# 06 · Fan out and collect

The first happy path that proves parallelism. N invocations submitted in
a list comprehension; the ids are collected with `q.result` in
submission order. There is no `gather` — the id list is the handle.

## Exercises

- Concurrent submission from one client.
- The dispatch plane distributing work across multiple workers.
- Invocation ids as the fan-out handles — the id list **is** the
  barrier; iterate it to collect in submission order.

## Requires

- Everything in [05 · Hello, queue-bound UDF](/python-sdk/happy-paths/05-hello-queue-udf.md).
- **At least two** workers on the same queue and the same plane
  (otherwise it parallelizes to 1 and only throughput differs from
  path 05):
  ```bash
  lakeshore worker start --queue compute &
  lakeshore worker start --queue compute &
  ```

## Verifies

The plane hands different invocations to different workers (atomic
claim — exactly one worker wins a given row), and collecting by id
preserves submission order regardless of completion order.

## Run

```python
# fan_out.py
import time
import dreamlake.lakeshore as dls

@dls.udf(queue="compute")
def square(x: int) -> int:
    time.sleep(1.0)          # make parallelism visible
    return x * x

q = dls.SyncQueue("compute")

t0 = time.monotonic()
ids = [square.submit(i) for i in range(8)]                 # fan out
results = [q.result(i, timeout=60.0) for i in ids]         # collect
print(results, f"{time.monotonic() - t0:.1f}s")
```

`examples/01_fan_out.py` in the SDK repo is the same shape on an
in-memory plane, with no worker terminals to start.

## Expected output

`[0, 1, 4, 9, 16, 25, 36, 49]` — in input order. Total wall clock should
be roughly `work_time / n_workers`, modulo overhead.

## If it fails

| Symptom | Likely cause |
| ------- | ------------ |
| Results out of order | Impossible by construction — you collect by id in the order you submitted. If the *values* are wrong, dig into the worker. |
| Wall clock ≈ `N × work_time` (no parallelism) | Only one worker is consuming. You need ≥ 2 workers on the same queue and the same plane. |
| Some invocations sit `queued` forever | Workers died mid-run — check their stderr. `q.count("queued")` shows the backlog on the local plane. |
| `DispatchError` partway through | A failing invocation raised at its `q.result` call — the message carries the worker error. |

## Status

Manual.

## Next

→ [07 · SSH provider smoke](/cli/happy-paths/07-ssh-provider-smoke.md)