# Object storage

> Blobs by key at any scale: S3-style storage, presigned URLs and why files don’t go in the database.

Concept · Storage & databases · https://levelcode.org/concepts/object-storage

Object storage — Amazon S3, Google Cloud Storage, Azure Blob Storage, and S3-compatible systems such as MinIO and Cloudflare R2 — stores files ("objects") of any size under keys in buckets. It's where images, video, uploads, backups, exports and data-lake files belong.

## What it's good at

- **Durability:** objects are replicated across devices and facilities; S3 is designed for 99.999999999% (eleven nines) durability.
- **Scale and cost:** effectively unlimited capacity at a few cents per gigabyte per month, with cheaper tiers for rarely read data.
- **Throughput:** many clients can read and write in parallel; large objects upload in parts.

## What it isn't

- Not a database: no queries beyond listing keys by prefix, and no transactions across objects.
- Not a filesystem: no in-place edits, and (outside a few specialised storage classes) no appends; you replace the whole object.
- Higher latency per request than a local disk (tens of milliseconds), so put a CDN in front for public content ([content delivery networks](https://levelcode.org/concepts/cdn)).

## Patterns

- **Metadata in the database, bytes in object storage.** The row holds the key, size, content type and owner; the object holds the content. Databases full of blobs are slow to back up, replicate and restore.
- **Presigned URLs.** Your server signs a short-lived URL that lets a client upload or download one object directly. Large uploads never pass through your API servers:

```text
1. client → API: "I want to upload photo.jpg (4 MB)"
2. API → client: presigned PUT URL for uploads/7/9f2c.jpg, valid 10 minutes
3. client → object storage: PUT the bytes directly
4. client → API: "done" (or an object-created event notifies you)
```

- **Immutable keys.** Write a new key for a new version (`avatars/7/v3.jpg`) instead of overwriting, which makes caching simple.
- **Lifecycle rules** move old objects to cheaper storage classes or delete them automatically.

Keep buckets private by default and grant access through presigned URLs or a CDN with origin authentication.