---
title: Resilient run start
description: Overhaul run start logic to tolerate world storage unavailability, as long as the queue is healthy, and significantly speeds up run start.
---

# Resilient `start()`

## Motivation

When `world` storage is unavailable but the queue is up,
`start()` previously failed entirely because `world.events.create(run_created)`
is called before `world.queue()`. This change decouples run creation from queue
dispatch so that runs can still be accepted when storage is degraded.

Additionally, the runtime previously called `world.runs.get(runId)` before
`run_started`, adding an extra round-trip. By always calling `run_started`
directly, we save that round-trip and can return pre-loaded events in the
response to skip the initial `events.list` call, reducing TTFB.

## Design

### `start()` changes (packages/core)

- `world.events.create` (run_created) and `world.queue` are now called **in parallel**
  via `Promise.allSettled`.
- If `events.create` errors with **429 or 5xx**, we log a warning saying that run
  creation failed but the run was accepted — creation will be re-tried async by the
  runtime when it processes the queue message. The returned `Run` instance is marked
  with `resilientStart = true`.
- If `events.create` errors with **409** (EntityConflictError), the run already exists
  (e.g., the queue handler's resilient start path created it first due to a cold-start
  race). This is treated as success.
- If `world.queue` fails, we still throw — the run truly failed and was not enqueued.
- The queue invocation now receives all the run inputs (`input`, `deploymentId`,
  `workflowName`, `specVersion`, `executionContext`) via `runInput` so the runtime can
  create the run later if needed.
- When the runtime re-enqueues itself, it does **not** pass these inputs — only the
  first queue cycle carries them.

### `workflowEntrypoint` changes (packages/core)

- When calling `world.events.create` with `run_started`, we now also always pass the
  run input that was sent through the queue, if available. The response will still be on off:
  - **200 with event (now running)**: As usual, but the server could have used the run input to create the run if it didn't exist yet. The response will be opaque to the runtime.
  - **200 without event (already running)**: As usual
  - **409 or 410 (already finished)**: As usual

### `Run.returnValue` polling (packages/core)

- When `resilientStart` is true on the Run instance (run_created failed), the
  `pollReturnValue` loop retries on `WorkflowRunNotFoundError` up to 3 times
  (1s + 3s + 6s = 10s total) to give the queue time to deliver and the runtime
  to create the run via `run_started`.
- When `resilientStart` is false (normal path), 404 fails immediately — no delay
  for the common case of a wrong run ID.

### World / workflow-server changes

- Posting `run_started` to a **non-existent** run is now allowed when the run input is
  sent along with the payload. The server:
  1. Creates a `run_created` event first (so the event log is consistent).
  2. Strips the input from the `run_started` event data (it lives on `run_created`).
  3. Then creates the `run_started` event normally.
  4. Emits a log and a Datadog metric (`workflow_server.resilient_start.run_created_via_run_started`)
     to track when this fallback path is hit.
- When `run_started` encounters an **already-running** run, all worlds return `{ run }`
  with `event: undefined` instead of throwing. No duplicate event is created.

### Queue transport changes

`Uint8Array` values (the serialized workflow input in `runInput`) don't survive plain
JSON serialization. Each world uses a transport that preserves binary data:

- **world-vercel**: CBOR transport — CBOR-encodes the entire queue payload into a
  `Buffer` and uses `BufferTransport` from `@vercel/queue`. Uint8Array survives natively.
- **world-local**: `TypedJsonTransport` — uses the existing `jsonReplacer`/`jsonReviver`
  from `fs.ts` that encode Uint8Array as `{ __type: 'Uint8Array', data: '<base64>' }`.
- **world-postgres**: Inline typed JSON transport — same tagged-envelope approach as
  world-local, inlined since world-postgres doesn't import from world-local.

## Decisions

1. **Parallel not sequential**: We chose `Promise.allSettled` over sequential calls to
   minimize latency in the happy path.

2. **Already-running returns run without event**: When `run_started` encounters an
   already-running run, all worlds return `{ run }` with `event: undefined` (no
   `events` array) instead of throwing. The runtime detects this by checking for
   `result.event === undefined`. This avoids an extra `world.runs.get` round-trip.

3. **Events in 200 response**: We only return events on the 200 path (first caller).
   On the already-running path, we fall back to the normal `events.list` call. This is
   correct because only on 200 can we be certain we know the full event history.

4. **Conditional 404 retry on Run.returnValue**: Only when `resilientStart = true`
   (run_created failed). Normal runs fail fast on 404.

## Known concerns

### Cold-start race on Vercel (observed in CI)

On Vercel, the parallel dispatch can cause the queue message to be processed before
`run_created` completes, if `run_created` hits a cold-start lambda. Confirmed via
Datadog: the `run_started` request hit a warm lambda (23ms) while `run_created` hit
a cold lambda (727ms), even though `run_created` arrived at the edge 116ms earlier.
When this happens:

1. The runtime's resilient start path creates the run from `run_started`.
2. The original `run_created` arrives and gets 409 (EntityConflictError).
3. `start()` treats the 409 as success (the run exists).

This is handled correctly. The `resilientStart` flag is NOT set on the Run instance
in this case (409 is not a retryable error), so `returnValue` fails fast on 404.

### Local Prod test flakiness (resolved)

On world-local, the queue's async IIFE can deliver the message before
`events.create(run_created)` finishes writing to the shared filesystem. The
resilient start path should handle this, but Local Prod tests showed occasional
runs stuck at `pending` (no `run_started` event), and Windows CI showed
"Unconsumed event in event log" errors from duplicate `run_created` events.

**Root cause:** A TOCTOU race between the normal `run_created` path and the
resilient start path. Both used `writeJSON` which checks existence with
`fs.access()` (non-atomic), so both could pass the check and write separate
`run_created` events with different event IDs. Fixed by switching both paths to
`writeExclusive` (O_CREAT|O_EXCL) — see retrospective items 12 and 16.

## Follow-up work

- [x] ~~Investigate Local Prod test flakiness~~ — resolved via `writeExclusive`
  for run entity creation (retrospective items 12, 16).
- [ ] Monitor the Datadog metric in production to understand how often the fallback is hit.
- [x] ~~Events optimization for re-enqueue cycles~~ — decided against. The
  already-running path returns early without writing an event, so preloading
  events there would require an extra filesystem/DB query on every re-enqueue.
  More importantly, on Vercel with at-least-once delivery, multiple lambdas can
  process the same run concurrently — the event snapshot could be stale or
  incomplete. The runtime's fallback to `events.list` is the correct behavior
  for re-enqueue cycles.
- [x] ~~CborTransport pass-through~~ — refactored. `encode()`/`decode()` now
  live inside `CborTransport.serialize()`/`deserialize()`, matching the pattern
  used by TypedJsonTransport (world-local) and the inline transport
  (world-postgres). Call sites pass plain objects instead of pre-encoded buffers.

## Development retrospective

Chronological log of mistakes, misunderstandings, and reverted approaches during
development. Included for future reference when working on similar cross-cutting
runtime changes.

### 1. Uint8Array corruption through JSON queue transport

The initial implementation passed `runInput.input` (a `Uint8Array`) directly through
the queue payload. `Uint8Array` doesn't survive `JSON.stringify` — it becomes
`{"0":72,"1":101,...}`. This corrupted the workflow input when the resilient start
path tried to recreate the run from the queue-delivered data.

Caught by the `spawnWorkflowFromStepWorkflow` e2e test and the `world-testing`
embedded tests, which failed with "Invalid input" from devalue's `unflatten()`.

Three approaches were tried before landing on the final solution:

1. **Base64 encoding** (`btoa`/`atob`) — worked but fragile. The decode side used
   `typeof runInput.input === 'string'` as a discriminant, which was flagged as
   dangerous since non-binary inputs could also be strings.
2. **`Array.from()`/`new Uint8Array()`** — replaced base64 with a plain number array.
   Two problems: (a) 3x JSON size regression vs base64, and (b) `Array.isArray()`
   false-positives on v1Compat runs where `dehydrateWorkflowArguments` returns
   devalue's flat Array format.
3. **CBOR + BufferTransport** (final) — world-vercel CBOR-encodes the queue payload;
   world-local and world-postgres use a `TypedJsonTransport` with a tagged envelope.

### 2. Forgot to commit world-postgres transport fix (twice)

After fixing world-local and world-vercel queue transports, the same `JsonTransport`
corruption bug existed in world-postgres. The fix was written during a session but
never committed — lost when the working directory was reset via stash/checkout. This
happened twice. The fix only landed on the third attempt when it was committed and
pushed immediately. All 14 Postgres e2e jobs failed each time.

### 3. Incorrect diagnosis of Vercel Prod 409 errors

Multiple Vercel Prod e2e tests failed with `EntityConflictError: Workflow run with
ID wrun_... already exists` on `run_created`. The initial assumption was that VQS
couldn't deliver the queue message fast enough to beat the `run_created` call.

Datadog logs showed otherwise: the `run_created` request arrived at Vercel's edge
116ms before `run_started`, but `run_created` hit a cold-start lambda (727ms) while
`run_started` hit a warm one (23ms). Cold starts can invert expected execution order.

### 4. Removed EntityConflictError catch, then had to restore it

The `workflowEntrypoint` error handler originally caught both `EntityConflictError`
and `RunExpiredError`. When adding the "already-running returns run without event"
behavior, `EntityConflictError` was removed from the catch since the new worlds
wouldn't throw it. Reviewer flagged this: old worlds or world-vercel hitting an
older workflow-server could still throw it. The catch was restored.

### 5. Duplicate `startedAt` check

After refactoring the `run_started` flow, a `workflowRun.startedAt` null check
existed both inside the `try` block and after the `catch` block. The second was
unreachable. Removed after review.

### 6. WORKFLOW_SERVER_URL_OVERRIDE left set

During development, `WORKFLOW_SERVER_URL_OVERRIDE` was set to a test URL pointing
at the workflow-server preview deployment and accidentally committed. The Vercel
bot flagged this. Reset to empty string.

### 7. e2e test assertion was wrong

The resilient start e2e test stubbed `world.events.create` and asserted
`createCallCount >= 2`. But the stub only intercepts calls from the test runner
process — the server uses its own world. `createCallCount` was always 1. Changed
to `expect(createCallCount).toBe(1)`.

### 8. Misattributed Local Prod timeouts as "pre-existing"

Local Prod tests showed 60-second timeouts across various tests. Initially dismissed
as CI flakes. Checking main's CI showed all Local Prod tests pass on main — the
timeouts are caused by our changes. Should have compared against main immediately.

### 9. Attempted to revert parallel dispatch

After identifying Local Prod timeouts, `start()` was partially reverted back to
sequential dispatch. The user pointed out that parallel dispatch is the core value
proposition of the PR. The revert was undone.

### 10. WorkflowRunNotFoundError retry was unconditional

The initial `pollReturnValue` retry on `WorkflowRunNotFoundError` applied to all
`Run` instances. A user calling `getRun()` with a wrong ID would wait 10 seconds
before getting a 404. Fixed by adding a `resilientStart` flag: only retries when
`run_created` actually failed.

### 11. Changeset `minor` vs `patch`

The changeset was created with `"@workflow/core": minor`. Reviewer flagged this as
violating repo rules ("all changes should be patch"). Changed after discussion.

### 12. world-local TOCTOU race causing duplicate `run_created` events (Windows CI)

The resilient start path AND the normal `run_created` path in `world-local/events-storage.ts`
both used `writeJSON` to create the run entity. `writeJSON` checks file existence with
`fs.access()` then writes via temp+rename — a classic TOCTOU race. On the local world,
the queue delivers via an async IIFE in the same event loop, so `events.create(run_created)`
and `events.create(run_started)` (with resilient start) run concurrently:

1. Both paths call `fs.access(runPath)` → ENOENT (file doesn't exist yet)
2. Both proceed to write → the last `fs.rename` wins
3. Both succeed → both write their own `run_created` event with different event IDs
4. During replay, the consumer sees two `run_created` events → "Unconsumed event" error

This caused consistent failures in `world-testing` embedded tests on Windows CI (`hooks`,
`supports null bytes in step results`, `retriable and fatal errors` — all timing out at
60s with "Unconsumed event in event log" errors). Linux CI was not affected because the
timing was different enough that the race window was rarely hit.

Fixed by switching BOTH paths to `writeExclusive` (O_CREAT|O_EXCL), which is atomic at
the OS level — exactly one writer wins, the other gets EEXIST. The normal `run_created`
path throws `EntityConflictError` on conflict (handled by `start()` as 409). The resilient
start path re-reads the run from disk on conflict. Either way, only one `run_created`
event is written.

### 13. Non-atomic run + run_created event in world-postgres resilient path

The resilient start path in `world-postgres/storage.ts` did two separate writes (run
insert, then event insert) without a transaction. If the process crashed between them,
the run would exist without a `run_created` event — an inconsistent event log.

A `drizzle.transaction()` wrapper was attempted but dropped due to TypeScript inference
issues with drizzle's transaction callback and the insert builder's overloads. The current
fix keeps the two writes sequential but adds the same conflict-aware re-read pattern as
world-local: when `onConflictDoNothing` produces no result (run already existed), the run
is re-read so downstream logic sees the real state. The narrow crash window between the
two writes is acceptable — if the run insert succeeds but the event insert crashes, the
run exists and `run_started` will still proceed normally (the event log will be missing a
`run_created` entry, but the run itself is functional).

### 14. Missing `WorkflowRunStatus` span attribute after parallel refactor

The `start()` span previously set `Attribute.WorkflowRunStatus(result.run.status)`, but
this was dropped in the parallel refactor because `result.run` is only available when
`runCreatedResult` fulfilled. The attribute is now conditionally set when the result is
available. In the resilient start case (run_created failed), the attribute is omitted
rather than erroring.

### 15. `run_started` eventData leak in world-postgres result

The `...data` spread in the result construction leaked `eventData` from `run_started`
into the returned event object. Storage was already correct (`storedEventData` is
`undefined` for `run_started`), but the returned result carried the input data. While
harmless (the runtime doesn't use `result.event.eventData`), it was restored to match
the pre-refactor behavior where eventData was explicitly stripped from the result.

### 16. Normal `run_created` path also needed `writeExclusive` (Windows CI)

The initial TOCTOU fix (item 12) only changed the resilient start path to use
`writeExclusive`. The normal `run_created` entity write still used `writeJSON` which
checks existence with `fs.access()` then writes via temp+rename — not atomic. On
Windows CI, the local queue's async IIFE delivered fast enough for both paths to pass
their existence checks simultaneously, producing two `run_created` events with different
event IDs. The events consumer saw the duplicate as "Unconsumed event in event log,"
causing `hooks`, `supports null bytes in step results`, and `retriable and fatal errors`
tests to time out at 60s. Fixed by also switching the normal `run_created` entity write to
`writeExclusive`, making both paths use the same atomic gate.

### 17. CborTransport was a pass-through wrapper

`world-vercel/queue.ts` had `CborTransport` implementing `Transport<Buffer>` with a
no-op `serialize` (identity function) and a `deserialize` that reassembled chunks into
a Buffer without decoding. The actual CBOR `encode()`/`decode()` calls happened at the
call sites — `queue()` pre-encoded before calling `client.send()`, and the handler
post-decoded after receiving from `client.handleCallback()`. This violated the transport
abstraction (every other transport does its encoding inside serialize/deserialize) and
meant the call site had to remember to pre-encode. Refactored to move `encode()`/`decode()`
into the transport methods and changed the type from `Transport<Buffer>` to
`Transport<unknown>`.

## Follow-up work (additional)

- [x] ~~**CborTransport is a pass-through**~~ — Resolved. Moved `encode()`/`decode()`
  into `CborTransport.serialize()`/`CborTransport.deserialize()`. The transport is now
  self-contained: call sites pass plain objects, and the handler receives decoded objects.
  See retrospective item 17.
