Direct-to-S3 multipart

Browser-direct S3 multipart upload

The browser uploads each part directly to Amazon S3 (or MinIO, Backblaze B2, Cloudflare R2, Wasabi â€” anything S3-API compatible) via presigned URLs your server signs on demand. The WebForms handler only signs â€” the bytes bypass IIS entirely. Survives reloads: the resumed task reuses the same uploadId and part ETags via IndexedDB.

Setup required. No IS3Signer is registered yet, so /ajaxupload.axd/s3/* currently returns 501. To activate this demo:

Install AWSSDK.S3 into bin/ (from NuGet).
Move v5-signers/DemoS3Signer.cs.template â†’ App_Code/DemoS3Signer.cs.
Fill in DemoS3:* values in web.config.
Recycle the app pool.

Full walk-through: v5-signers/README.md.

Client config

AjaxUploader.create('#uploader', {
    uploadUrl: '/ajaxupload.axd/upload',
    strategy: 's3',                 // selects _s3DirectUpload()
    chunkSize: 5 * 1024 * 1024,     // S3 5 MB minimum part size
    chunkConcurrency: 4,            // parallel PUTs
    persistState: true,
    persistAdapter: 'indexeddb'     // resume survives reload
});

Server-side (Global.asax.cs)

using AjaxUploader.Providers;

protected void Application_Start(object sender, EventArgs e)
{
    // Your IS3Signer wraps AmazonS3Client (or MinIO client).
    S3.Signer = new MyS3Signer(
        bucketName: "your-bucket",
        region: "us-east-1",
        accessKeyId: ConfigurationManager.AppSettings["AWS:AccessKeyId"],
        secretKey: ConfigurationManager.AppSettings["AWS:SecretKey"]);
}

Wire protocol

POST /ajaxupload.axd/s3/create â†’ { uploadId, key }
POST /ajaxupload.axd/s3/sign â† { uploadId, key, partNumbers } â†’ { urls }
PUT <signedUrl> â†’ S3 returns ETag per part (browser-direct)
POST /ajaxupload.axd/s3/complete â† { uploadId, key, parts:[{PartNumber, ETag}] }
POST /ajaxupload.axd/s3/abort on cancel / fatal error

Bucket CORS

The browser PUTs directly to S3, so the bucket must allow cross-origin PUT and expose the ETag header:

[{
  "AllowedOrigins": ["https://ajaxuploader.com"],
  "AllowedMethods": ["PUT"],
  "AllowedHeaders": ["*"],
  "ExposeHeaders": ["ETag"],
  "MaxAgeSeconds": 3000
}]