Direct Upload Architecture

Direct uploads let a browser send files to a storage provider without routing the file bytes through your application server.

Upload SDK fits the preparation step. It validates client-reported file information, resolves a typed upload asset, and returns a short-lived provider upload target.

Direct upload flow

A typical direct upload has two server-visible phases:

1. The browser asks your application for a prepared upload.
2. The browser sends the file directly to AWS S3 or ImageKit.

Your server signs intent. The provider receives the file.

Browser -> Application server -> Upload SDK -> Provider signing fields
Browser -> Storage provider

This keeps provider credentials on the server while avoiding a large file transfer through your application server.

When direct uploads fit

Direct uploads are a good fit when:

• Files can be uploaded directly to object storage or an asset provider
• The application server should avoid handling large request bodies
• The browser can submit multipart FormData
• The provider can enforce enough constraints for the upload
• The application can verify and persist the result after upload when needed

Examples include avatars, document attachments, media drafts, generated exports, and user-submitted images.

When server-proxied uploads may fit better

A server-proxied upload may be better when:

• The server must inspect the file before storage
• The application needs synchronous virus scanning before accepting the object
• The provider does not support the required browser upload flow
• The server must transform the file before it reaches storage
• The upload policy depends on data that cannot be safely represented in signed provider fields

Upload SDK does not replace these workflows. It is focused on preparing signed direct upload targets.

What Upload SDK prepares

Upload SDK handles the shared preparation work:

• Typed asset selection
• Storage profile resolution
• Filename and key-prefix sanitization
• Reported MIME, extension, and size validation
• Expiration validation
• Provider-specific signed fields
• Consistent PrepareUploadOutput shape

The same application-facing flow works across AWS S3 and ImageKit, while each provider receives the fields it expects.

What your application still owns

Your application remains responsible for:

• Authentication and authorization
• Deciding which asset a user may upload
• Rate limits and abuse controls
• Client-side upload progress
• Completion checks
• Database persistence
• File inspection, moderation, or malware scanning
• Cleanup of abandoned or invalid uploads

A prepared upload is permission to attempt an upload. It is not proof that the file exists or is safe.

Provider tradeoffs

AWS S3 and ImageKit use the same SDK preparation flow, but their provider behavior differs.

Choose the provider based on storage, delivery, transformation, and operations requirements. Upload SDK keeps the preparation contract consistent.

Production checklist

Before shipping a direct upload flow, decide how your application will handle:

• User authorization before signing
• Accepted asset types and size limits
• Provider CORS or upload-origin configuration
• Expiration and retry behavior
• Database records and ownership
• Upload completion confirmation
• Invalid file cleanup
• Post-upload content verification when needed

The strictness depends on what users can upload and how much trust the stored file needs.

Next steps

• Next.js Direct Uploads: Implement the flow with a route handler.

• S3 Presigned Browser Uploads: Understand the S3-specific signing and browser upload flow.

• ImageKit Browser Uploads: Understand the ImageKit-specific token and upload fields.