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Node.js Backend Development Bootcamp · Lezione

RPC unary, streaming server, client e bidirezionale

Implementi i quattro tipi di chiamata gRPC e scelga il pattern più adatto per ogni interazione.

RPC unary, streaming server, client e bidirezionale è una lezione Node.js Backend Development Bootcamp gratuita su CoddyKit. Questa è la lezione 2 di 4. Puoi leggere la lezione completa qui gratuitamente — poi esercitati direttamente nel browser con un editor di codice integrato e un tutor IA disponibile 24/7. Fa parte del percorso di apprendimento Node.js Backend Development Bootcamp, e i tuoi progressi si sincronizzano tra il web e l'app CoddyKit. Il corso Node.js Backend Development Bootcamp include 4 lezioni in totale.

Parti di questa lezione non sono ancora state tradotte e vengono mostrate in inglese.

Four Ways to Call an RPC

gRPC defines four kinds of method in your .proto file, and each one maps to a different streaming shape:

  • Unary — one request, one response (like a normal function call).
  • Server streaming — one request, a stream of responses.
  • Client streaming — a stream of requests, one response.
  • Bidirectional streaming — both sides stream independently.

The stream keyword in the service definition is what decides the shape. In this lesson you'll implement all four in Node.js with @grpc/grpc-js and learn when to reach for each.

syntax = "proto3";
package chat;

service ChatService {
  // unary
  rpc GetUser (UserRequest) returns (User);
  // server streaming
  rpc ListMessages (RoomRequest) returns (stream Message);
  // client streaming
  rpc UploadLogs (stream LogLine) returns (UploadSummary);
  // bidirectional streaming
  rpc Chat (stream Message) returns (stream Message);
}

Loading the Proto in Node.js

Before you implement any handler, you load the .proto definition at runtime with @grpc/proto-loader and turn it into a gRPC service object with @grpc/grpc-js.

The loaded packageDefinition mirrors your proto packages and services. You attach handlers to the service with server.addService() — the names you provide must match the rpc names exactly.

const grpc = require('@grpc/grpc-js');
const protoLoader = require('@grpc/proto-loader');

const pkgDef = protoLoader.loadSync('chat.proto', {
  keepCase: true,
  longs: String,
  enums: String,
  defaults: true,
  oneofs: true,
});

const proto = grpc.loadPackageDefinition(pkgDef).chat;
const server = new grpc.Server();

// handlers get attached here
server.addService(proto.ChatService.service, { /* ... */ });

Unary RPC — One In, One Out

A unary handler receives (call, callback). The request payload is on call.request, and you reply by invoking the Node-style callback(error, response).

  • Pass null as the first argument for success.
  • Pass a { code, details } error object to signal failure.

Unary is the right choice for classic request/response work: fetching a record, validating input, performing a single mutation.

function getUser(call, callback) {
  const { id } = call.request;
  if (!id) {
    return callback({
      code: grpc.status.INVALID_ARGUMENT,
      details: 'id is required',
    });
  }
  const user = { id, name: 'Ada Lovelace' };
  callback(null, user);
}

server.addService(proto.ChatService.service, { GetUser: getUser });

Server Streaming — One In, Many Out

A server-streaming handler receives only (call) — there is no callback. You push each response with call.write(message), then signal completion with call.end().

This pattern shines when the server produces a sequence the client can consume incrementally: paginated results, log tails, progress events, or large result sets you don't want to buffer in memory.

function listMessages(call) {
  const { roomId } = call.request;
  const messages = loadMessagesForRoom(roomId); // array

  for (const msg of messages) {
    call.write({ id: msg.id, text: msg.text });
  }
  call.end(); // closes the stream to the client
}

server.addService(proto.ChatService.service, {
  ListMessages: listMessages,
});

Backpressure in Server Streaming

call.write() returns a boolean. When it returns false, the internal buffer is full and you should wait for the 'drain' event before writing more. Ignoring this on large streams can balloon memory usage.

The robust pattern wraps writes in a promise that resolves on drain, so an async loop naturally pauses when the consumer is slow.

function writeAsync(call, msg) {
  return new Promise((resolve) => {
    if (call.write(msg)) resolve();
    else call.once('drain', resolve);
  });
}

async function streamBigResult(call) {
  for (let i = 0; i < 100000; i++) {
    await writeAsync(call, { id: i, text: 'row ' + i });
  }
  call.end();
}

Client Streaming — Many In, One Out

A client-streaming handler receives (call, callback). You listen for incoming items with call.on('data', ...), do final work on call.on('end', ...), and send the single response via the callback.

Use it when the client feeds many items but you only need one aggregate result: bulk uploads, batched metrics ingestion, or computing a summary/average over a stream.

function uploadLogs(call, callback) {
  let count = 0;
  let bytes = 0;

  call.on('data', (logLine) => {
    count += 1;
    bytes += Buffer.byteLength(logLine.text || '');
  });

  call.on('end', () => {
    callback(null, { received: count, totalBytes: bytes });
  });

  call.on('error', (err) => console.error('client stream error', err));
}

Bidirectional Streaming — Many In, Many Out

A bidirectional handler receives only (call) and treats it as a duplex stream: read incoming items with call.on('data', ...) and emit responses with call.write(...) at any time. Both directions are fully independent.

This is ideal for chat, multiplayer state sync, or live request/response negotiation where either side may speak first or out of lockstep.

function chat(call) {
  call.on('data', (msg) => {
    // echo every message back to the sender, augmented
    call.write({ id: msg.id, text: 'echo: ' + msg.text });
  });

  call.on('end', () => call.end());
  call.on('error', (err) => console.error('chat error', err));
}

server.addService(proto.ChatService.service, { Chat: chat });

Calling From a gRPC Client

The client API mirrors the server shapes. Unary returns via a callback; server streaming returns a readable stream you iterate with 'data'; client streaming gives you a writable stream you write() to and then end(); bidirectional gives you both at once.

The single rule: the stream keyword on each side of the proto determines whether you get a callback or a stream object.

const client = new proto.ChatService(
  'localhost:50051',
  grpc.credentials.createInsecure(),
);

// unary
client.GetUser({ id: '1' }, (err, user) => console.log(user));

// server streaming
const stream = client.ListMessages({ roomId: 'general' });
stream.on('data', (m) => console.log(m.text));
stream.on('end', () => console.log('done'));

Mental Model: Pick by Cardinality

Choosing the right RPC type is almost always a question of cardinality on each side:

  • One request, one response → unary.
  • One request, results arrive over time → server streaming.
  • Many inputs collapsed into one result → client streaming.
  • Continuous, independent two-way flow → bidirectional.

Don't reach for streaming just because data is large — pagination over unary calls is often simpler. Reach for streaming when the data is open-ended in time or genuinely incremental.

A Runnable Cardinality Picker

Here's a tiny, framework-free helper that encodes the decision rule above. It takes whether each side streams and returns the gRPC method type — useful as a sanity check when designing a service.

This is plain JavaScript with no gRPC dependency, so an online judge can run it directly.

function rpcType(clientStreams, serverStreams) {
  if (!clientStreams && !serverStreams) return 'unary';
  if (!clientStreams && serverStreams) return 'server-streaming';
  if (clientStreams && !serverStreams) return 'client-streaming';
  return 'bidirectional';
}

console.log(rpcType(false, false)); // unary
console.log(rpcType(false, true));  // server-streaming
console.log(rpcType(true, false));  // client-streaming
console.log(rpcType(true, true));   // bidirectional

Errors, Deadlines, and Cleanup

Across all four types, a few production habits matter:

  • Always attach call.on('error', ...) on streaming handlers — an unhandled stream error can crash the process.
  • Respect deadlines: check call.cancelled in long loops and stop writing if the client gave up.
  • For client/bidi streams, do final work in 'end', not after the first 'data'.
  • Send domain errors with grpc.status codes (e.g. NOT_FOUND, INVALID_ARGUMENT) rather than throwing raw exceptions.
function listMessages(call) {
  let i = 0;
  const timer = setInterval(() => {
    if (call.cancelled) { clearInterval(timer); return; }
    if (i >= 10) { clearInterval(timer); return call.end(); }
    call.write({ id: i, text: 'tick ' + i++ });
  }, 100);

  call.on('error', () => clearInterval(timer));
}

Quick Check

Test your understanding of choosing the right RPC type.

Recap

You now know all four gRPC call shapes and how to implement them in Node.js with @grpc/grpc-js:

  • Unary — (call, callback); read call.request, reply once.
  • Server streaming — (call); call.write() repeatedly, then call.end(), minding backpressure via 'drain'.
  • Client streaming — (call, callback); aggregate on 'data', respond once on 'end'.
  • Bidirectional — (call); read and write independently for live, two-way flows.

Choose by cardinality and whether the data is open-ended in time, always handle stream errors, and respect client deadlines with call.cancelled.

Domande Frequenti

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Implementi i quattro tipi di chiamata gRPC e scelga il pattern più adatto per ogni interazione. Eserciti Node.js Backend Development Bootcamp con codice pratico che esegui direttamente nel browser, e un tutor IA 24/7 risponde alle tue domande mentre lavori sulla lezione.

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Tutte le lezioni di questo corso

  1. Definizione di servizi e messaggi con Protobuf IDL
  2. RPC unary, streaming server, client e bidirezionale
  3. Interceptor, deadline e metadati
  4. Evoluzione dei proto e compatibilità con le versioni precedenti
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