Are UUIDs really unique?

Effectively, yes — you can stop worrying. With 122 bits of randomness per v4 UUID, you'd need to generate roughly 2.7 quintillion (2.7 × 10^18) of them before there's even a 50% chance of one collision, courtesy of the birthday paradox. For any normal application, treat them as unique forever — writing collision-handling code for v4 UUIDs is wasted effort that would never trigger in your lifetime.

Should I use UUID as a database primary key?

Yes, with one real caveat. UUIDs are the right answer for distributed systems — no central coordinator, no auto-increment race conditions, no leaked customer count from sequential IDs in your URLs. But they're 128 bits wide (twice a bigint), and on large tables the randomness scatters inserts across the B-tree index, hurting write performance. If you hit that wall, switch to UUID v7 — time-ordered prefix plus random suffix, sortable inserts, same uniqueness guarantee.

What's the difference between v1 and v4?

v1 packs a timestamp plus your machine's MAC address — sortable, but it leaks identity (which is why it fell out of favour). v4 is pure randomness — no leaked info, no time ordering. v7 is the newer compromise — a timestamp prefix followed by a random suffix, so you get sortable inserts AND privacy. This generator produces v4, which is still the most common choice across modern frameworks.

Why are some UUIDs uppercase and some lowercase?

Both are valid — RFC 4122 calls lowercase the canonical output form, but parsers must accept either. The split is platform-cultural: .NET, Windows, and SQL Server tools traditionally emit UPPERCASE; Linux, Python, JavaScript, and most modern languages use lowercase. Pick whichever your downstream system expects and stay consistent — mixing them in storage is fine for parsers, but it makes grep and diff annoying.

What's the {braced} format?

It's the Microsoft GUID convention — the UUID wrapped in curly braces, like {550e8400-e29b-41d4-a716-446655440000}. You'll need it when working with Windows COM components, certain registry APIs, or any .NET interop that expects the braced literal form. Stripping the braces gets you back to standard UUID format.

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UUID v4 collisions are mathematically negligible. You'd need ~2.7 quintillion UUIDs before there's a 50% chance of one collision. For any normal application, treat them as unique forever — collision-handling code is wasted effort here.

Random UUIDs hurt B-tree indexes. UUID v4 as a primary key is correct for distributed systems but inserts scatter across the index, killing write performance on huge tables. If you're hitting that wall, switch to UUID v7 (time-ordered prefix + random suffix) — sortable inserts, same uniqueness guarantee.

Pick a format and stick with it across your stack. Lowercase with hyphens is canonical RFC 4122. UPPERCASE is .NET / Windows convention. {braced} is Microsoft COM. Mixing formats is fine for storage (parsers accept all), but consistency makes grep and diff a lot easier.

Idempotency keys are the killer use case nobody mentions in tutorials. Stamp every outgoing payment, webhook delivery, or non-idempotent API call with a fresh UUID and pass it as a header — the receiver dedupes if your retry logic fires twice. Saves you from charging a customer twice during a network blip.

Don't use UUIDs as secrets. They're random, but they're meant to be visible — they show up in URLs, logs, and database dumps. If you need an unguessable token (session, password reset, API key), use a real CSPRNG-generated token of the length the use case demands, not a UUID someone might log.

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