Avro + Grpc

Using Avro with gRPC involves integrating Avro serialization for data exchange in gRPC services instead of using the default Protocol Buffers. Although gRPC is designed around protobuf by default, you can still customize the serialization layer to use Avro. This requires some adjustments since the gRPC framework doesn’t natively support Avro. Here is how you can implement this.

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Steps to Use Avro with gRPC

1. Define Your Avro Schema

Create an Avro schema file (e.g., user.avsc), which defines the data model you want to exchange using gRPC.

{
  "namespace": "com.example",
  "type": "record",
  "name": "User",
  "fields": [
    { "name": "id", "type": "string" },
    { "name": "name", "type": "string" },
    { "name": "email", "type": "string" }
  ]
}

Use this schema to generate the Java classes required for serialization and deserialization.

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2. Generate Avro Classes from Schema

You can use the Avro Maven plugin or command-line tools to generate Java classes from your .avsc file.

Using Maven:

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.avro</groupId>
            <artifactId>avro-maven-plugin</artifactId>
            <version>1.11.0</version>
            <executions>
                <execution>
                    <phase>generate-sources</phase>
                    <goals>
                        <goal>schema</goal>
                    </goals>
                    <configuration>
                        <sourceDirectory>${project.basedir}/src/main/avro</sourceDirectory>
                        <outputDirectory>${project.build.directory}/generated-sources/avro</outputDirectory>
                    </configuration>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

Place your user.avsc schema in the src/main/avro directory. Then run:

mvn clean compile

This will generate a User.java class in the target/generated-sources/avro directory.

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3. Define gRPC Service Interfaces

Create a gRPC service definition in a user.proto file to define the service API. Although we are using Avro for serialization, we still need the proto file to define the service interface.

user.proto:

syntax = "proto3";

service UserService {
  rpc GetUser(UserIdRequest) returns (UserResponse);
}

message UserIdRequest {
  string id = 1;
}

message UserResponse {
  bytes user_data = 1;  // This will contain serialized Avro data
}

Here, we are using bytes to carry the Avro-serialized data in the response.

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4. Implement gRPC Service with Avro Serialization

Now, implement the gRPC service in Java. In the service implementation, we will use Avro to serialize and deserialize the data.

Here’s an example:

UserServiceImpl.java:

import com.example.User;
import io.grpc.stub.StreamObserver;
import user.UserServiceGrpc;
import user.UserIdRequest;
import user.UserResponse;
import org.apache.avro.io.BinaryEncoder;
import org.apache.avro.io.EncoderFactory;
import org.apache.avro.specific.SpecificDatumWriter;
import java.io.ByteArrayOutputStream;
import java.io.IOException;

public class UserServiceImpl extends UserServiceGrpc.UserServiceImplBase {

    @Override
    public void getUser(UserIdRequest request, StreamObserver<UserResponse> responseObserver) {
        // Create a sample User object
        User user = User.newBuilder()
                .setId(request.getId())
                .setName("John Doe")
                .setEmail("john.doe@example.com")
                .build();

        // Serialize User object to Avro
        byte[] avroData = serializeToAvro(user);

        // Create a gRPC response with Avro-encoded data
        UserResponse response = UserResponse.newBuilder()
                .setUserData(com.google.protobuf.ByteString.copyFrom(avroData))
                .build();

        responseObserver.onNext(response);
        responseObserver.onCompleted();
    }

    private byte[] serializeToAvro(User user) {
        try (ByteArrayOutputStream out = new ByteArrayOutputStream()) {
            BinaryEncoder encoder = EncoderFactory.get().binaryEncoder(out, null);
            SpecificDatumWriter<User> writer = new SpecificDatumWriter<>(User.class);
            writer.write(user, encoder);
            encoder.flush();
            return out.toByteArray();
        } catch (IOException e) {
            e.printStackTrace();
            return new byte[0];
        }
    }
}

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5. Create the gRPC Server

Next, we need to create the gRPC server that uses the UserServiceImpl.

GrpcServer.java:

import io.grpc.Server;
import io.grpc.ServerBuilder;

public class GrpcServer {
    public static void main(String[] args) throws Exception {
        Server server = ServerBuilder.forPort(8080)
                .addService(new UserServiceImpl())
                .build();

        System.out.println("Starting gRPC server...");
        server.start();
        server.awaitTermination();
    }
}

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6. Implement the gRPC Client with Avro Deserialization

Now let’s create a gRPC client that makes a request to the gRPC server and decodes the Avro response.

GrpcClient.java:

import io.grpc.ManagedChannel;
import io.grpc.ManagedChannelBuilder;
import user.UserIdRequest;
import user.UserResponse;
import user.UserServiceGrpc;
import org.apache.avro.io.BinaryDecoder;
import org.apache.avro.io.DecoderFactory;
import org.apache.avro.specific.SpecificDatumReader;
import com.example.User;
import java.io.ByteArrayInputStream;
import java.io.IOException;

public class GrpcClient {
    public static void main(String[] args) {
        ManagedChannel channel = ManagedChannelBuilder.forAddress("localhost", 8080)
                .usePlaintext()
                .build();

        UserServiceGrpc.UserServiceBlockingStub stub = UserServiceGrpc.newBlockingStub(channel);

        // Create a request
        UserIdRequest request = UserIdRequest.newBuilder().setId("123").build();

        // Get the response
        UserResponse response = stub.getUser(request);

        // Deserialize the Avro-encoded data
        User user = deserializeFromAvro(response.getUserData().toByteArray());

        System.out.println("Received User: " + user);

        channel.shutdown();
    }

    private static User deserializeFromAvro(byte[] avroData) {
        try (ByteArrayInputStream in = new ByteArrayInputStream(avroData)) {
            BinaryDecoder decoder = DecoderFactory.get().binaryDecoder(in, null);
            SpecificDatumReader<User> reader = new SpecificDatumReader<>(User.class);
            return reader.read(null, decoder);
        } catch (IOException e) {
            e.printStackTrace();
            return null;
        }
    }
}

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7. Run the gRPC Server and Client

1. Start the gRPC server:

   java GrpcServer
   

2. Run the gRPC client in another terminal:

   java GrpcClient
   

You should see the client print the user details retrieved from the server, proving that Avro serialization and deserialization work properly over gRPC.

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Summary

In this example:

1. We defined the Avro schema to describe the data.
2. We generated Avro classes using the Avro Maven plugin.
3. We used gRPC with protobuf messages carrying Avro-encoded data in the bytes field.
4. We implemented both the server and client, handling Avro serialization and deserialization manually.

This approach allows you to leverage Avro as the data serialization format while still using gRPC for communication.