Blob Storage

Trigger a File Download from a Blob

Generating a CSV, JSON export, or screenshot client-side and saving it without a server round-trip is a five-line trick: build a `Blob`, mint an object URL, click a hidden `<a download>`, and revoke the URL. This snippet covers the canonical helper, a JSON export wrapper, and the cleanup pattern that prevents memory leaks during long-running sessions.

Blob Storage, Object Stores & CDNs

Databases are wrong for storing large unstructured files - photos, videos, backups, logs. Object stores like S3 give you cheap, durable, infinitely scalable storage for blobs, while CDNs cache that content at edges close to users. This lesson covers the object-storage data model, multi-part upload, storage classes, presigned URLs, and how a CDN turns a globally slow origin into a globally fast experience. By the end you can design the media layer for any social, video, or e-commerce system.

Design Pastebin

Design a service like Pastebin or GitHub Gist where users dump up to 10 MB of text and share a link. The interview twist over a URL shortener: pastes are big, so you store them in object storage (S3) and only keep metadata in your database. This lesson covers the metadata vs blob split, expiration via S3 lifecycle policies, presigned URLs for direct uploads, syntax highlighting strategy, and how to handle the read pattern when most pastes are read once and never again.

Design an Email Service (Gmail)

Design an email service like Gmail handling 1.8B users storing 500EB of email, accepting ~300B inbound messages per day from the public SMTP network while filtering 90%+ as spam, and serving full-text search over a user's entire inbox in sub-200ms. The interview centerpiece is the asymmetric architecture: SMTP is an untrusted public protocol with hostile traffic patterns (spam, phishing, sender forgery) that needs heavy gateway-side filtering, while the user-facing IMAP/web layer needs cheap reads, pagination of huge mailboxes, and per-user inverted indexes for search. We cover the SMTP MX gateway, the spam pipeline (SPF/DKIM/DMARC + ML), the per-user inverted index for search, and how mailboxes scale when one user holds 50GB of email.