WhatsApp forgets your messages. On purpose.

transcript

WhatsApp's servers forget your messages. On purpose. And that one choice saves 50 terabytes a day.

Keep every chat forever? An encrypted message is about 500 bytes. More than 100 billion of them, every day. That's 50 terabytes of new storage. Every single day. An archive that only grows. To shard, back up and protect.

So WhatsApp flips it. Alice sends hello. The server holds the blob. Bob's phone receives it. His phone sends an ack. That's Alice's second tick. And for the server, delivered means deleted.

Keeping everything: 50 terabytes, every day. Delete on ack, and only about 5 percent are ever undelivered. Add about 10 terabytes for offline queues. And about 2 point 5 terabytes active. That's it.

But here's the catch. What if Bob's phone never comes back? After 30 days with no ack, the message expires. Alice never sees a double tick. And the queue can't grow forever.

Less to store. Less to back up. Less to protect. The server is a delivery queue. Not an archive.

Full breakdown in the lecture.