---
title: Reacting to Changes
slug: guides/reacting-to-changes
public: true
category: guides
template: guide
order: 11
description: Subscribe to every committed mutation with `brain.onChange` — the in-process change feed behind live UIs, cache invalidation, and realtime sync. Covers the event shape, delivery guarantees, and catch-up patterns.
next:
  - guides/optimistic-concurrency
  - guides/snapshots-and-time-travel
---

# Reacting to Changes

`brain.onChange(cb)` is Brainy's in-process change feed: subscribe once and
receive one event per committed mutation — **every** mutation, regardless of
how it happened. Direct calls, batch methods, `transact()`, imports, and
Virtual Filesystem writes all funnel through the same commit point the feed is
emitted from, so nothing slips past it.

```ts
const off = brain.onChange((e) => {
  if (e.kind === 'entity') {
    console.log(`${e.op} ${e.entity?.type} ${e.id} @ generation ${e.generation}`)
  }
})

await brain.add({ data: 'Ada Lovelace', type: 'person' })
// → "add person 0198... @ generation 42"

off() // unsubscribe when done
```

## The event

```ts
interface BrainyChangeEvent {
  kind: 'entity' | 'relation' | 'store'
  op: 'add' | 'update' | 'remove' | 'relate' | 'unrelate' | 'updateRelation'
    | 'clear' | 'restore'
  id?: string
  entity?:   { id: string; type: string; subtype?: string;
               metadata: Record<string, unknown>; service?: string }
  relation?: { id: string; from: string; to: string; type: string;
               metadata?: Record<string, unknown> }
  generation?: number
  timestamp: number
}
```

- **Entity events** (`add` / `update` / `remove`) carry the post-commit indexed
  view — `type`, `subtype`, and the full custom `metadata`, so you can match
  your own `where`-style filters against events without a read.
- **Deletes are fully described.** A `remove` or `unrelate` event carries the
  record's *last committed state* (sourced from the commit's own history
  record), not just an id.
- **Batches emit per item.** `addMany` / `updateMany` / `relateMany` /
  `removeMany` emit one event per affected record; a `transact()` batch emits
  one event per item, all sharing the batch's single `generation`.
- **Cascades are visible.** Removing an entity also emits `unrelate` for each
  relationship the delete cascaded to.
- **Store-level events** (`kind: 'store'`) fire for the two wholesale
  operations — `clear()` and `restore()` — and mean *"everything may have
  changed; refetch what you care about."*

## Delivery guarantees

- **Post-commit only.** An aborted write — a losing
  [`ifRev` compare-and-swap](optimistic-concurrency.md), a rejected
  transaction — never emits. If you received the event, the write is durable.
- **Commit-ordered.** Events arrive in the order writes committed;
  `generation` is monotonic.
- **Asynchronous, never blocking.** Delivery happens in a microtask after the
  write completes. A slow listener cannot delay a write; a throwing listener
  is logged and isolated from other listeners.
- **Zero overhead when unused.** With no subscribers, the write path does no
  event work at all.
- **Fire-and-forget.** There is no replay or backpressure. For catch-up after
  a disconnect, use the `generation` on each event together with
  [`asOf()` / the transaction log](snapshots-and-time-travel.md): record the
  last generation you processed, and on reconnect diff from there. For file
  content specifically, `vfs.readFile(path, { asOf })` and
  `vfs.history(path)` are the temporal read — see
  [Snapshots & Time Travel](snapshots-and-time-travel.md).

## Patterns

**Cache invalidation** — drop cached reads for whatever changed:

```ts
brain.onChange((e) => {
  if (e.kind === 'store') return cache.clear()
  if (e.id) cache.delete(e.id)
})
```

**Live queries (notify-and-refetch)** — re-run a query when a relevant change
lands, rather than diffing incrementally:

```ts
brain.onChange((e) => {
  if (e.kind === 'entity' && e.entity?.type === 'order') {
    refreshOpenOrdersView() // debounce as needed
  }
})
```

**Forwarding to other processes** — the feed is in-process by design. To push
changes to browsers or other services, forward events through your own
transport (WebSocket, SSE) from the process that owns the brain.

## Lifecycle

`onChange` returns an unsubscribe function — call it when tearing down a
subscriber (for example, when evicting a pooled instance). `brain.close()`
drops all listeners; no events are delivered for or after `close()`.
