TCP transport in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Understanding TCP Transport Client Behavior in NestJS

Consider a NestJS microservice client configured with TCP transport connecting to a server. What will happen if the server is not running when the client tries to send a message?

NestJS

const client = ClientProxyFactory.create({ transport: Transport.TCP, options: { host: '127.0.0.1', port: 3001 } });

async function sendMessage() {
  return client.send('test_pattern', { data: 'hello' }).toPromise();
}

sendMessage().catch(err => console.error(err));

A The client will throw an error immediately because the server is unreachable.

B The client will connect to a default fallback server automatically.

C The client will silently drop the message without any error or retry.

D The client will queue the message and retry sending until the server becomes available.

💡 Hint

Think about how NestJS TCP clients handle connection retries by default.

By default, NestJS TCP clients will attempt to reconnect and retry sending messages if the server is not available. This means the message is queued and retried until the server responds or the client times out.

📝 Syntax

intermediate

Correct TCP Transport Server Setup in NestJS

Which of the following code snippets correctly creates a TCP microservice server in NestJS listening on port 4000?

A

const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { port: 4000 } });
await app.listen();

B

const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, port: 4000 });
await app.listen();

C

const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { host: 'localhost' } });
await app.listen();

D

const app = await NestFactory.createMicroservice(AppModule, { transport: 'tcp', options: { port: 4000 } });
await app.listen();

💡 Hint

Check how the port is specified inside the options object.

The correct way to configure a TCP microservice server in NestJS is to specify the transport as Transport.TCP and provide the port inside the options object.

🔧 Debug

advanced

Diagnosing TCP Transport Connection Refused Error

A NestJS TCP client throws a "connect ECONNREFUSED" error when trying to send a message. Which of the following is the most likely cause?

A The client and server have mismatched message payload formats.

B The client is using the wrong message pattern name.

C The server is not running or not listening on the specified port.

D The client forgot to import the Transport module.

💡 Hint

ECONNREFUSED means the connection was actively refused by the target machine.

"ECONNREFUSED" indicates that the client tried to connect but no server was listening on the target port, so the connection was refused.

❓ state_output

advanced

Output of TCP Client Send with Timeout

Given a NestJS TCP client configured with a 2-second timeout, what will be the output if the server takes 5 seconds to respond?

NestJS

const client = ClientProxyFactory.create({
  transport: Transport.TCP,
  options: { host: '127.0.0.1', port: 3001, timeout: 2000 }
});

async function send() {
  try {
    const response = await client.send('delayed_response', {}).toPromise();
    console.log('Response:', response);
  } catch (err) {
    console.log('Error:', err.message);
  }
}
send();

A Logs: 'Error: Timeout has occurred' after 2 seconds.

B Logs: 'Error: Connection refused' immediately.

C Logs: 'Response: <server response>' after 5 seconds.

D No output; the client waits indefinitely.

💡 Hint

Consider the effect of the timeout option on client requests.

The timeout option causes the client to throw a timeout error if the server does not respond within the specified time.

🧠 Conceptual

expert

TCP Transport Message Pattern Matching in NestJS

In a NestJS TCP microservice, how does the server determine which handler to invoke for an incoming message?

A It uses the message payload type to select the handler automatically.

B It matches the message's pattern property to the @MessagePattern decorator's pattern on handlers.

C It invokes all handlers and returns the first non-null response.

D It relies on the client to specify the handler function name explicitly.

💡 Hint

Think about how NestJS uses decorators to route messages.

The server uses the pattern string sent by the client to find the matching @MessagePattern handler method.