# Events & Time Travel (/build/concepts/events)



Immutable Event Log [#immutable-event-log]

Every mutation appends to an immutable log:

```
Contact.Created   → { id, name, email, stage: 'Lead' }
Contact.Qualified → { id, stage: 'Lead' → 'Qualified', qualifiedBy: 'agent_1' }
```

Events are never deleted or modified.

Time Travel [#time-travel]

Reconstruct any point in time:

```typescript
import { $ } from '@headlessly/sdk'
import { Contact } from '@headlessly/crm'

const contacts = await Contact.find(
  { stage: 'Lead' },
  { asOf: '2026-01-15T10:00:00Z' }
)

await Contact.rollback('contact_uLoSfycy', { asOf: '2026-02-06T15:00:00Z' })
```

Subscriptions [#subscriptions]

Code-as-Data (~0ms) [#code-as-data-0ms]

```typescript
import { Deal } from '@headlessly/crm'

Deal.closed((deal, $) => {
  $.Subscription.create({ plan: 'pro', contact: deal.contact })
})
```

WebSocket (~10ms) [#websocket-10ms]

```typescript
import { $ } from '@headlessly/sdk'

$.events.subscribe('Contact.Qualified', event => {
  console.log(`${event.entity.name} was qualified`)
})
```

Webhook (~100ms) [#webhook-100ms]

```typescript
import { Workflow } from '@headlessly/platform'

await Workflow.create({
  trigger: 'Deal.Closed',
  action: 'webhook',
  url: 'https://my-app.com/hooks/deal-closed',
})
```

Metric Watches [#metric-watches]

React when numbers cross thresholds:

```typescript
import { Metric } from '@headlessly/analytics'
import { Campaign } from '@headlessly/marketing'

Metric.watch('churn_rate', { threshold: 3.0, direction: 'above' }, () => {
  Campaign.create({ name: 'Win-back', type: 'Email', segment: 'churning' })
})
```

Alerts [#alerts]

The status endpoint surfaces anomalies agents can act on:

```typescript
import { $ } from '@headlessly/sdk'

const { alerts } = await $.status()
// [{ type: 'churn_spike', severity: 'high', action: 'retain' }]
```