# Multipart Uploads

Multipart upload lets you upload large objects in parts, enabling parallelism, retries of failed parts, and resuming interrupted transfers.

## When to use multipart upload

- Objects larger than 100 MB (recommended)
- Unreliable networks where retrying parts is beneficial
- Parallelizing uploads to increase throughput

> Available S3 endpoint: `a-s3.aur.lu` (France, Albi).

## Using AWS CLI

### Simple approach (aws s3)

The `aws s3 cp` command automatically uses multipart for large files:

```bash
aws --profile aurora --endpoint-url https://a-s3.aur.lu s3 cp ./bigfile.tar s3://my-bucket/bigfile.tar
```

### Fine-grained control (aws s3api)

```bash
# 1) Create the multipart upload
aws --profile aurora --endpoint-url https://a-s3.aur.lu s3api create-multipart-upload   --bucket my-bucket   --key bigfile.tar

# Save the returned UploadId as UPLOAD_ID

# 2) Upload parts (example shows 3 parts)
aws --profile aurora --endpoint-url https://a-s3.aur.lu s3api upload-part   --bucket my-bucket   --key bigfile.tar   --part-number 1   --body ./part1.bin   --upload-id $UPLOAD_ID

aws --profile aurora --endpoint-url https://a-s3.aur.lu s3api upload-part   --bucket my-bucket   --key bigfile.tar   --part-number 2   --body ./part2.bin   --upload-id $UPLOAD_ID

aws --profile aurora --endpoint-url https://a-s3.aur.lu s3api upload-part   --bucket my-bucket   --key bigfile.tar   --part-number 3   --body ./part3.bin   --upload-id $UPLOAD_ID

# 3) List uploaded parts (optional)
aws --profile aurora --endpoint-url https://a-s3.aur.lu s3api list-parts   --bucket my-bucket   --key bigfile.tar   --upload-id $UPLOAD_ID

# 4) Complete the upload (provide ETags and part numbers)
aws --profile aurora --endpoint-url https://a-s3.aur.lu s3api complete-multipart-upload   --bucket my-bucket   --key bigfile.tar   --multipart-upload file://parts.json   --upload-id $UPLOAD_ID

# Abort a stuck multipart upload
aws --profile aurora --endpoint-url https://a-s3.aur.lu s3api abort-multipart-upload   --bucket my-bucket   --key bigfile.tar   --upload-id $UPLOAD_ID
```

## Best practices

- Prefer at least 5–25 MB part sizes; 8–64 MB is a common sweet spot.
- Upload parts in parallel; 4–16 workers fit many networks.
- Keep a manifest of part ETags to safely resume and complete.
- Regularly list and abort stale multipart uploads.
- Incomplete multipart uploads are automatically deleted after 24 hours.