A user uploads a photo. The platform needs: a thumbnail at 150x150, a medium preview at 800x600, the original compressed to 80% quality, EXIF data extracted, content safety scanning, and a perceptual hash for deduplication. None of these should block the upload response. All of them need to happen reliably. The Fan-Out After Upload The upload endpoint does one thing: store the raw file, acknowledge the upload, publish an event. Everything else is async.