Persistent snapshot

Serialize state and expirations deterministically and restore while ignoring expired keys. Keep commands added earlier compatible.

Observable contract

The harness reads JSON; raw RESP values are escaped inside strings. The same parser and executor should later be usable with a real socket.

{
  "op": "snapshot",
  "nowMs": 100,
  "entries": [
    ["b", "2", null],
    ["a", "1", 200]
  ]
}

produces:

{
  "records": [
    ["a", "1", 200],
    ["b", "2", null]
  ]
}

Emit compact JSON with a final newline. An invalid request leaves stdout empty, writes error to stderr, and exits non-zero.

Implementation

Separate the incremental RESP parser, command dispatch, store, and serialization. RESP sizes count bytes. Every client needs its own input buffer and transaction state; the data dictionary must never hold a half-built reply.

Test fragmentation at every byte, null values, wrong arity, and mixed command casing. Whenever time is involved, use only the clock supplied by the harness.

Acceptance criteria

• The example produces the exact response.
• A failed command does not corrupt data.
• Collections and snapshots use deterministic ordering.