Erlang backend
Connection servers written in Erlang. One process per connected client. Millions of concurrent connections per box.
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Erlang backend
Connection servers written in Erlang. One process per connected client. Millions of concurrent connections per box.
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Signal Protocol E2E
Every message encrypted with recipient's public key. Server sees ciphertext + metadata only. Group messages use sender keys.
Offline delivery
Recipient offline → message parked in temp store. Push notification wakes device. Client reconnects, pulls queued messages, ACKs, server deletes.
Media path bypasses main flow
Photo/video uploaded to encrypted blob store. Client sends URL + decryption key in-band. Recipient downloads separately.
Presence is best-effort
Last-seen is soft-state, cached per user. Rebuilt on reconnect. Not critical path.