# T407

> handler on a snapshot source

**Severity** Error

`data.book`, `data.volume` and `data.hl` are snapshot sources: they do not emit events, they are queried. A handler such as `on book.update` therefore has nothing to react to.

Read the snapshot from a handler of a bar or trade subscription, for example `on chart.update { let d = book.depth(0.5) }`.

## Failing example

```flowscope
script "Book imbalance"
data book = subscribe(data.book)
pane p = pane(title: "Imbalance", height: 0.2)
plot imb = plot.line(title: "Imbalance 0.5%", on: p)
on book.update {
  let d = book.depth(0.5)
  if d != null {
    imb.plot(d.imbalance)
  }
}
```

## Fixed example

```flowscope
script "Book imbalance"
data (
  chart = subscribe(data.ohlcv)
  book = subscribe(data.book)
)
pane p = pane(title: "Imbalance", height: 0.2)
plot imb = plot.line(title: "Imbalance 0.5%", on: p)
on chart.update {
  let d = book.depth(0.5)
  if d != null {
    imb.plot(d.imbalance)
  }
}
```

## Related

- [T405](/docs/scripting/reference/diagnostics/t405)
- [T422](/docs/scripting/reference/diagnostics/t422)