The journal beside the bus: an outbox, a last word, a replay.

02:15:17.826 [info] EchoData: contract self-check passed, codec=pure
header: Valkey on 6390 | SQLite 3.46.0 via exqlite, WAL, synchronous=NORMAL | Elixir 1.14.0 OTP 25 | schedulers 1
H1 files ok -- one journal file per group, named by the group's branded id -- two groups, two files on disk -- a non-branded group is refused at the door, and the schema carries the two memories: intents (the outbox) and applied (the lane's last word per name)
derive (windows): the writer's flow is record, enqueue, mark -- two crash seams; a death before the enqueue leaves a pending intent that replay enqueues; a death after the enqueue but before the mark leaves the bus holding the job, and replay's reuse of the recorded job id lets the bus's own admission dedup absorb it; full coverage by the applied memory makes replay a no-op
H2 windows ok -- both crash seams closed by machinery that already exists: the never-enqueued intent replayed onto the bus, the enqueued-but-unmarked one answered :duplicate at admission and was counted, both ended in the applied memory -- and once coverage is total, replay is exactly %{replayed: 0, deduplicated: 0}
derive (memory): the applied table lives in the file, so it survives the table, the node, and the bus; after a full restart with an empty L1, a replayed old version must answer :remembered_stale from the journal alone -- no cache row consulted, none created -- while a genuinely newer version passes through and updates the memory
H3 memory ok -- the journal remembered v5 across a full stop of table and journal; the replayed old version answered :remembered_stale without touching the cache -- the table's verdict counters did not move and no row appeared -- and the genuinely newer v6 passed through and became the new last word
derive (loss): D-2 keeps the bus volatile, so a bus restart erases queued coherence jobs; 50 intents recorded and enqueued, 20 applied before the loss, the lane's queue keys flushed -- replay must re-enqueue exactly the 30 uncovered intents in seq order, and a consumer must drain them to a remembered count of 50
H4 loss ok -- the bus restart erased the queue and the journal replayed the lane back: exactly 30 uncovered intents re-enqueued in seq order under their recorded job ids, the consumer drained them, and the applied memory closed at 50 of 50 names holding their final versions
derive (compaction): an intent is retired when its name carries an applied version at least as new -- coverage, not acknowledgment -- so after H4 all 50 intents are deletable, the applied memory keeps all 50 names, replay finds nothing, and a reopen still remembers
H5 compaction ok -- all 50 intents retired by coverage in one pass, the applied memory kept its 50 names, replay over the compacted journal found nothing to do, and a fresh open of the same file still answers the last word -- the outbox empties, the memory does not
derive (price): on prepared-once statements -- the single writer's privilege, with bind resetting the statement -- the writer's pair is two WAL commits and one cached rowid read at synchronous=NORMAL, so expect between 20 and 250 us on this disk; the remembered lane's end-to-end median should land between 200 us and 2 ms against the bare lane's committed 148 us: dearer, bounded, and the chapter's reason the journal is declared per group rather than assumed
H6 price ok -- the memory's price on this disk: 143 us per record-and-mark pair at the writer's edge, and a remembered lane end-to-end median of 524 us against the bare lane's committed 148 us -- 3.5 times the latency buys an outbox, a last word per name, and a replay that survives the bus
PASS 6/6