Singleton workflows | pg-workflows

singleton: true allows at most one pending or running run of a workflow ID. Starting a second run throws WorkflowRunInProgressError.

import { WorkflowRunInProgressError, workflow } from 'pg-workflows'

const nightlySync = workflow(
  'nightly-sync',
  async ({ step }) => {
    await step.run('pull', async () => ({ pulled: true }))
  },
  { singleton: true },
)

await engine.startWorkflow({ workflowId: 'nightly-sync', input: {} })

try {
  await engine.startWorkflow({ workflowId: 'nightly-sync', input: {} })
} catch (error) {
  if (error instanceof WorkflowRunInProgressError) {
    // the first run is still pending or running
  }
}

A run releases the slot when it pauses (waitFor, pause, delay, waitUntil, poll), completes, fails, or is cancelled. Resuming a paused run while another run holds the slot also throws WorkflowRunInProgressError.

WorkflowClient doesn't load workflow definitions, so it can't see singleton on the definition. Set it on the ref instead:

const nightlySyncRef = workflow.ref('nightly-sync', { singleton: true })
await client.startWorkflow(nightlySyncRef, {})