NestJS Enterprise Backend APIs · Lección

Cargas directas a S3 con URL prefirmadas

Traslade las cargas pesadas al almacenamiento de objetos emitiendo URL prefirmadas de corta duración desde la API.

Cargas directas a S3 con URL prefirmadas es una lección gratuita de NestJS Enterprise Backend APIs en CoddyKit. Esta es la lección 3 de 4. Puedes leer la lección completa abajo gratuitamente — luego la practicas en el navegador con un editor de código integrado y un tutor de IA 24/7. Forma parte de la ruta de aprendizaje de NestJS Enterprise Backend APIs, y tu progreso se sincroniza en la web y la app de CoddyKit. El curso de NestJS Enterprise Backend APIs incluye 4 lecciones en total.

Partes de esta lección aún no han sido traducidas y se muestran en inglés.

Why Not Proxy Uploads Through the API?

When a client uploads a large file, the naive design sends the bytes to your NestJS API, which then forwards them to object storage. This makes your server a bottleneck.

Memory & CPU pressure — every upload occupies a request thread and buffers/streams through your process.
Doubled bandwidth — bytes travel client → API → S3, so you pay for the same data twice.
Request timeouts — load balancers (e.g. ALB, Nginx) cap request duration; multi-GB uploads stall.

The fix: let the browser upload directly to S3. Your API only issues a short-lived, signed URL that grants permission for one specific operation.

What Is a Presigned URL?

A presigned URL is a normal S3 object URL with extra query parameters that encode a temporary, cryptographically-signed grant. Anyone holding the URL can perform exactly one operation (e.g. PutObject) on exactly one key, until it expires.

Signed with your AWS credentials, but the credentials are never exposed — only the signature is.
Scoped to a single HTTP method, bucket, and object key.
Has a hard expiry (seconds), after which S3 rejects it with 403.

Because S3 validates the signature itself, your API does not touch the file bytes at all.

The Upload Flow

The end-to-end flow has three actors: the browser, your NestJS API, and S3.

1. Request: Browser asks the API, "I want to upload avatar.png, 240 KB, image/png."
2. Sign: API validates the request, generates a unique key, and returns a presigned PUT URL.
3. Upload: Browser PUTs the raw bytes straight to that URL on S3.
4. Confirm: Browser tells the API the upload succeeded; API persists the key in the database.

Your API stays fast and stateless — it never proxies the payload.

Configuring the S3 Client

Use the AWS SDK v3 modular packages. Create a single S3Client instance and share it via a NestJS provider so credentials and region are configured in one place.

Credentials come from environment variables (or, in production, an IAM role). Never hardcode them.

import { Injectable } from '@nestjs/common';
import { ConfigService } from '@nestjs/config';
import { S3Client } from '@aws-sdk/client-s3';

@Injectable()
export class S3Provider {
  readonly client: S3Client;
  readonly bucket: string;

  constructor(private readonly config: ConfigService) {
    this.bucket = config.getOrThrow<string>('S3_BUCKET');
    this.client = new S3Client({
      region: config.getOrThrow<string>('AWS_REGION'),
      credentials: {
        accessKeyId: config.getOrThrow<string>('AWS_ACCESS_KEY_ID'),
        secretAccessKey: config.getOrThrow<string>('AWS_SECRET_ACCESS_KEY'),
      },
    });
  }
}

Generating a Presigned PUT URL

The @aws-sdk/s3-request-presigner package signs a command without executing it. You build a PutObjectCommand describing the target key and content type, then call getSignedUrl with an expiresIn.

ContentType in the command is enforced: the browser must send a matching Content-Type header.
expiresIn is in seconds — keep it short (60–300s) so leaked URLs die quickly.

import { Injectable } from '@nestjs/common';
import { PutObjectCommand } from '@aws-sdk/client-s3';
import { getSignedUrl } from '@aws-sdk/s3-request-presigner';
import { S3Provider } from './s3.provider';

@Injectable()
export class UploadsService {
  constructor(private readonly s3: S3Provider) {}

  async createUploadUrl(key: string, contentType: string): Promise<string> {
    const command = new PutObjectCommand({
      Bucket: this.s3.bucket,
      Key: key,
      ContentType: contentType,
    });
    return getSignedUrl(this.s3.client, command, { expiresIn: 120 });
  }
}

Generating Safe, Unique Object Keys

Never trust the client's filename as the S3 key. A user could send ../../etc/passwd or collide with another user's file. Generate the key server-side.

Namespace by owner: uploads/{userId}/... so authorization is easy to reason about.
Use a random UUID to guarantee uniqueness.
Preserve only a sanitized extension for content-type hints and tooling.

import { randomUUID } from 'node:crypto';
import { extname } from 'node:path';

function buildObjectKey(userId: string, originalName: string): string {
  const ext = extname(originalName).toLowerCase().replace(/[^.a-z0-9]/g, '');
  const safeExt = /^\.[a-z0-9]{1,8}$/.test(ext) ? ext : '';
  return `uploads/${userId}/${randomUUID()}${safeExt}`;
}

console.log(buildObjectKey('user-42', 'My Vacation Photo.PNG'));
console.log(buildObjectKey('user-42', 'sneaky/../../etc/passwd'));

The Controller Endpoint

Expose a guarded endpoint that takes the file metadata, validates it with a DTO, and returns the presigned URL plus the final key. The client needs the key later to confirm or to build the public/read URL.

Authentication matters here: the signing endpoint is your access-control gate. S3 itself trusts any valid signature, so all checks (who, what size, what type) must happen before you sign.

import { Body, Controller, Post, UseGuards, Req } from '@nestjs/common';
import { JwtAuthGuard } from '../auth/jwt-auth.guard';
import { UploadsService } from './uploads.service';
import { CreateUploadDto } from './dto/create-upload.dto';

@UseGuards(JwtAuthGuard)
@Controller('uploads')
export class UploadsController {
  constructor(private readonly uploads: UploadsService) {}

  @Post('presign')
  async presign(@Req() req, @Body() dto: CreateUploadDto) {
    return this.uploads.presignForUser(req.user.id, dto);
  }
}

Validating the Upload Request DTO

Validate metadata before signing. Reject disallowed MIME types and oversized files at the API layer — but remember the client could lie, so this is a first line of defense, not the last.

Whitelist contentType against an allow-list.
Cap the declared size to fail fast on obviously huge uploads.

import { IsIn, IsInt, IsString, Max, Min } from 'class-validator';

const ALLOWED = ['image/png', 'image/jpeg', 'image/webp', 'application/pdf'] as const;

export class CreateUploadDto {
  @IsString()
  filename: string;

  @IsIn(ALLOWED)
  contentType: (typeof ALLOWED)[number];

  @IsInt()
  @Min(1)
  @Max(10 * 1024 * 1024) // 10 MB
  size: number;
}

Enforcing Size with a Signed Content-Length

The DTO size check is advisory — the browser still controls how many bytes it actually PUTs. To make S3 itself reject oversized uploads, bind a content-length range into the signature.

For a single PUT, sign ContentLength so S3 enforces an exact byte count. For more flexible limits (a min/max range), use a presigned POST policy instead, which supports content-length-range conditions.

import { createPresignedPost } from '@aws-sdk/s3-presigned-post';

async function presignPost(client, bucket: string, key: string) {
  return createPresignedPost(client, {
    Bucket: bucket,
    Key: key,
    Conditions: [
      ['content-length-range', 1, 10 * 1024 * 1024], // 1 byte – 10 MB
      ['starts-with', '$Content-Type', 'image/'],
    ],
    Fields: { 'Content-Type': 'image/png' },
    Expires: 120,
  });
}

The Browser-Side Upload

With the presigned PUT URL in hand, the browser uploads with a plain fetch. There is no SDK and no AWS credentials on the client — just the bytes and a matching Content-Type.

The header must equal the ContentType you signed, or S3 returns 403 SignatureDoesNotMatch.
Do not send Authorization — the signature lives in the query string.

async function uploadFile(presignedUrl: string, file: File): Promise<void> {
  const res = await fetch(presignedUrl, {
    method: 'PUT',
    headers: { 'Content-Type': file.type },
    body: file,
  });
  if (!res.ok) {
    throw new Error(`Upload failed: ${res.status} ${res.statusText}`);
  }
}

Confirming the Upload & Reading Back

Because S3 doesn't notify your API, the client calls back after a successful PUT so you can persist the key. For extra safety, the API can HeadObject to verify the object truly exists and check its real size/type before trusting it.

To serve the file later, either keep the bucket private and issue a presigned GET URL on demand, or (for public assets) store and return the public URL.

import { GetObjectCommand, HeadObjectCommand } from '@aws-sdk/client-s3';
import { getSignedUrl } from '@aws-sdk/s3-request-presigner';

async confirm(userId: string, key: string) {
  const head = await this.s3.client.send(
    new HeadObjectCommand({ Bucket: this.s3.bucket, Key: key }),
  );
  if ((head.ContentLength ?? 0) > 10 * 1024 * 1024) {
    throw new BadRequestException('Object exceeds size limit');
  }
  await this.files.save({ userId, key, size: head.ContentLength });
}

async downloadUrl(key: string) {
  return getSignedUrl(
    this.s3.client,
    new GetObjectCommand({ Bucket: this.s3.bucket, Key: key }),
    { expiresIn: 300 },
  );
}

Quick Check

Test your understanding of where access control lives in this pattern.

Recap

You learned how to offload heavy uploads to S3 using presigned URLs:

Why: proxying bytes through the API wastes bandwidth, memory, and hits request timeouts.
How: the API signs a short-lived PutObjectCommand with getSignedUrl; the browser PUTs straight to S3.
Keys: always generate them server-side (UUID + namespaced by user); never trust client filenames.
Security: the presign endpoint is the access-control gate — authenticate, whitelist MIME types, and cap size there. Use presigned POST content-length-range to let S3 enforce size.
After upload: confirm with HeadObject, persist the key, and serve later via presigned GetObjectCommand URLs.

Preguntas frecuentes

¿La lección «Cargas directas a S3 con URL prefirmadas» es gratis?

Sí — el texto completo de «Cargas directas a S3 con URL prefirmadas» es gratis para leer aquí en la web. Para practicarla de forma interactiva (editor de código integrado y tutor de IA 24/7) y desbloquear el resto del curso de NestJS Enterprise Backend APIs, actualiza a CoddyKit PRO. El curso de NestJS Enterprise Backend APIs incluye 4 lecciones en total.

¿Qué aprenderé en «Cargas directas a S3 con URL prefirmadas»?

Traslade las cargas pesadas al almacenamiento de objetos emitiendo URL prefirmadas de corta duración desde la API. Practicas NestJS Enterprise Backend APIs con código real que ejecutas directamente en el navegador, y un tutor de IA 24/7 responde tus preguntas mientras trabajas en la lección.

¿Necesito experiencia previa para empezar NestJS Enterprise Backend APIs?

No se requiere experiencia previa. NestJS Enterprise Backend APIs en CoddyKit está estructurado para principiantes hasta estudiantes avanzados, así que puedes empezar aquí o desde el inicio y avanzar a tu ritmo. Esta es la lección 3 de 4.

¿Cuánto tiempo toma la lección «Cargas directas a S3 con URL prefirmadas»?

La mayoría de las lecciones de CoddyKit toman alrededor de 5–10 minutos. Cada una es compacta e interactiva, así que avanzas constantemente y retomas exactamente por donde dejaste en la web y la app.

¿Puedo escribir y ejecutar código en esta lección de NestJS Enterprise Backend APIs?

Sí. Cada lección de NestJS Enterprise Backend APIs incluye un editor de código integrado, así que escribes y ejecutas código real directamente en tu navegador y obtienes retroalimentación instantánea de IA — sin configuración local necesaria.

Todas las lecciones de este curso

Cargas multipart con interceptors de Multer
Streaming de respuestas grandes con StreamableFile
Cargas directas a S3 con URL prefirmadas
Pipelines de procesamiento de imágenes con Sharp

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