Provider scope (default, request, transient) in NestJS - Performance & Optimization

Performance: Provider scope (default, request, transient)

MEDIUM IMPACT

This affects server response time and memory usage by controlling how often instances of providers are created during request handling.

Managing service instances for HTTP requests

NestJS

import { Injectable, Scope } from '@nestjs/common';

@Injectable({ scope: Scope.REQUEST })
export class UserService {
  // stateful data per request
}

// New instance per HTTP request avoids shared state

Creates a fresh instance per request, avoiding shared state and reducing memory leaks.

📈 Performance GainImproves response consistency and controls memory usage by limiting instance lifetime.

Managing service instances for HTTP requests

NestJS

import { Injectable, Scope } from '@nestjs/common';

@Injectable({ scope: Scope.DEFAULT })
export class UserService {
  // stateful data stored here
}

// Used in many requests, but state is kept per instance

Using default scope with stateful providers can cause shared state bugs and unnecessary memory retention.

📉 Performance CostIncreases memory usage and can cause slower response times due to unintended state sharing.

Performance Comparison

Pattern	Instance Creation Frequency	Memory Usage	Response Time Impact	Verdict
Default Scope (Singleton)	Once per app lifecycle	Low	Fast	[OK] Good
Request Scope	Once per HTTP request	Medium	Moderate	[!] OK
Transient Scope	Every injection	High	Slower if overused	[X] Bad if misused

Rendering Pipeline

Provider scope affects how NestJS creates and manages service instances during request processing, impacting memory allocation and CPU usage before sending response.

→Dependency Injection

→Request Handling

→Memory Management

⚠️ BottleneckMemory Management due to instance creation and garbage collection

Optimization Tips

1Use default (singleton) scope for stateless services to minimize instance creation.

2Use request scope only when you need per-request state isolation.

3Use transient scope sparingly for unique short-lived instances to avoid memory bloat.

Performance Quiz - 3 Questions

Which provider scope creates a new instance for every HTTP request in NestJS?

A Default scope

B Request scope

C Transient scope

D Singleton scope

DevTools: Node.js Profiler or NestJS Debug Logs

How to check: Enable debug logs for provider lifecycle or use Node.js profiler to monitor object creation and memory usage during requests.

What to look for: Look for excessive instance creations or memory spikes indicating inefficient provider scope usage.

Practice

1. In NestJS, what does the default provider scope mean for a service instance?

easy

A. A new instance is created for each HTTP request.

B. A single instance is shared across the entire application.

C. A new instance is created every time the service is injected.

D. The service instance is destroyed after each method call.

Solution

1. Step 1: Understand default provider scope behavior

   By default, NestJS creates one instance of a provider and shares it across the whole app.

2. Step 2: Compare with other scopes

   Request scope creates per HTTP request instances, transient creates per injection instances, so default is the shared singleton.

3. Final Answer:

   A single instance is shared across the entire application. -> Option B

4. Quick Check:

   Default scope = singleton instance [OK]

Hint: Default scope means one shared instance app-wide [OK]

Common Mistakes:

• Confusing default with request scope
• Thinking default creates new instances per injection
• Assuming default destroys instance after use

2. Which of the following is the correct way to set a provider's scope to transient in NestJS?

easy

A. @Injectable({ scope: Scope.REQUEST })

B. @Injectable({ scope: 'transient' })

C. @Injectable({ scope: Scope.TRANSIENT })

D. @Injectable({ transient: true })

Solution

1. Step 1: Recall correct syntax for setting scope

   In NestJS, the scope is set using the Scope enum inside @Injectable decorator like { scope: Scope.TRANSIENT }.

2. Step 2: Eliminate incorrect options

   @Injectable({ scope: Scope.REQUEST }) uses REQUEST scope, @Injectable({ scope: 'transient' }) uses string instead of enum, @Injectable({ transient: true }) uses invalid property.

3. Final Answer:

   @Injectable({ scope: Scope.TRANSIENT }) -> Option C

4. Quick Check:

   Use Scope.TRANSIENT enum for transient scope [OK]

Hint: Use Scope.TRANSIENT enum inside @Injectable decorator [OK]

Common Mistakes:

• Using string 'transient' instead of Scope enum
• Confusing transient with request scope
• Using invalid decorator properties

3. Consider this NestJS service with transient scope:

@Injectable({ scope: Scope.TRANSIENT })
export class MyService {
  id = Math.random();
}

If this service is injected twice in the same controller, what will be true about the id values?

medium

A. Both injections share the same id value.

B. The service will throw an error on multiple injections.

C. The id value is always zero.

D. Each injection has a different id value.

Solution

1. Step 1: Understand transient scope behavior

   Transient scope creates a new instance every time the service is injected, so each injection gets a fresh object.

2. Step 2: Apply to the id property

   Since id is set to a random number on instance creation, each injection will have a different id.

3. Final Answer:

   Each injection has a different id value. -> Option D

4. Quick Check:

   Transient scope = new instance per injection [OK]

Hint: Transient scope means new instance each injection [OK]

Common Mistakes:

• Assuming transient shares instance like default
• Thinking id is constant or zero
• Expecting errors on multiple injections

4. You set a service to request scope but notice it behaves like a singleton. What is the most likely cause?

medium

A. The controller injecting the service is not handling HTTP requests.

B. The service was not decorated with @Injectable.

C. The service scope was set incorrectly using a string instead of Scope.REQUEST.

D. The service is injected only once in the entire app.

Solution

1. Step 1: Understand request scope requirements

   Request scope creates a new instance per HTTP request, so it only works if the provider is injected in a context handling requests.

2. Step 2: Analyze the controller context

   If the controller is not handling HTTP requests (e.g., injected in a service or singleton), the request scope behaves like singleton.

3. Final Answer:

   The controller injecting the service is not handling HTTP requests. -> Option A

4. Quick Check:

   Request scope needs HTTP request context [OK]

Hint: Request scope needs HTTP request context to work [OK]

Common Mistakes:

• Forgetting request scope needs HTTP context
• Confusing decorator absence with scope behavior
• Assuming string scope works like enum

5. You have a service with request scope injected into a transient-scoped service, which is then injected into a controller. How many instances of the request-scoped service are created per HTTP request?

hard

A. One instance per HTTP request.

B. One instance per injection of the transient service.

C. One instance per controller instance.

D. Multiple instances per HTTP request depending on injections.

Solution

1. Step 1: Understand request scope lifetime

   Request-scoped services create one instance per HTTP request regardless of how many times they are injected.

2. Step 2: Analyze injection chain

   Even if injected into a transient service (new instance per injection), the request-scoped service instance remains the same within the same HTTP request.

3. Final Answer:

   One instance per HTTP request. -> Option A

4. Quick Check:

   Request scope = one instance per HTTP request [OK]

Hint: Request scope always one instance per HTTP request [OK]

Common Mistakes:

• Thinking transient causes multiple request-scoped instances
• Confusing transient and request scope lifetimes
• Assuming controller scope affects request-scoped service count