# `Storehouse.Plug.Presign`
[🔗](https://github.com/hattoriai/storehouse/blob/v0.2.0/lib/storehouse/plug/presign.ex#L1)

Signs uploads straight to the store for a frontend that is not a
LiveView: a JSON endpoint in front of `Storehouse.Batch.presign/2`.

    # lib/my_app_web/router.ex, behind your authentication
    scope "/files" do
      pipe_through [:files, :require_user]  # the guide's, with its CSRF check

      forward "/presign", Storehouse.Plug.Presign,
        owner: &MyAppWeb.Files.owner/1,
        room: &MyAppWeb.Files.room/1,
        existing: &MyAppWeb.Files.existing/2,
        accept: Storehouse.Check.images(),
        max_bytes: 50_000_000

      forward "/register", Storehouse.Plug.Register, ...
    end

A `POST` of

    {"uploads": [{"name": "harbour.jpg", "type": "image/jpeg", "bytes": 35367, "checksum": "e3b0…"}]}

is answered `200` with one result per upload, in order:

    {"uploads": [
      {"upload": {"key": "accounts/42/2026/10/…-harbour.jpg", "method": "PUT", "url": "https://…", "headers": {"content-type": "image/jpeg", …}}},
      {"existing": {"key": …, "url": …, …}},
      {"error": "storage_full"}
    ]}

The browser sends each file to its `url` with its `method` and exactly
its `headers`, then gives the keys to `Storehouse.Plug.Register`. A body
that is not that JSON gives `400`, one that is not `application/json`
`415` (a form of another site is never read), and a batch larger than `:max_items`
`413`, with `{"error": code}`.

## Options

  * `:owner` - a function of the conn that gives the owner whose files
    these are (`Storehouse.store/2`'s `:owner`), or `nil`.
  * `:room` - a function of the conn that gives the bytes the owner may
    still store, shared by the batch, or `:infinity`.
  * `:existing` - a function of the conn and a checksum that gives the
    file the app has already with it, or `nil` (`:existing` of
    `Storehouse.presign_upload/4`).
  * the other options of `Storehouse.Batch.presign/2` (`:accept`,
    `:max_bytes`, `:key`, `:cache`, `:expires_in`, `:adapter`,
    `:max_items`), as they are.

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*Consult [api-reference.md](api-reference.md) for complete listing*