Skip to content
An experiment in creating a simple markdown editor supplemented with the ability to programmatically insert img tags, handling upload attempts when the user is offline, or sufferring from a dodgy connection
Branch: master
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.


An experiment in creating a simple markdown editor supplemented with the ability to programmatically insert img tags, handling upload attempts when the user is offline, or sufferring from a dodgy connection:

MobileMarkdown is a Phoenix app exposing the following endpoints:


The entrypoint into the user-facing application. On this view we render a textarea and file input, allowing the user to compose markdown, and insert img tags for newly uploaded files programmatically.


This endpoint returns a time-limited credential allowing the user to upload files to Amazon S3.


An endpoint for development, returning a mock AWS signature response.

Implementation details

The Elixir application has 2 responsibilities:

  • A simple web applicaition implemented using Phoenix;
  • An AWSSigV4 context, handling generation of values required for authenticated uploads to Amazon S3.
    • AWSSigV4 is the main public interface for this functionality. Calling get_credential/5 returns a %Credential{} populated with values allowing upload to S3, with a simple default policy.
    • AWSSigV4.Credential knows nothing about S3, but handles constructing a credential string, and encoded signature based upon provided values (for instance date, AWS service, string to be signed)
    • AWSSigV4.S3 handles S3-specific functionality; exposing functions for returning a policy document given some conditions, and a date string formatted correctly for this service.

On the front end, we have some vanilla js, and an Elm application:


  • On loading the editor, we initialize the Elm application in a target div;
  • We then register a service worker, and subscribe to Elm ports for upload attempt, and upload success;
    • The upload success port, waitForUploadAtPosition, gets the current cursor position, and writes the passed file json to a new row in IndexedDB. We then pass the cursor and row ID back to Elm, across the receiveCursorAndId port.
    • Next, we call the Service Worker's sync method with the ID. Sync makes use of retries and backoffs, to handle offline or dodgy connections:
      • First, get the row from IndexedDB, by ID;
      • Next check if this upload is already complete, by looking at the value of a status field.
      • If this is succeeded, we have finished this upload already, so we resolve.
      • Otherwise, we want to attempt the upload, so send a message back to the Service Worker's client, with the file value.
      • When the client receives this message, we send the data back to Elm, over the performUpload port.
    • When we receive the upload success message over uploadSuccessful, we update the row found by the provided ID to have succeeded status.


The Elm app is responsible for rendering the editor, keeping track of and updating editor state, fetching credentials and attempting uploads.

A writeup of a similar but simplified approach to this can be seen on my blog.

  • When a user selects a file for the file input, we parse this value to JSON, then send a message through the waitForUploadAtPosition port, asking javascript to get the current position of the cursor.
  • When we receive a message through the receiveCursorAndId port, we insert a temporary img tag at the provided position, using the passed ID as a temporary key instead of an image URL.
  • When receiving a message over the performUpload port, we decode the file json into a NativeFile value, and then attempt the upload:
    • first get the credentials
    • now use this response in an attempt to upload to S3
    • If this is successful, we update the temporary img tag with the returned image URL, and send a message to js over the uploadSuccessful port.

Running the app

You'll need to set some AWS values in your config—some are secret, so don't commit this!


use Mix.Config

config :mobile_markdown, :s3_post_config,
  endpoint: "",
  bucket: "",
  region: "us-east-1",
  ttl: 30,
  public_key: "PUBLIC",
  private_key: "SECRET"

To start your Phoenix server:

  • Install dependencies with mix deps.get
  • Create and migrate your database with mix ecto.create && mix ecto.migrate
  • Install Node.js dependencies with cd assets && npm install
  • Start Phoenix endpoint with mix phx.server

Now you can visit localhost:4000 from your browser.

You can’t perform that action at this time.