Multipart HTTP response

Question

The goal is for a Node.js / hapi API server to respond to a browser's AJAX request with two things:

• A media file (e.g. an image)
• A JSON object with metadata about the file

These are two separate items only because binary data cannot easily be stored in JSON. Otherwise, this would be a single resource. Nevertheless, it is preferable that they be sent in a single response.

We upload these in a single request with multipart/form-data. In that case, browsers provide a built-in mechanism to serialize the body and most server-side frameworks know how to parse it. But how does one do the same for a response, in the opposite direction? Namely, how should a server serialize the body to transmit it to a client?

From what I can tell, multipart/mixed could be a useful content type. But there is very little talk of this. Most people seem to resort to providing two separate GET routes, one for each piece. I dislike that because it opens you up to race conditions, amongst other things. What am I missing?

See also my question in hapijs/discuss#563.

Answer 1

You can serve the response as multipart/form-data and use Response.formData() to read response at client

fetch("/path/to/server", {method:"POST", body:formData})
.then(response => response.formData())
.then(fd => {
  for (let [key, prop] of fd) {
    console.log(key, prop)
  }
})

let fd = new FormData();
fd.append("json", JSON.stringify({
  file: "image"
}));
fetch("data:image/gif;base64,R0lGODlhEAAQAPIAAP///wAAAMLCwkJCQgAAAGJiYoKCgpKSkiH+GkNyZWF0ZWQgd2l0aCBhamF4bG9hZC5pbmZvACH5BAAKAAAAIf8LTkVUU0NBUEUyLjADAQAAACwAAAAAEAAQAAADMwi63P4wyklrE2MIOggZnAdOmGYJRbExwroUmcG2LmDEwnHQLVsYOd2mBzkYDAdKa+dIAAAh+QQACgABACwAAAAAEAAQAAADNAi63P5OjCEgG4QMu7DmikRxQlFUYDEZIGBMRVsaqHwctXXf7WEYB4Ag1xjihkMZsiUkKhIAIfkEAAoAAgAsAAAAABAAEAAAAzYIujIjK8pByJDMlFYvBoVjHA70GU7xSUJhmKtwHPAKzLO9HMaoKwJZ7Rf8AYPDDzKpZBqfvwQAIfkEAAoAAwAsAAAAABAAEAAAAzMIumIlK8oyhpHsnFZfhYumCYUhDAQxRIdhHBGqRoKw0R8DYlJd8z0fMDgsGo/IpHI5TAAAIfkEAAoABAAsAAAAABAAEAAAAzIIunInK0rnZBTwGPNMgQwmdsNgXGJUlIWEuR5oWUIpz8pAEAMe6TwfwyYsGo/IpFKSAAAh+QQACgAFACwAAAAAEAAQAAADMwi6IMKQORfjdOe82p4wGccc4CEuQradylesojEMBgsUc2G7sDX3lQGBMLAJibufbSlKAAAh+QQACgAGACwAAAAAEAAQAAADMgi63P7wCRHZnFVdmgHu2nFwlWCI3WGc3TSWhUFGxTAUkGCbtgENBMJAEJsxgMLWzpEAACH5BAAKAAcALAAAAAAQABAAAAMyCLrc/jDKSatlQtScKdceCAjDII7HcQ4EMTCpyrCuUBjCYRgHVtqlAiB1YhiCnlsRkAAAOwAAAAAAAAAAAA==")
  .then(response => response.blob())
  .then(blob => {
    fd.append("file", blob);
    new Response(fd)
      .formData()
      .then(formData => {
        for (let [key, data] of formData) {
          console.log(key, data)
        }
      })
  })