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NestJS Microservice quizz

Getting started

In order to run the application:

npm ci
npm run start:dev

You should have the appropriate configuration and services running on your machine in order to ensure the application is launching properly.

Telemetry

This application is based on @opentelemetry-js, so metrics and traces are generated through it. Moreover, logs (in production environment) have fields (trace.id and span.id) in order to correlate logs with traces. You can find the configuration files for tracing in tracing.config.ts. When forking the boilerplate, you should update tracing.config.ts:12 to match your service.

Learn more about how you can use spans in your code with nestjs-otel. And more globally, to learn about telemetry, see this CNCF article.

Configuration

The service configuration is handled over YAML and environment variables.

In src/resources, you'll find a file, application.yml which contains the service configuration in YAML format. This file is the first file loaded by the service..

For example, by default the src/resources/application.yml contains :

server:
  port: 8080

You can add all the configuration required by your service here. Every key/values will be loaded into your application context.

To access a configuration value in your code, you can simply do the following :

const config = app.get<ConfigService>(ConfigService);
const port = config.get<number>("server.port", 3000);

console.log(port); // 8080

If you need to define environment specific variables, you can simply create another file, in the following format : application.${ENVIRONMENT}.yml. For example, for a production environment, you'll create the file src/resources/application.production.yml :

By default, the service will assume that your configuration files are located into resources. If you're in a Docker container for example, and that your configuration files were copied into /etc/app/config, you can simply use the special environment variable NEST_ADDITIONAL_CONFIG_LOCATION=/etc/app/config to specify the service to check for configurations into the directory specified.

# src/resources/application.production.yml
server:
  tls: true
const config = app.get<ConfigService>(ConfigService);
const port = config.get<number>("server.port", 3000);
const useTls = config.get<boolean>("server.tls", false);

console.log(port); // 8080
console.log(useTls); // true

The configuration keys from different sources will be merged at the runtime. So you can define some common configurations into the application.yml, and add more environment specific variables into application.${ENVIRONMENT}.yml.

The last thing about configuration is that everything is overridable by environment. If you wish to override the server.port for example, you can simply export the variable NEST_SERVER_PORT.

Be careful, environment variables take precedence over each other configuration method. You'll find below how the service will load it's configuration :

  • application.yml
  • application.${ENVIRONMENT}.yml
  • Environment variables

Kafka

Official documentation: https://docs.nestjs.com/microservices/kafka

If you need to use Kafka, you must inject the client in your class.

  constructor(
    @InjectKafkaClient() private readonly client: ClientKafka
  ) {}

You can use the @Expose() decorator to set only a specific property to be sent to the topic. In the example below, only the id and title will be sent to the topic.

export class Foo {
  @Expose()
  id: string;

  @Expose()
  title: string;

  views: string;
}

In order to use the @Expose() decorator, you must use the KafkaEventBuilder.

const message = new KafkaEventBuilder(id, element)
  .withTrigger(TriggerType.CREATE)
  .build();

this.kafkaClient.emit(FOO_TOPICS, message);