Event patterns (event-based) in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an event pattern in NestJS event-based communication?

An event pattern is a unique identifier string used to listen for or emit specific events between microservices or modules in NestJS. It helps match events with their handlers.

beginner

How do you define an event handler for a specific event pattern in NestJS?

You use the @EventPattern('patternName') decorator on a method inside a controller or service. This method will run when an event with that pattern is received.

intermediate

What is the difference between @EventPattern and @MessagePattern in NestJS?

@EventPattern listens for events that do not expect a response (fire-and-forget). @MessagePattern listens for messages that expect a response (request-response).

beginner

How do you emit an event with a pattern in NestJS?

You use the ClientProxy's emit method with the event pattern string and the data payload. For example: client.emit('order_created', { id: 1 }).

intermediate

Why are event patterns useful in microservice architectures?

They allow services to communicate asynchronously and loosely coupled by sending and listening to events identified by patterns, improving scalability and flexibility.

Which decorator do you use to listen for an event pattern in NestJS?

A @SubscribeEvent

B @MessagePattern

C @EventPattern

D @ListenEvent

The @EventPattern decorator is used to listen for events matching a specific pattern.

What method do you call to emit an event from a NestJS client proxy?

A emit()

B dispatch()

C send()

D trigger()

The emit() method sends an event without expecting a response.

What is the main difference between @EventPattern and @MessagePattern?

A @EventPattern expects a response, @MessagePattern does not

B @EventPattern is fire-and-forget, @MessagePattern expects a response

C @EventPattern is for HTTP, @MessagePattern is for TCP

D They are the same

@EventPattern handles fire-and-forget events; @MessagePattern handles request-response messages.

In NestJS, what does an event pattern usually look like?

A A string identifier

B A function

C A boolean value

D A number

Event patterns are string identifiers used to match events with handlers.

Why use event patterns in microservices?

A To avoid using events

B To enable synchronous communication

C To tightly couple services

D To allow asynchronous, loosely coupled communication

Event patterns enable asynchronous and loosely coupled communication between services.

Explain how to create and handle an event pattern in NestJS.

Think about how you listen and send events using patterns.

Describe the benefits of using event patterns in a microservice system.

Consider how events help services work independently.

Practice

1. What is the main purpose of using @EventPattern in a NestJS application?

easy

A. To create database entities

B. To define HTTP routes for REST APIs

C. To listen for specific events and handle their data

D. To style components with CSS

Solution

1. Step 1: Understand the role of @EventPattern

   This decorator is used to mark a method that listens for a specific event in an event-based system.

2. Step 2: Compare with other options

   Options B, C, and D relate to other parts of NestJS or frontend development, not event listening.

3. Final Answer:

   To listen for specific events and handle their data -> Option C

4. Quick Check:

   @EventPattern listens to events [OK]

Hint: Remember: @EventPattern listens to events, not routes or styles [OK]

Common Mistakes:

• Confusing @EventPattern with @Controller for routes
• Thinking @EventPattern creates database models
• Mixing event listening with styling tasks

2. Which of the following is the correct way to receive event data inside an event handler method in NestJS?

easy

A. Use @Payload() decorator on a method parameter

B. Use @Body() decorator on a method parameter

C. Use @Query() decorator on a method parameter

D. Use @Param() decorator on a method parameter

Solution

1. Step 1: Identify the decorator for event data

   In NestJS event handlers, @Payload() extracts the data sent with the event.

2. Step 2: Differentiate from HTTP decorators

   @Body(), @Query(), and @Param() are used in HTTP controllers, not event handlers.

3. Final Answer:

   Use @Payload() decorator on a method parameter -> Option A

4. Quick Check:

   @Payload() gets event data [OK]

Hint: Event data comes from @Payload(), not HTTP decorators [OK]

Common Mistakes:

• Using @Body() in event handlers instead of @Payload()
• Confusing HTTP request decorators with event decorators
• Omitting the decorator and expecting automatic data injection

3. Given the following NestJS event handler code, what will be logged when an event named 'order_created' with payload { id: 42 } is emitted?

@EventPattern('order_created')
handleOrder(@Payload() data: any) {
  console.log(`Order ID: ${data.id}`);
}

medium

A. No output because event name is wrong

B. Order ID: undefined

C. SyntaxError due to missing return

D. Order ID: 42

Solution

1. Step 1: Understand event matching

   The handler listens for 'order_created' events, so it will trigger when such an event is emitted.

2. Step 2: Check payload usage

   The payload has an object with id: 42, so data.id is 42 and will be logged.

3. Final Answer:

   Order ID: 42 -> Option D

4. Quick Check:

   Payload id 42 logs correctly [OK]

Hint: Match event name exactly and use @Payload() to access data [OK]

Common Mistakes:

• Assuming event name is case-insensitive
• Forgetting to use @Payload() and getting undefined
• Expecting a return value to log output

4. Identify the error in this NestJS event handler code:

@EventPattern('user_registered')
handleUser(@Payload data: any) {
  console.log(data.username);
}

medium

A. Missing parentheses in @Payload decorator

B. Incorrect event name format

C. Method name must be 'onUserRegistered'

D. Payload parameter must be last in method

Solution

1. Step 1: Check decorator syntax

   The @Payload decorator requires parentheses even if no arguments are passed, so it should be @Payload().

2. Step 2: Verify other parts

   Event name format is correct, method name can be anything, and parameter order is flexible.

3. Final Answer:

   Missing parentheses in @Payload decorator -> Option A

4. Quick Check:

   @Payload() needs parentheses [OK]

Hint: Always write @Payload() with parentheses, even if empty [OK]

Common Mistakes:

• Omitting parentheses in decorators
• Thinking method names must follow event names
• Assuming parameter order matters for decorators

5. You want to create a NestJS service that listens to two different events: 'payment_success' and 'payment_failed'. Which of the following is the best way to implement this using event patterns?

hard

A. Use @Controller with two routes named after events

B. Create two methods, each decorated with @EventPattern for each event name

C. Create one method with @EventPattern('payment') and check event type inside

D. Use @Payload() decorator twice in one method for both events

Solution

1. Step 1: Understand event pattern usage

   Each event should have its own handler method decorated with @EventPattern specifying the event name.

2. Step 2: Evaluate other options

   Create one method with @EventPattern('payment') and check event type inside is less clear and not idiomatic; Use @Controller with two routes named after events is for HTTP routes, not events; Use @Payload() decorator twice in one method for both events is invalid syntax.

3. Final Answer:

   Create two methods, each decorated with @EventPattern for each event name -> Option B

4. Quick Check:

   Separate methods for separate events [OK]

Hint: Use one @EventPattern per event with separate methods [OK]

Common Mistakes:

• Trying to handle multiple events in one method without checking
• Confusing event handlers with HTTP controllers
• Using multiple @Payload decorators in one method