shortly

Shortly: Link Shortener

Reading material

• Rebar3 build tool
• Rebar3 hex
• Cowboy http server
• Cowboy user-guide

Exercise

This exercise will be divided in multiple parts:

1. Shortly application
2. Create two nodes
3. Mnesia
4. Syn

Shortly application

Create an OTP application using rebar3 and cowboy that is capable of receiving long links and returning shorts ones:

• Receive a HTTP POST at http://localhost:8080/<LONG_URL> returning a shortened link.
• Receive a HTTP GET at http://localhost:8080/<SHORT_URL> returning the original long link.
• Accept websocket connections at http://localhost:8080/news and notify every time a new link is shortened.

BONUS: Create similar endpoints (GET and POST), but using cowboy_rest handler.

Create two nodes

Now that you have a working application for a single node we need to make it distributed. For this you will need to do the following:

• Receive cowboy's port as either a configuration variable or an environment variable.
• Set the node name when starting it.

Mnesia

So we can have two nodes of our application running, but what is the point if they don't actually comunicate. To fix this we will make use of Mnesia, which will let us have a distributed database across our nodes. To achieve this do the following:

• Create the Mnesia schema.
• Create the shortly_urls table.
• Add the Mnesia application as a dependency (no more manual starts).
• Change code to use Mnesia and abstract its usage to its own module shortly_db.

If everything went smoothly you should be able to create a short URL in one node, ask the other node for the long URL, and get your answer.

Syn

For the final touch we need to let our users connect (websocket) to any node and still receive the notifications of link creations in all other nodes. For this task we will use Syn and do the following:

• Add the Syn application as a dependency.
• Change code to use Syn and abstract it to a shortly_presence module.

Now no matter to what node your users are connected whenever a URL is shortened by their node or any other they should receive the notification.

Solution

• src/bitly_short_link_handler.erl handles POST and GET requests.
• src/bitly_shortener is a gen_server that uses ets for storing the links in memory. Being a separated process from the normal cowboy request handler is very important because the ets data is lost if the process which created it dies, and the cowboy handlers processes are created and destroyed in every request.

To run the solution first star the server with make server and then make test for running simple requests.