View Source External Uploads

This guide continues from the configuration started in the server Uploads guide.

Uploads to external cloud providers, such as Amazon S3, Google Cloud, etc., can be achieved by using the :external option in allow_upload/3.

You provide a 2-arity function to allow the server to generate metadata for each upload entry, which is passed to a user-specified JavaScript function on the client.

Typically when your function is invoked, you will generate a pre-signed URL, specific to your cloud storage provider, that will provide temporary access for the end-user to upload data directly to your cloud storage.

Chunked HTTP Uploads

For any service that supports large file uploads via chunked HTTP requests with Content-Range headers, you can use the UpChunk JS library by Mux to do all the hard work of uploading the file. For small file uploads or to get started quickly, consider uploading directly to S3 instead.

You only need to wire the UpChunk instance to the LiveView UploadEntry callbacks, and LiveView will take care of the rest.

Install UpChunk by saving its contents to assets/vendor/upchunk.js or by installing it with npm:

$ npm install --prefix assets --save @mux/upchunk

Configure your uploader on Phoenix.LiveView.mount/3:

def mount(_params, _session, socket) do
  {:ok,
   socket
   |> assign(:uploaded_files, [])
   |> allow_upload(:avatar, accept: :any, max_entries: 3, external: &presign_upload/2)}
end

Supply the :external option to Phoenix.LiveView.allow_upload/3. It requires a 2-arity function that generates a signed URL where the client will push the bytes for the upload entry. This function must return either {:ok, meta, socket} or {:error, meta, socket}, where meta must be a map.

For example, if you were using a context that provided a start_session function, you might write something like this:

defp presign_upload(entry, socket) do
  {:ok, %{"Location" => link}} =
    SomeTube.start_session(%{
      "uploadType" => "resumable",
      "x-upload-content-length" => entry.client_size
    })

  {:ok, %{uploader: "UpChunk", entrypoint: link}, socket}
end

Finally, on the client-side, we use UpChunk to create an upload from the temporary URL generated on the server and attach listeners for its events to the entry's callbacks:

import * as UpChunk from "@mux/upchunk"

let Uploaders = {}

Uploaders.UpChunk = function(entries, onViewError){
  entries.forEach(entry => {
    // create the upload session with UpChunk
    let { file, meta: { entrypoint } } = entry
    let upload = UpChunk.createUpload({ endpoint: entrypoint, file })

    // stop uploading in the event of a view error
    onViewError(() => upload.pause())

    // upload error triggers LiveView error
    upload.on("error", (e) => entry.error(e.detail.message))

    // notify progress events to LiveView
    upload.on("progress", (e) => {
      if(e.detail < 100){ entry.progress(e.detail) }
    })

    // success completes the UploadEntry
    upload.on("success", () => entry.progress(100))
  })
}

// Don't forget to assign Uploaders to the liveSocket
let liveSocket = new LiveSocket("/live", Socket, {
  uploaders: Uploaders,
  params: {_csrf_token: csrfToken}
})

Direct to S3

The largest object that can be uploaded to S3 in a single PUT is 5 GB according to S3 FAQ. For larger file uploads, consider using chunking as shown above.

This guide assumes an existing S3 bucket is set up with the correct CORS configuration which allows uploading directly to the bucket.

An example CORS config is:

[
    {
        "AllowedHeaders": [ "*" ],
        "AllowedMethods": [ "PUT", "POST" ],
        "AllowedOrigins": [ "*" ],
        "ExposeHeaders": []
    }
]

You may put your domain in the "allowedOrigins" instead. More information on configuring CORS for S3 buckets is available on AWS.

In order to enforce all of your file constraints when uploading to S3, it is necessary to perform a multipart form POST with your file data. You should have the following S3 information ready before proceeding:

1. aws_access_key_id
2. aws_secret_access_key
3. bucket_name
4. region

We will first implement the LiveView portion:

def mount(_params, _session, socket) do
  {:ok,
    socket
    |> assign(:uploaded_files, [])
    |> allow_upload(:avatar, accept: :any, max_entries: 3, external: &presign_upload/2)}
end

defp presign_upload(entry, socket) do
  uploads = socket.assigns.uploads
  bucket = "phx-upload-example"
  key = "public/#{entry.client_name}"

  config = %{
    region: "us-east-1",
    access_key_id: System.fetch_env!("AWS_ACCESS_KEY_ID"),
    secret_access_key: System.fetch_env!("AWS_SECRET_ACCESS_KEY")
  }

  {:ok, fields} =
    SimpleS3Upload.sign_form_upload(config, bucket,
      key: key,
      content_type: entry.client_type,
      max_file_size: uploads[entry.upload_config].max_file_size,
      expires_in: :timer.hours(1)
    )

  meta = %{uploader: "S3", key: key, url: "http://#{bucket}.s3-#{config.region}.amazonaws.com", fields: fields}
  {:ok, meta, socket}
end

Here, we implemented a presign_upload/2 function, which we passed as a captured anonymous function to :external. It generates a pre-signed URL for the upload and returns our :ok result, with a payload of metadata for the client, along with our unchanged socket.

Next, we add a missing module SimpleS3Upload to generate pre-signed URLs for S3. Create a file called simple_s3_upload.ex. Get the file's content from this zero-dependency module called SimpleS3Upload written by Chris McCord.

Tip: if you encounter errors with the :crypto module or with S3 blocking ACLs, please read the comments in the gist above for solutions.

Next, we add our JavaScript client-side uploader. The metadata must contain the :uploader key, specifying the name of the JavaScript client-side uploader. In this case, it's "S3", as shown above.

Add a new file uploaders.js in the following directory assets/js/ next to app.js. The content for this S3 client uploader:

let Uploaders = {}

Uploaders.S3 = function(entries, onViewError){
  entries.forEach(entry => {
    let formData = new FormData()
    let {url, fields} = entry.meta
    Object.entries(fields).forEach(([key, val]) => formData.append(key, val))
    formData.append("file", entry.file)
    let xhr = new XMLHttpRequest()
    onViewError(() => xhr.abort())
    xhr.onload = () => xhr.status === 204 ? entry.progress(100) : entry.error()
    xhr.onerror = () => entry.error()
    xhr.upload.addEventListener("progress", (event) => {
      if(event.lengthComputable){
        let percent = Math.round((event.loaded / event.total) * 100)
        if(percent < 100){ entry.progress(percent) }
      }
    })

    xhr.open("POST", url, true)
    xhr.send(formData)
  })
}

export default Uploaders;

We define an Uploaders.S3 function, which receives our entries. It then performs an AJAX request for each entry, using the entry.progress() and entry.error() functions to report upload events back to the LiveView. The name of the uploader must match the one we return on the :uploader metadata in LiveView.

Finally, head over to app.js and add the uploaders: Uploaders key to the LiveSocket constructor to tell phoenix where to find the uploaders returned within the external metadata.

// for uploading to S3
import Uploaders from "./uploaders"

let liveSocket = new LiveSocket("/live",
   Socket, {
     params: {_csrf_token: csrfToken},
     uploaders: Uploaders
  }
)

Now "S3" returned from the server will match the one in the client. To debug client-side javascript when trying to upload, you can inspect your browser and look at the console or networks tab to view the error logs.

Direct to S3-Compatible

This section assumes that you installed and configured ExAws and ExAws.S3 correctly in your project and can execute the examples in the page without errors.

Most S3 compatible platforms like Cloudflare R2 don't support POST when uploading files so we need to use PUT with a signed URL instead of the signed POSTand send the file straight to the service, to do so we need to change the presign_upload/2 function and the Uploaders.S3 that does the upload.

The new presign_upload/2:

def presign_upload(entry, socket) do
  config = ExAws.Config.new(:s3)
  bucket = "bucket"
  key = "public/#{entry.client_name}"

  {:ok, url} =
    ExAws.S3.presigned_url(config, :put, bucket, key,
      expires_in: 3600,
      query_params: [{"Content-Type", entry.client_type}]
    )
   {:ok, %{uploader: "S3", key: key, url: url}, socket}
end

The new Uploaders.S3:

Uploaders.S3 = function (entries, onViewError) {
  entries.forEach(entry => {
    let xhr = new XMLHttpRequest()
    onViewError(() => xhr.abort())
    xhr.onload = () => xhr.status === 200 ? entry.progress(100) : entry.error()
    xhr.onerror = () => entry.error()

    xhr.upload.addEventListener("progress", (event) => {
      if(event.lengthComputable){
        let percent = Math.round((event.loaded / event.total) * 100)
        if(percent < 100){ entry.progress(percent) }
      }
    })

    let url = entry.meta.url
    xhr.open("PUT", url, true)
    xhr.send(entry.file)
  })
}