writeonce/docs/stories/language-runtime-database/18-memory-db-features.md

Iteration 18 — memory-rich framework features over the embedded database

Format: product/story-iteration-template. Part of Story — one language, one runtime, one database, one binary.

Inserted 2026-08-20, forks settled the same day (developer decisions below). Next step: spec + plan, implementation on approval — the iteration-17 discipline.

Why this iteration exists

The single binary owns its memory AND its durable store — the class of features other stacks buy with Redis, a message broker, and an outbox pattern falls out of what is already here: an in-process map needs no serialization format or cache-invalidation protocol (one process, one node); a @table is a durable queue with WAL recovery for free; and the WAL already stages multi-write batches internally (wal_append* → wal_commit) — db.c merely commits per statement today, so exposing the batch as a language transaction makes "enqueue a job and write the order in ONE commit" true, which is the outbox problem dissolved rather than worked around.

Ground truths the design stands on (verified 2026-08-20)

• time.now exists (Timestamp) — TTL/expiry is expressible; there are no timers, so expiry must piggyback on requests (lazy), never on a clock.
• The runtime is single-threaded and net.accept blocks without a timeout: an IDLE server executes nothing. Any "background" work runs only when requests give it time — disclosed, not hidden.
• Long-lived state lives on the instances the app wires (App, middleware, handler fields) — they survive across requests; GC (7b) handles the churn. No globals needed, none exist.
• WAL commit is per statement in db.c; the staged-batch machinery beneath it is already transactional in shape (RAM apply → append → commit=write+fdatasync).

Settled decisions (2026-08-20)

1. Scope: TTL cache, feature flags, background jobs. Pub/sub for WebSockets is REJECTED for now — WebSockets do not exist, and long-lived connections on a single-threaded accept loop is the iteration-16 starvation lesson magnified; both wait for shards/fibers (8/11). A channel data structure without delivery is mechanism without a consumer.
2. Jobs execution model: drain-on-request. A bounded job budget runs after each served response, in-process, adjacent to the app's own writes. Honest limit stated everywhere it matters: an idle server drains nothing until the next request arrives. Fibers (11) later replaces the scheduler; the queue table and job shape stay. (Rejected for v1: a second worker process over 9c attach — real parallelism but blocks on finishing 9c; parking jobs entirely — the queue-plus-drain is useful today.)
3. transaction { } ships in this iteration. Language block deferring wal_commit to block end; a trap unwinding out of the block aborts the staged batch (nothing committed, RAM state rolled back or rebuilt per the spec's choice — the spec must settle recovery semantics precisely). This is the headline: job enqueue + business write, one fdatasync, no outbox.
4. Recorded as iteration 18; spec + plan before any code.

Goals (draft — the spec refines)

• framework/cache.wo: a TTL + size-cap cache class (lazy time.now expiry on read, evict-on-write over capacity) usable as a field on any long-lived instance. Pure .wo, no engine change.
• Feature flags: @table-backed flags with a cached read-through map and bump-on-write invalidation — a framework pattern (and helper) proving "table + cached read" with zero cross-node invalidation problem.
• Background jobs: a @table queue (durable, WAL-recovered, restart-run proven like the storefront's) + a framework drain seam — the serve loop offers a bounded after-response tick to a job runner the app registers; job handlers are classes satisfying an interface (the Handler doctrine).
• transaction { }: multi-statement atomicity exposed in the language, engine-backed by the existing staged batch; enqueue-with-write becomes one commit. Trap = abort.
• The web-app demonstrates: an order-confirmation job enqueued in the same transaction as the order insert, drained after later requests.

Open questions for the spec (not forks — details)

• Transaction semantics under trap: RAM apply happens before append — abort must undo RAM state; the spec settles whether the engine keeps an undo list or applies RAM changes only at commit.
• Nested transaction { }: reject (WO-E1xx) or flatten; leaning reject.
• Job table shape: id, kind, payload (json Text), attempts, not_before — the spec fixes it; retries/backoff policy stays app-side in v1.
• Drain seam shape: interface on the Dispatcher, or a second registration on App (app.jobs(runner, budget)); leaning the App registration.
• Cache eviction order: exact LRU needs an ordered structure — the spec decides between approximate (FIFO of keys) and true LRU cost.

Acceptance Criteria (draft)

• Given a cache with TTL 1 and capacity N, when a request reads an expired key or writes past capacity, then the entry is gone / evicted — proven by a probe without sleeping the server (stamps injected, not waited).
• Given two writes inside transaction { } and a crash (SIGKILL) between block end and the next request, when the server restarts, then both rows exist; given a trap inside the block, then neither row exists and the server keeps serving.
• Given an order POST that enqueues a job transactionally, when the response has been sent and a subsequent request arrives, then the job has run within the drain budget; given a SIGKILL before the drain, then the job survives restart and runs after the next request.
• Given a flag flipped through its table, when the next request reads it, then the cached read reflects the write (bump invalidation), and just web-app stays green throughout.

Out Of Scope

Pub/sub and WebSockets (behind 8/11); streaming job payloads; cross-node anything (there is one node by doctrine); job priorities/cron scheduling (the log-watcher run mode already covers time-based execution externally); exposing the WAL batch API beyond transaction { }; distributed cache invalidation (does not exist to invalidate).

Proposed Solution

Spec next: transaction semantics (the one engine+language seam), the four framework pieces as .wo (cache, flags, queue+drain, web-app demo), and just web-app extended as the gate — including the SIGKILL/restart transactional-jobs proof. Plan follows the spec; implementation on approval.