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docs(workflow): add deterministic execution requirement #81

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38 changes: 38 additions & 0 deletions docs/sdk/services/workflow.md
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Expand Up @@ -115,6 +115,44 @@ export const mainJob = createWorkflowJob({

**Important:** On the Tailor runtime, job triggers are executed synchronously. This means `Promise.all([jobA.trigger(), jobB.trigger()])` will not run jobs in parallel.

### Deterministic Execution Requirement

Workflow jobs use a **suspend/resume execution model**. When a job calls `.trigger()`, the runtime suspends the current job, executes the triggered job, and then **re-executes the calling job from the beginning** with cached results from previous triggers.

This means that **job code must be deterministic** — every re-execution must produce the same sequence of `.trigger()` calls with the same arguments in the same order.

Using `.trigger()` inside a loop works correctly, as long as the loop is deterministic:

```typescript
// ✅ OK: deterministic loop — same calls in the same order on every execution
const regions = ["us", "eu", "ap"];
for (const region of regions) {
const result = await fetchData.trigger({ region });
results.push(result);
}
```

```typescript
// ❌ Bad: non-deterministic — argument changes between executions
await processJob.trigger({ timestamp: Date.now() });
```

```typescript
// ❌ Bad: non-deterministic — external data may change between executions
const items = await fetch("https://api.example.com/items").then((r) => r.json());
for (const item of items) {
await processItem.trigger({ id: item.id });
}
```
Comment on lines +135 to +146

@toiroakr toiroakr Apr 9, 2026 •
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I thought it might be even better if there were examples of how to rewrite the "bad" examples.

// ❌ Bad: non-deterministic — argument changes between executions
await processJob.trigger({ timestamp: Date.now() });

↓

// ✅ OK: call Date.now() in separated job
const timestamp = await timestampJob.trigger();
await processJob.trigger({ timestamp });
// ❌ Bad: non-deterministic — external data may change between executions
const items = await fetch("https://api.example.com/items").then((r) => r.json());
for (const item of items) {
  await processItem.trigger({ id: item.id });
}

↓

// ✅ OK: call fetch("https://api.example.com/items").then((r) => r.json()); in separated job
const items = await fetchItemsJob.trigger();
for (const item of items) {
  await processItem.trigger({ id: item.id });
}


If the runtime detects that a `.trigger()` call at the same position has different arguments than the previous execution, it will throw an **argument hash mismatch error**.

**Guidelines:**

- Do not use non-deterministic values (random numbers, timestamps, external API responses) as `.trigger()` arguments.
- Do not use conditions that may change between executions to decide whether to call `.trigger()`.
- Any data that varies between executions should be fetched **inside the triggered job**, not passed as an argument from the calling job.

## Workflow Definition

Define a workflow using `createWorkflow` and export it as default:
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