# Files: an export's pieces

A large export is written a page of rows at a time, and each page is a
piece of the file. This topic is what that asks of the `files`
capability ([How an export's run is kept](export-batch-runs.md);
[Data formats](data-formats-json-csv-tsv-and-json-lines.md)).

## Writing the pieces

Each page's rows are written by an ordinary keyed `put`, under a key
made from the file's own key (`jadpo_std::files::piece_key`). That key
has the runtime's shape, so a driver refuses no piece, and finds each
piece from the file's key alone.

Two more operations go through the seam, each a keyed call:

- `join(key, pieces, type)` joins the pieces, in order, into the file
  under `key`. Repeating it once the file is whole does nothing.
- `discard(key, pieces)` drops a cancelled export's pieces, and its file
  if they were joined.

A store with no room answers `storage_full`, and a join with a piece
missing answers `not_found`.

## The disk driver joins whole

The disk driver copies the pieces in order into a temporary file
readable by the application's user only, a window at a time. It syncs
the file, renames it into place and syncs its folder, then removes the
pieces, so the file appears whole or not at all. The in-memory store a
test's files are in does the same in memory.

## `FileType.json_lines`

`FileType.json_lines` is a written JSON Lines file's type,
`application/jsonl`, as jsonlines.org names it. It comes after `json`.