Hybrid applications in NestJS - Step-by-Step Execution

Concept Flow - Hybrid applications

Start NestJS App

↓

Create HTTP Server

↓

Create Microservice Server

↓

Combine Servers in Hybrid App

↓

Listen to HTTP Requests

↓

Listen to Microservice Messages

↓

Handle Requests & Messages

↓

Shutdown Servers

The NestJS hybrid app starts by creating both HTTP and microservice servers, then combines them to listen and handle requests simultaneously.

Execution Sample

NestJS

import { NestFactory } from '@nestjs/core';
import { Transport } from '@nestjs/microservices';
import { AppModule } from './app.module';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.connectMicroservice({ transport: Transport.TCP });
  await app.startAllMicroservices();
  await app.listen(3000);
}

bootstrap();

This code creates a NestJS app that runs both an HTTP server and a TCP microservice server together.

Execution Table

Step	Action	Evaluation	Result
1	Call NestFactory.create(AppModule)	Creates HTTP app instance	HTTP server ready
2	Call app.connectMicroservice({ transport: Transport.TCP })	Adds microservice config	Microservice server configured
3	Call app.startAllMicroservices()	Starts microservice server	Microservice server listening
4	Call app.listen(3000)	Starts HTTP server on port 3000	HTTP server listening
5	Receive HTTP request	Route to HTTP handler	Response sent
6	Receive microservice message	Route to microservice handler	Message processed
7	Shutdown app	Close HTTP and microservice servers	Servers stopped

💡 Execution stops when servers are shut down or app is terminated

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
app	undefined	HTTP app instance	HTTP + microservice config	Microservice started	HTTP + microservice listening	Servers stopped

Key Moments - 3 Insights

Why do we call connectMicroservice before starting microservices?

Because connectMicroservice sets up the microservice configuration needed before starting it, as shown in steps 2 and 3 of the execution_table.

Does app.listen start both HTTP and microservice servers?

No, app.listen only starts the HTTP server (step 4). Microservices start separately with startAllMicroservices (step 3).

Can the app handle HTTP and microservice requests at the same time?

Yes, after both servers start (steps 3 and 4), the app listens and handles both kinds of requests concurrently (steps 5 and 6).

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of 'app' after step 2?

A HTTP app instance only

B HTTP app with microservice configured

C Microservice started

D Servers stopped

💡 Hint

Check variable_tracker column 'After Step 2' for 'app' state.

At which step does the HTTP server start listening on port 3000?

A Step 1

B Step 3

C Step 4

D Step 2

💡 Hint

Look at execution_table row with action 'Call app.listen(3000)'.

If we skip startAllMicroservices(), what happens?

A Microservice server will not start listening

B HTTP server will not start

C Both servers start normally

D App will crash immediately

💡 Hint

Refer to execution_table steps 3 and 4 about starting servers.

Concept Snapshot

NestJS Hybrid Applications:
- Create HTTP app with NestFactory.create()
- Add microservice with connectMicroservice()
- Start microservices with startAllMicroservices()
- Start HTTP server with listen(port)
- Both servers run concurrently handling requests
- Shutdown closes all servers cleanly

Full Transcript

This visual trace shows how a NestJS hybrid application runs. First, the app is created as an HTTP server. Then, a microservice is connected and started separately. After both servers are running, the app listens for HTTP requests on port 3000 and microservice messages simultaneously. The variable 'app' changes from undefined to an HTTP app instance, then to a hybrid app with microservice configured and started. Key moments clarify why microservices start separately and how both servers handle requests concurrently. The quiz tests understanding of app state after each step and the order of server startup. This helps beginners see the step-by-step flow of hybrid app execution in NestJS.