Ephemeral P2P

See example implementation running here. Discussion on Hacker News here.

This app hosts "P2P" pages that are "ephemeral". We say "P2P" because the clients host the page; new visitors retrieve the page contents from other visitors on the same page. It's "ephemeral" in that the server does not store the contents of any page, so once the last visitor leaves a particular page, it is gone.

How it works

Ephemeral P2P is an Elixir/Phoenix app, taking advantage of Phoenix's excellent Channel functionality. All of the logic lies in two channels, the HaveChannel for clients who have a particular bit of content, and the WantChannel for visitors who want it. The content is addressed by its SHA256 (let us say abc123), and the two topics associated with that content are have:abc123 and want:abc123.

A new page is created from the homepage. A visitor fills in the textarea with whatever content they desire, and presses the "Submit" button (which is not a form submission). The client hashes the content (let's call the hash abc123) and uses the HTML5 history api to change the URL to /abc123 for easy copy/paste-ability. The client then joins the "have:abc123" topic and begins listening for "content_request" messages, ready to respond with a "content" message that includes the page content which it has in memory.

A subsequent visitor who loads /abc123 joins the want:abc123 topic and tries to obtain the content. First it listens for a "content" message that another visitor may have provoked. If the visitor does not receive it in 2 seconds, it will send a "content_request" message itself. The server will re-broadcast this message to all have:abc123 subscribers, except that a handle_out will allow the message with probability 1/subscriber_count and drop it otherwise. Any have:abc123 subscribers who receive the message will respond with the content and the server will broadcast it to all want:abc123 subscribers. The new visitor will send a "content_request" message every 2 seconds until it gets the content (for the case where the handle_out drops the message to everyone, or a have subscriber fails to respond for some reason.)

When a want:abc123 subscriber gets the content, it leaves the want:abc123 topic and joins the have:abc123 topic, ready to pass it along to newer visitors.

Lastly, whenever a subscriber joins or leaves have:abc123, the new visitor count is broadcast, so all clients know the "health" of the page and how close it is to going away.

Run it yourself

First you'll need to install Elixir. Once you have elixir set up, it should be as easy as:

$ git clone git@github.com:losvedir/ephemeral2.git
$ cd ephemeral2
$ mix deps.get
$ mix phoenix.server

That should serve a copy of the app locally at localhost:4000. If you would like to modify the CSS or Javascript, you'll need to install npm and then brunch with:

$ npm install -g brunch