This is an implementation of an application for Reddit AMA style Q&A sessions.

Motivation

While I think Modernator is itself a pretty useful and neat application, my primary purpose for creating this is to experiment and push the boundaries of how to design a React application. If you're familiar with client-side Javascript development, you might be familiar with TodoMVC. It's a small little application that can be built from scratch in a few hours to help experiment with client-side frameworks and libraries. It's very useful, but I've found that it's not complex enough to accurately weigh what different frameworks bring to the table. With that in mind I created Modernator. The Modernator specification is intended to be complex enough such that it requires careful design and forethought during its implementation on both client and server. However, it's not so complex as to require a major time investment in understanding and implementing it. A functioning, complete implementation (without polish and optimization) should be possible in a few days to a week for reasonably experienced developers. Whereas something like TodoMVC might be doable in a few hours.

Building

Nix

The easiest way to build this is to download the Nix package manager and use one of the following commands:

• nix-env --install -f default.nix - build and install the application, making it available from the command line under modernator-haskell. Use this to test deployments.
• nix-build default.nix - build the application, resulting in an executable under ./result/bin/modernator-haskell. Use this to test builds.
• nix-shell --pure shell.nix - Enter a shell environment where all compile and run time dependencies are installed and available. Use this for active development. You can use cabal build and cabal repl to build and enter an interactive REPL in this environment, respectively.

Each of the above commands will automatically download and install all dependencies required by the project.

I've pinned the application to a specific Nixpkgs version, so anyone building with Nix should have exactly the same libraries availabile.

Using the Binary Cache

I've set up a binary cache that can be used to speed up the build time of the project. If you're on NixOS you can add the following lines to /etc/nixos/configuration.nix:

nix.trustedBinaryCaches = [ "http://nix-cache.danielwilsonthomas.com" ];
nix.binaryCachePublicKeys = [ "nix-cache.danielwilsonthomas.com-1:QM4lCC4Z8uywH15CMs0Rt+0EvuqGL1kqxBFritTwIMY=" ];

If you already have those keys set up you can add them to the existing list.

If you're just using the Nix package manager, you can add the following to your nix.conf file:

trusted-binary-caches = http://nix-cache.danielwilsonthomas.com
binary-cache-public-keys = nix-cache.danielwilsonthomas.com-1:QM4lCC4Z8uywH15CMs0Rt+0EvuqGL1kqxBFritTwIMY=

In either case when you run any of nix-build, nix-env --install, or nix-shell you should pass --option extra-binary-caches http://nix-cache.danielwilsonthomas.com

Cabal

You can also build the project using the traditional Haskell cabal configure && cabal build, using cabal install to make the package available in your executable paths. However this will not make sure any runtime dependencies are installed.

Deployment

Before deploying, you'll want to follow the directions in env.nix.sample. Talk to the project maintainer about appropriate values if necessary.

I've set up the project to be easily deployable using NixOps. You can deploy and manipulate the application in a local VirtualBox VM with the following set of commands.

• nixops create network.nix infrastructure-vbox.nix --name modernator - Create the deployment.
• nixops deploy -d modernator - Perform the provisioning and build the instance
• nixops destroy -d modernator - Destroy the instance
• nixops delete -d modernator - Delete the deployment

Again, for NixOS users you might need a more recent set of Nix package definitions. When creating the deployment, do nixops create -I nixpkgs=/path/to/your/pkgs network.nix infrastructure-vbox.nix --name modernator.

You can deploy to an EC2 instance by first running nixops create network.nix infrastructure-ec2.nix --name modernator-ec2 and then repeating the deploy command.

Prerequisites for EC2 deployment

1. You'll require a set of AWS credentials. Talk to the project maintainer about getting those.
2. You'll want to follow the directions in ec2-config.nix.sample. Talk to the project maintainer about the appropriate values if necessary.

Useful Commands

• cabal repl - Launch a GHCi session with all source files loaded
• cabal run - Build (if necessary) and run the application
• cabal test - Build (if necessary) and run the test suite

Environment Variables

• MODERNATOR_PORT - Specify the port to bind to, defaults to 8080
• MODERNATOR_STATE_DIR - Specify the location of the acid state directory, defaults to current working directory
• MODERRNATOR_KEY_DIR - Specify the location of cookie encryption keys, defaults to current working directory
• MODERNATOR_CORS - Specify any extra CORS domains, space separated, defaults to empty string

Dependencies

• GHC 7.10.2

Helpful Links

• Servant Documentation
• AcidState Documentatoin