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Shut the Box

Multiplayer web-based version of Shut the Box with real-time video chat. Created in April 2020 to bring friends and family together during the pandemic.

Uses ClojureScript on both the client (re-frame) and the server (Macchiato). Twilio's Network Traversal Service provides STUN/TURN for the WebRTC connections. shadow-cljs is used for builds, GitHub Actions for CI, and nginx for deployment. The player icons and dice are from the Kenney Animal and Boardgame packs.

Why is there no public version of this game available? Because there are some open issues related to STUN/TURN auth (or the lack thereof...) which means that the current game acts as an open TURN relay. That is not ideal, since it costs money to relay those packets. Long term the right approach would be to require user accounts and time limits on usage, but this is a prototype and those are not prototype-y things.

Development Environment

Building & Running

Make sure you have a local HTTPS certificate, created a Twilio account, and set up your config.edn file. You then have to edit :devtools-url in the shadow-cljs.edn.

Compile and watch the client and server builds:

$ clojure -A:watch

Run the Node.js server in another window:

$ node server.js

Point your web browser at the (HTTPS!) version of the domain name that you created earlier, port 3000. You can do that from multiple devices in order to test the multiplayer aspects of the game.

Local HTTPS Certificates

The game has an embedded video chat feature, which means that we need access to the user's camera. Web browsers only allow access to that API from a secure context (such as a page hosted over HTTPS). This significantly complicates the development experience, since you need to host the local app from an HTTPS server.

One way to do that is to create a self-signed certificate and then add it to your computer's trust store. That would have to be done on every device that you plan to use with the game, which, since this is a multiplayer game might be difficult (on mobile devices, for example).

The mechanism that I went with is to use a Let's Encrypt certificate that maps to my development machine, which I can then access from anywhere on my home network. This approach requires you to have both a public DNS server (for the Let's Encrypt validation) and an internal server (so that your computers can find each other). These could be the same DNS server, depending on your network configuration.

Here are the steps that I used to create that certificate (on NixOS, but the resulting certificates can be used on any OS):

  1. Get a Nix shell with the Certbot, OpenSSL, and Java.
  2. Perform a Let's Encrypt manual DNS challenge. Pick a domain name like mydevbox.example.com (where mydevbox is the name of your computer on your local network and example.com is your domain name).
  3. The resulting key and certificate can be used as-is with Macchiato's Node.js-based HTTPS server. (put the files in certs/dev-fullchain.pem and certs/dev-privkey.pem)
  4. Shadow CLJS also has a web server (used to communicate with the client for triggering reloads) and that server also needs the key and certificate, but in a Java KeyStore file. You can use OpenSSL to convert the PEM files into a Java KeyStore.

This repo includes a shell script that can do all of that for you. Here is how you use it on NixOS:

$ nix-shell -p certbot jdk14 openssl
$ ./certs/generate.sh

Twilio Account

The game uses Twilio's Network Traversal Service to connect clients together even when they are behind NATs. This requires you to have a Twilio account, which then automatically provisions a Twilio "Programmable Video" Network Traversal Service. There are other options for STUN/TURN services, including open source self-hosted options like coturn. Twilio is cheap and very easy to get running, so I went with that.

config.edn

The server uses macchiato-env to load runtime configuration data. Configuration can be sourced from environment variables, a config file, or both. For development, using a config.edn file is the simplest approach (but do not check this in!).

Here is a sample file:

{:dev true
 :host "0.0.0.0"
 :twilio-account-sid "TWILIO SID HERE"
 :twilio-auth-token "TWILIO AUTH TOKEN HERE"}

REPL

When watch is running, you can access the REPL from vim-fireplace (more info here):

:Piggieback :client
cqp (js/alert "Hi")

View re-frame app DB:

(in-ns 'shut-the-box.client.views)
@re-frame.db/app-db

Send events from the REPL:

(in-ns 'shut-the-box.client.views)
(re-frame/dispatch [:initialize-db])

CLJS DevTools

CLJS DevTools only works in normal browser mode, not in the "device emulation" (aka responsive browser) mode. You can disable the check in CLJS DevTools that prevents that though (or just use CLJS DevTools in normal mode):

:dev {:compiler-options
      {:external-config
       {:devtools/config
        {:bypass-availability-checks true}}}

Production Deployment

This will be heavily dependent on your deployment approach. I use nginx with a custom NixOS Module to run the Node.js server. The continuous integration action in this repo builds a batteries-included tarball that can be run as-is in many environments, since it includes the production node_modules used by the server.

You will need to inject your production config as well, which may use different Twilio credentials, ports, etc. This can be done with environment variables or via a config.edn file. See macchiato-env for more options.

The following keys are mandatory and have no defaults:

  • :twilio-account-sid (aka TWILIO_ACCOUNT_SID as an environment variable)
  • :twilio-auth-token (aka TWILIO_AUTH_TOKEN as an environment variable)

If you use nginx (and a UNIX socket for communicating with the Macchiato application), then you will need a proxy section like this in your virtual host config:

proxyPass = "http://unix:/run/shutthebox/socket/ipc.socket";
proxyWebsockets = true;

Open Issues

  • There is no authentication at all. Anyone can join anyone else's game (which is trivial, because game ids are monotonically-increasing integers).
  • No authentication also means that the service ultimately acts as an open STUN/TURN relay, so someone could rack up significant charges against your Twilio account.

Resources

ClojureScript

clojure.spec:

CSS

STUN/TURN

Browser Audio/Video

WebRTC Libraries

  • Good overview of WebRTC, with a discussion of all of the protocols: https://hpbn.co/webrtc/
  • Alternatives to simple-peer:
    • PeerJS is a peer-to-peer (only) helper for easily connecting WebRTC clients, but does require an open-source "PeerServer" to connect those clients. I don't think that this helps us, because we need the server anyway for sign in, notifications, etc. and so we can just exchange ICE data ourselves (and trigger new peer connections).
    • Geckos.io looks like a "batteries included" approach to building client-server games using WebRTC. It uses the same node-webrtc library used by simple-peer for the Node.js server. I don't think we need this either, because we are not terminating any WebRTC connections on our server.
  • node-webrtc is the underlying Node.js library that powers all of the server-side WebRTC termination. It uses Google's WebRTC native library to do all of the complex stuff (DTLS, SCTP, WebRTC, etc.). This might be an interesting way to try out a server-based peer (as opposed to WebSockets).
    • Important bug fix/issue (which I think has now been integrated?) related to large numbers of WebRTC connections: node-webrtc/node-webrtc#362 and here and this. Note that the Google WebRTC library apparently uses 7 file descriptors per connection, which seems really high...