Custom adapters

The Storage interface is technology-agnostic on purpose: you can back Postel with anything — libSQL, Turso, D1, CockroachDB, a backend the first-party adapters don't cover — by implementing it yourself. Wire your adapter in exactly like a first-party one:

const postel = Postel({
  outbound: {
    storage: myAdapter,
  },
});

What you implement

Storage is operation-shaped, not CRUD. The hot-path operations carry semantics a plain update can't express — reserveBatch is lock-acquire + lease-assign + return in one atomic step (FOR UPDATE SKIP LOCKED on Postgres, BEGIN IMMEDIATE on SQLite). You implement that operation set plus the endpoints / secrets / tenants sub-namespaces, dedup, and transaction(cb). Declare a capabilities object so the worker scheduler degrades gracefully (poll when notify is absent, run sequentially when transactional is false).

Don't reimplement the glue — use @postel/storage-helpers

@postel/storage-helpers has zero database dependencies and carries everything an adapter would otherwise hand-roll: dialect-aware timestamp / JSON / bytes codecs, the canonical capability flag sets, idempotency-key formatting, message / attempt / endpoint / secret row encode-decode, the schema-version constant, and the code-side filter/transform callback registry (those callbacks aren't serializable, so adapters keep them in memory keyed by endpoint id and re-attach them on read).

Prove it with the conformance battery

@postel/storage-testkit exports runStorageTests(factory) — the same behavior battery the in-memory reference and every first-party adapter pass. Point it at your adapter from a test file:

import { makeFakeClock, runStorageTests } from "@postel/storage-testkit";
import { myAdapter } from "../src/index.js";

runStorageTests({
  name: "my-adapter",
  capabilities: { notify: false, txIsolation: true },
  async create() {
    const clock = makeFakeClock();
    return { storage: myAdapter({ clock }), clock };
  },
});

The capabilities flags let the battery skip scenarios your backend genuinely can't honor (e.g. cross-connection notify on a single-connection store). Everything else must pass — that's the bar for being a real Postel storage backend.