Uploading to S3 from Elm

Storing binary data on S3 is a typical front-end need, and often has be to implemented with authorisation to ensure that only designated users can upload. To do that, uploads need to be ‘signed’ with a secret key. There are several possible approaches:

This blog focuses on the latter.

Http - POST Form

The Amazon docs provide an example of an HTML form that - on Submit - POSTs to S3. In their case the server populates the form fields in advance with all the necessary credentials, and only the file to upload is required from the user. Notionally we could have Elm render the same HTML, and leave it to the browser to do the form submission, but that would clearly be bizarre.

A HTML webpage form ultimate creates a multi-part POST object, and Elm can be cajoled into doing the same. The trick is to use a new feature of file-reader library. The library, which is available on Github only because it uses native code, provides access to the FileReader APIs, and includes a function filePart for making a Http.Part from a blob. Examples are included of the FileReader APIs, but this post builds on filePart to upload signed content to S3.

Passing files to Elm

We have the choice of Drag-n-Drop and the HTML <input type='file'> tag, and the FileReader library provides Json Decoders to read the event objects that these generate. In the case of Drag-n-Drop, we get the file contents directly; while with the input tag we get a File object that is ready automatically by the FormData library used later. (As a result the FileReader apis are not needed in this instance.)

The following code snippet shows a simple Drag-n-Drop interface that keeps track of dragenter and dragleave events to provide users with visual feedback as they drag files onto the browser window. On Drop, we store the NativeFiles (see the FileReader library for details) in our Model.

update : Msg -> Model -> Model
update message model =
    case message of
        DragEnter ->
            { model | dnd = model.dnd + 1 }

        DragLeave ->
            { model | dnd = model.dnd - 1 }

        Drop nfs ->
            { model | dnd = 0, nativeFiles = nfs }

view : Model -> Html Msg
view {dnd } =
        (class "drop-zone-container" :: dropZoneEventHandlers)
        [ if dnd == 0 then
            text ""
            div [ class "drop-zone" ] [ text "Drop" ]


To upload the files we will need authorisation in the form of signed credentials (a ‘Policy’ in S3 parlance). I will not cover the backend that uses a secret key to generate these, and in this instance, we will not encode any of the file details in the Policy. So we use a simple GET request to obtain credentials.

getSigningData : Cmd Msg
getSigningData =
    Http.get "/api/signature" (signatureDecoder SigningData)
        |> Http.send SigningDataResult

signatureDecoder constructor =
    map6 constructor
        (field "stem" string)
        (field "host" string)
        (field "credential" string)
        (field "date" string)
        (field "policy" string)
        (field "signature" string)

Creating the POST form

With all the elements of the Policy in hand, we can now build the form to POST to S3. Most of the elements are Strings that can be encoded with Http.stringPart but for the file content itself we need to rely on the FileReader library, for a little native code trick to get past the Elm compiler.

Looking at the source code we have:

filePart : String -> NativeFile -> Part
filePart name nf =
    Native.FileReader.filePart name nf.blob


var filePart = function(name, blob) {
    return {
        _0: name,
        _1: blob

With that we can use the following to complete the job. Http.multipartBody will happily use the result of FilePart to add binary content to the native browser FormData object it is creating (FormData also reads a File loaded with the file input tag).

sendSignedData : Model -> Cmd Msg
sendSignedData { signingData, dropzone } =
    case dropzone.nativeFiles of
        [] ->

        nf :: _ ->
            Http.post bucket (makeMultiPart signingData nf) etagDecoder
                |> Http.send S3Confirmation

makeMultiPart : SigningData -> FR.NativeFile -> Http.Body
makeMultiPart signingData nf =
        [ stringPart "key" (signingData.stem ++ "/" ++ nf.name)
        , stringPart "x-amz-algorithm" "AWS4-HMAC-SHA256"
        , stringPart "x-amz-credential" signingData.credential
        , stringPart "x-amz-date" signingData.date
        , stringPart "success_action_redirect" (signingData.host ++ "/success")
        , stringPart "policy" signingData.policy
        , stringPart "x-amz-signature" signingData.signature
        , FR.filePart "file" nf

Finally, we need to handle on the backend the GET /success when the upload succeeds. What I do is have that read the Url parameters (the Etag) and return them as json to Elm to complete the Command.

Full code on Github, with Elixir backend.