How Gmail checks username availability instantly (billions of users)

[Swati]

[Rajendra]

Conditional if  (just being sarcastic)

[Yash]

The username disappears before you even finish typing it.

[Sarika]

Usually they use Bloom filters.

[Pooja]

Bloom filter.

[Himani]

Bloom filters or tries.

[Nitin]

I search anything on Google and the results show up in about a second, even though there are trillions of data sources across the web.

[Sneha]

Not millions, billions of accounts.

[Kavitha]

1] Indexed database
2] Hashing
3] Distributed systems like Google Spanner

These make lookup possible in O(log n), insanely fast.

Bonus:
They cache hot usernames - popular ones and recent queries.

All of this together makes it possible.

[Harini]

Many comments have already mentioned Bloom filters, but that's only part of the picture since they can tell if a username definitely doesn't exist. They can give false positives, saying a username may exist, so they're used as a fast pre‑check, not the final source of truth. There's a ByteMonk video https://youtu.be/_l5Q5kKHtR8 that explains this flow in detail - a must‑watch for me.

[Shashi]

Bloom filter.