Durable step stdlib

step.run(key, input?, handler, options?) memoizes a handler result in Harn's active EventLog. On a later process restart or script replay, Harn re-executes the program from the top, but a matching step returns the persisted result without invoking the handler again.

Use it around side-effectful or expensive work that must happen at most once for a deterministic input.

fn main(harness: Harness) {
  let user = step.run("load-user", {user_id: harness.event.id}, { input ->
    return load_user(input.user_id)
  }, {namespace: "signup-workflow"})

  let enriched = step.run("enrich", {user_id: user.id}, { ->
    return call_enrichment(user)
  }, {namespace: "signup-workflow"})

  return enriched
}

Contract

• key is the authored step name.
• The optional input is hashed and persisted as the deterministic input for that step occurrence.
• The first run invokes handler, persists the JSON-shaped result, and returns it.
• A replay with the same key, occurrence number, namespace, and input hash returns the persisted result without invoking handler.
• A replay with the same key and occurrence number but a different input hash throws a deterministic input mismatch error.

Repeated uses of the same key are tracked by occurrence number in execution order. Keep keys stable and do not branch around earlier step.run calls unless the branch condition is itself deterministic for the namespace.

Namespaces

Pass options.namespace when the same script can run for independent logical workflow instances:

let result = step.run("charge-card", {order_id: order.id}, { input ->
  return charge_order(input.order_id)
}, {namespace: "order-" + order.id})

If omitted, Harn uses the entry source path as the namespace. That is convenient for local scripts, but production workflows should supply an event, session, or order scoped namespace explicitly.

Inspection

step.inspect(namespace_or_options?) returns completed records from the active step log:

let records = step.inspect({namespace: "order-123"})
log(json_stringify(records))

Each record includes event_id, namespace, key, sequence, deterministic_inputs_hash, result, and occurred_at_ms.

Persistence and Privacy

Step records are EventLog entries on topic step.run.<sanitized namespace>; the raw namespace is also persisted in each record and participates in replay identity. Under harn run, records use the same .harn state root as store, checkpoints, triggers, and waitpoints. The default EventLog backend is SQLite unless HARN_EVENT_LOG_BACKEND overrides it.

Inputs and results are persisted. Do not pass secrets or raw private payloads as step inputs/results unless the selected EventLog storage is allowed to hold them. Persist hashes or stable opaque ids when that is enough for replay.