What is gRPC?

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syntax = "proto3";
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package userservice.v1;
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// User service for managing user accounts
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service UserService {
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  // Create a new user account
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  rpc CreateUser(CreateUserRequest) returns (User);
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  // Get user by ID
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  rpc GetUser(GetUserRequest) returns (User);
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  // List users with pagination
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  rpc ListUsers(ListUsersRequest) returns (ListUsersResponse);
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  // Update user information
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  rpc UpdateUser(UpdateUserRequest) returns (User);
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  // Delete a user account
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  rpc DeleteUser(DeleteUserRequest) returns (google.protobuf.Empty);
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  // Stream user events (server streaming)
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  rpc StreamUserEvents(StreamUserEventsRequest) returns (stream UserEvent);
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  // Upload user data (client streaming)
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  rpc UploadUserData(stream UploadUserDataRequest) returns (UploadUserDataResponse);
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  // Real-time chat (bidirectional streaming)
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  rpc Chat(stream ChatMessage) returns (stream ChatMessage);
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}
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// Messages for user operations
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message User {
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  string id = 1;
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  string email = 2;
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  string name = 3;
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  int32 age = 4;
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  google.protobuf.Timestamp created_at = 5;
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  UserStatus status = 6;
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}
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message CreateUserRequest {
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  string email = 1;
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  string name = 2;
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  int32 age = 3;
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}
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message GetUserRequest {
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  string id = 1;
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}
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message ListUsersRequest {
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  int32 page_size = 1;
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  string page_token = 2;
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  string filter = 3;
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}
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message ListUsersResponse {
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  repeated User users = 1;
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  string next_page_token = 2;
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  int32 total_count = 3;
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}
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message UpdateUserRequest {
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  string id = 1;
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  User user = 2;
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  google.protobuf.FieldMask update_mask = 3;
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}
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message DeleteUserRequest {
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  string id = 1;
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}
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message StreamUserEventsRequest {
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  string user_id = 1;
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  repeated UserEventType event_types = 2;
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}
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message UserEvent {
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  string id = 1;
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  string user_id = 2;
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  UserEventType type = 3;
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  google.protobuf.Timestamp timestamp = 4;
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  google.protobuf.Any data = 5;
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}
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message UploadUserDataRequest {
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  oneof data {
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    UserDataChunk chunk = 1;
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    UserDataMetadata metadata = 2;
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  }
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}
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message UserDataChunk {
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  bytes data = 1;
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  int64 offset = 2;
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}
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message UserDataMetadata {
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  string filename = 1;
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  int64 total_size = 2;
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  string content_type = 3;
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}
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message UploadUserDataResponse {
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  string file_id = 1;
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  int64 bytes_uploaded = 2;
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}
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message ChatMessage {
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  string id = 1;
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  string user_id = 2;
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  string room_id = 3;
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  string content = 4;
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  google.protobuf.Timestamp timestamp = 5;
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  ChatMessageType type = 6;
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}
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// Enums
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enum UserStatus {
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  USER_STATUS_UNSPECIFIED = 0;
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  USER_STATUS_ACTIVE = 1;
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  USER_STATUS_INACTIVE = 2;
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  USER_STATUS_SUSPENDED = 3;
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}
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enum UserEventType {
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  USER_EVENT_TYPE_UNSPECIFIED = 0;
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  USER_EVENT_TYPE_CREATED = 1;
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  USER_EVENT_TYPE_UPDATED = 2;
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  USER_EVENT_TYPE_DELETED = 3;
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  USER_EVENT_TYPE_LOGIN = 4;
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  USER_EVENT_TYPE_LOGOUT = 5;
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}
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enum ChatMessageType {
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  CHAT_MESSAGE_TYPE_UNSPECIFIED = 0;
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  CHAT_MESSAGE_TYPE_TEXT = 1;
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  CHAT_MESSAGE_TYPE_IMAGE = 2;
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  CHAT_MESSAGE_TYPE_FILE = 3;
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  CHAT_MESSAGE_TYPE_SYSTEM = 4;
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}