more about Java virtual threads in general and the Quarkus Virtual Thread support.

By default, the Quarkus gRPC extension invokes service methods on an event-loop thread. See the Quarkus Reactive Architecture documentation for further details on this topic. But, you can also use the @Blocking annotation to indicate that the service is blocking and should be run on a worker thread.

The idea behind Quarkus Virtual Thread support for gRPC services is to offload the service method invocation on virtual threads, instead of running it on an event-loop thread or a worker thread.

To enable virtual thread support on a service method, simply add the @RunOnVirtualThread annotation to the method. If the JDK is compatible (Java 19 or later versions) then the invocation will be offloaded to a new virtual thread. It will then be possible to perform blocking operations without blocking the platform thread upon which the virtual thread is mounted.

## Configuring gRPC services to use virtual threads

Let’s see an example of how to implement a gRPC service using virtual threads. First, make sure to have the gRPC extension dependency in your build file:

pom.xml

    <dependency>
      <groupId>io.quarkus</groupId>
      <artifactId>quarkus-grpc</artifactId>
    </dependency>

build.gradle

    implementation("io.quarkus:quarkus-grpc")

You also need to make sure that you are using Java 19 or later, this can be enforced in your `pom.xml` file with the following:

pom.xml

    <properties>
        <maven.compiler.source>19</maven.compiler.source>
        <maven.compiler.target>19</maven.compiler.target>
    </properties>

Virtual threads are still a preview feature, so you need to start your application with the `--enable-preview` flag:

    java --enable-preview -jar target/quarkus-app/quarkus-run.jar

or to use the Quarkus Dev mode, insert the following to the `quarkus-maven-plugin` configuration:

pom.xml

    <plugin>
        <groupId>io.quarkus</groupId>
        <artifactId>quarkus-maven-plugin</artifactId>
        <version>${quarkus.version}</version>
        <executions>
            <execution>
                <goals>
                    <goal>build</goal>
                    <goal>generate-code</goal>
                    <goal>generate-code-tests</goal>
                </goals>
            </execution>
        </executions>
        <configuration>
          <source>19</source>
          <target>19</target>
          <compilerArgs>
            <arg>--enable-preview</arg>
          </compilerArgs>
        </configuration>
    </plugin>

| The `--enable-preview` flag is not necessary with Java 21+.

Then you can start using the annotation `@RunOnVirtualThread` in your service implementation:

    package io.quarkus.grpc.example.streaming;

    import com.google.protobuf.ByteString;
    import com.google.protobuf.EmptyProtos;

    import io.grpc.testing.integration.Messages;
    import io.grpc.testing.integration.TestService;
    import io.quarkus.grpc.GrpcService;
    import io.smallrye.common.annotation.RunOnVirtualThread;
    import io.smallrye.mutiny.Multi;
    import io.smallrye.mutiny.Uni;

    @GrpcService
    public class TestServiceImpl implements TestService {

        @RunOnVirtualThread
        @Override
        public Uni<EmptyProtos.Empty> emptyCall(EmptyProtos.Empty request) {
            return Uni.createFrom().item(EmptyProtos.Empty.newBuilder().build());
        }

        @RunOnVirtualThread
        @Override
        public Uni<Messages.SimpleResponse> unaryCall(Messages.SimpleRequest request) {
            var value = request.getPayload().getBody().toStringUtf8();
            var resp = Messages.SimpleResponse.newBuilder()
                    .setPayload(Messages.Payload.newBuilder().setBody(ByteString.copyFromUtf8(value.toUpperCase())).build())
                    .build();
            return Uni.createFrom().item(resp);
        }

        @Override
        @RunOnVirtualThread
        public Multi<Messages.StreamingOutputCallResponse> streamingOutputCall(Messages.StreamingOutputCallRequest request) {
            var value = request.getPayload().getBody().toStringUtf8();
            return Multi.createFrom().<String> emitter(emitter -> {
                emitter.emit(value.toUpperCase());
                emitter.emit(value.toUpperCase());
                emitter.emit(value.toUpperCase());
                emitter.complete();
            }).map(v -> Messages.StreamingOutputCallResponse.newBuilder()
                    .setPayload(Messages.Payload.newBuilder().setBody(ByteString.copyFromUtf8(v)).build())
                    .build());
        }
    }

| Limitations The gRPC methods receiving streams, such as a `Multi` cannot use `@RunOnVirtualThread`, as the method must not be blocking and produce its result (`Multi` or `Uni`) immediately.