---
title: Watch
description: Subscribe to external file changes on any mount. Events arrive only after Mirage has invalidated its caches, so reacting to a change always reads fresh content.
icon: bell
---

## What It Does

Files on a mounted backend change outside your workspace all the time: a
teammate uploads to Nextcloud, a pipeline rewrites an S3 object, a cron job
deletes a report. `watch` turns those external changes into an async event
stream, so an agent reacts instead of rescanning and diffing directories.

```python
from mirage import Workspace
from mirage.resource.nextcloud import NextcloudConfig, NextcloudResource

ws = Workspace({"/nc": NextcloudResource(NextcloudConfig(...))})

async for event in ws.watch("/nc/Documents"):
    print(event.kind, event.path.virtual)   # e.g. "update /nc/Documents/report.txt"
    result = await ws.execute(f"cat {event.path.virtual}")  # guaranteed fresh
```

No setup call is needed: the watch runtime attaches lazily on first use, and an
idle workspace carries no watch state at all. Mirage runs **no server and no
background loop**. Detection is yours (a webhook receiver or a small poll
loop); Mirage's job is everything after the signal: cache invalidation, scope
matching, and delivery.

```mermaid
flowchart LR
    NC[Backend<br/>Nextcloud, S3, ...] -->|webhook| RX[Your receiver]
    NC -->|delta pull| PL[Your poll loop]
    RX --> N["ws.notify(event)"]
    PL --> N
    N --> INV[invalidate caches<br/>path + ancestor listings]
    INV --> Q[per-watch queues<br/>scope matching]
    Q --> W["async for event in ws.watch(...)"]
```

## Scopes

The root's shape defines the depth, GNU shell glob style. `*` never crosses
`/`, and matching happens at delivery time, so files created after the watch
started still match.

| root | scope |
| --- | --- |
| `/nc/data` | the whole subtree |
| `/nc/data/*` | the entries at that level only (shallow, no descent) |
| `/nc/data/*/` | everything inside child directories (GNU `*/` matches directories) |
| `/nc/data/*.txt` | the `.txt` entries at that level |
| `/nc/data/*/reports/` | everything inside each child's `reports` directory |
| `["/nc/docs", "/nc/cfg/app.yaml"]` | any of several roots, one event stream |

```python
async for event in ws.watch("/nc/data/*.pdf"):   # pattern
    ...

async for event in ws.watch(["/nc/inbox/*", "/nc/config"]):  # list
    ...
```

## The Event

```python
@dataclass(frozen=True, slots=True)
class FileEvent:
    kind: FileChangeKind                   # create / update / delete / move / unknown
    path: PathSpec                         # virtual path of the changed entry
    timestamp: datetime                    # UTC time the change was observed
    previous_path: PathSpec | None = None  # prior path for MOVE events
    metadata: FileMetadata | None = None   # post-change metadata, when the source has it
```

`FileMetadata` carries `fingerprint` (the backend's ETag/rev, or an
`mtime|size` composite), `size`, and `modified`. Producers fill only what their
signal honestly knows: a listing walk fills all three, a webhook payload none.

`unknown` is the overflow signal: if a burst exceeds the queue cap, pending
events collapse into one `unknown` event per watch root, meaning
"re-inventory this subtree". Precision degrades; dirtiness is never lost.

Events are **level-triggered**: an event says *what is dirty*, not every
intermediate edit. Read current content through the workspace after receiving
one. That read is guaranteed fresh, because Mirage invalidates the changed
path's cache entries, and every cached ancestor listing up to the mount root,
*before* the event reaches any subscriber.

## Push Mode

Host a small endpoint in your own service, map the provider payload to a
`FileEvent`, and inject it with `ws.notify`. For Nextcloud, the
`webhook_listeners` app (Nextcloud 30+) POSTs on every file event:

```python
async def handle(request):                     # your aiohttp/FastAPI route
    payload = await request.json()
    kind = KIND_BY_CLASS.get(payload["event"]["class"])   # NodeCreatedEvent -> create ...
    node = payload["event"]["node"]["path"]               # /admin/files/data/report.txt
    virtual = "/nc/" + node.removeprefix("/admin/files/") # -> /nc/data/report.txt
    await ws.notify(FileEvent(kind=kind,
                              path=PathSpec.from_str_path(virtual),
                              timestamp=datetime.now(timezone.utc)))
    return web.json_response({"ok": True})
```

The full runnable version, including the one-time `occ` registration commands,
is [`examples/python/nextcloud/watch.py`](https://github.com/strukto-ai/mirage/blob/main/examples/python/nextcloud/watch.py).

## Pull Mode

Backends that implement `delta_hook()` (Nextcloud today) answer one question:
*what changed under this root since the last checkpoint?* A baseline pull
(`checkpoint=None`) emits nothing; every later pull diffs against the
checkpoint you hand back. The whole consumer poller is:

```python
hook = ws.registry.mount_for("/nc").resource.delta_hook()
checkpoint = None
while True:
    delta = await hook.pull(root, checkpoint)
    checkpoint = delta.checkpoint
    for event in delta.changes:
        await ws.notify(event)
    await asyncio.sleep(30)
```

Pull is self-healing: it diffs current backend state against your checkpoint,
so events missed while your service was down surface on the next pull. A
common production shape is push-first with a pull at startup as recovery, or
webhook-as-doorbell: on any webhook, pump the pull once and trust its diff
rather than the payload.

## Queues

Each `watch()` owns its own delivery queue: `notify` fans one event out into
every queue whose scope matches, so two watches on overlapping scopes each
consume at their own pace and a slow consumer only overflows its own queue.

The default `RAMWatchQueue` coalesces per path with level-triggered semantics
(create then update stays create; create then delete cancels; delete then
create becomes update), so pending size is bounded by distinct dirty paths,
not event volume. Overflow policy is `collapse` (default, the `unknown`
rescan event), `drop_oldest`, or `error`.

To customize, attach the runtime explicitly before the first watch:

```python
from functools import partial
from mirage.watch import RAMWatchQueue, Watcher

ws.attach_watch_runtime(
    Watcher(ws.registry,
            queue_factory=partial(RAMWatchQueue, max_pending=64)))
```

Any object satisfying the async `WatchQueue` protocol (`push` / `pop` /
`pending` / `clear` / `close`) drops in, so a Redis- or SQS-backed queue needs
no interface changes. See
[`examples/python/nextcloud/watch_custom_queue.py`](https://github.com/strukto-ai/mirage/blob/main/examples/python/nextcloud/watch_custom_queue.py)
for a runnable custom-queue setup.

To tear down without closing the workspace, `await
ws.detach_watch_runtime()`: every subscriber queue closes (active `watch`
loops end cleanly) and the workspace returns to its idle state; the next
`watch` lazily attaches a fresh default runtime.

## Scope Notes

- Watch roots are fixed at subscription time: there is no API to add or
  remove roots on a live watch. To change scope, start a new `watch()` with
  the updated list and stop iterating the old one (its pending events are
  dropped when it closes). A mutable subscription handle is future work.
- A watch may span mounts, including a mount nested inside another
  mount's subtree: scope matching is on virtual paths, and each event is
  invalidated on the mount that owns its path (longest prefix). Detection
  stays per backend, so each mount needs its own signal feeding
  `ws.notify`.
- Python only for now; the TypeScript mirror is planned.
- Nextcloud is the first backend with a `delta_hook`; push mode works with any
  backend today since `ws.notify` accepts events from any detection you run.
  See the [Watch Matrix](/python/watch-matrix) for per-resource support.
- Events are in-memory and at-most-once per subscriber; durable queues and
  acknowledgement are future work.
