How Chat works

Before you start

• Requirements

  Each worked system starts from a scoped list of verbs and numbers.

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• Pick the key every read has

  Messages are partitioned by conversation and ordered by a sequence number.

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• Queues

  Slow work leaves the request path through a queue.

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Why it matters

Ana may send while Bo is offline, and either phone can reconnect after missing several messages. A live connection says only where a device is now; it cannot prove what that device has already received or preserve a common order after a network change. Sending must not wait for Bo’s phone, and reconnecting must recover every missed message without gaps or duplicates.

Two people exchange text. If the recipient is online, they see it immediately. If not, they see it the next time they connect. History is kept.

Input

log = {}

Output

[[2, "bo", "hey"], [3, "ana", "lunch?"]]

For conversation c1, append Ana’s “hi,” Bo’s “hey,” and Ana’s offline “lunch?” as seq 1–3; acknowledge from that log, push only when connected, and let Bo reconnect with after=1 to receive seq 2 and 3.

Conversation c1 owns one durable log. Ana’s “hi” is seq 1, Bo’s “hey” is seq 2, and Ana’s offline “lunch?” is seq 3. The server acknowledges each send after its append, then uses Bo’s live connection only as an immediate delivery path when one exists.

Bo stores cursor 1 before disconnecting. sync(c1, after=1) returns seq 2 and 3 in order; after applying both, Bo commits cursor 3. Retrying with cursor 3 returns nothing. If a response is replayed from after=1, the client deduplicates seq 2 and 3 before advancing.