sfmc-servicecloud-customsplit | Salesforce Marketing Cloud - Custom journey builder split

Prepare deployment (gather knowledge of the URL the application will be hosted, etc.) to a server you manage or any Node.js-ready cloud service like heroku.com. (Note: this example-application is already prepared to be deployed to heroku, so this is the easiest option. Furthermore you don't have to worry about SSL-setup when using heroku.)
Before this example can be used, a package containing a Journey Builder Activity needs to be created in Salesforce Marketing Cloud App Center. A documentation for this task can be found here: Create a Marketing Cloud App. Important: The activity's "Public Extension"-setting needs to be set to "This application and other installed Applications" and the example needs to be available via HTTPS which has to be running on the default port 443.
To secure the backend and make sure only requests from your marketing cloud instance are processed, you need to create a salt key in marketing cloud. Information can be found in the documentation on Key Management and Encode with Customer Key. Follow these steps and add your salt key's ascii representation to package.json under options.salesforce.marketingCloud.jwtSecret, as well as the external key seen in Key Management to all the customerKey-properties in public/config.json. Important: According to the documentation the salt key can be converted by piping it to xxd -p and adding 0x as prefix. However in my tests on MacOS and CentOS the last two characters 0a had to be removed, so I propose to use echo "Hello world" | xxd -p | sed 's/^/0x/;s/0a$//' instead, where Hello world would be your salt.
The unique key of the created Journey Builder Activity needs to be added to public/config.json properties key and configurationArguments.applicationExtensionKey
Replace <URL OF THE SERVER RUNNING THE BACKEND> in public/config.json with the URL where your split's backend can be reached (unless deployed to heroku or a similar service, you need to make sure this express app is eihter altered to run using https on port 443 or proxy it on your own (e.g. using nginx)).
Adapt the outcomes of the split in public/config.json according to your needs.
Add your Service Cloud credentials to package.json under options.salesforce.serviceCloud.username and options.salesforce.serviceCloud.password.
Adapt backed/lib/sfdc.js and backend/server.js so the desired object and field from Service Cloud is read and the outcome is returned accordingly.
Change the module name of the backend in package.json
Optional: Replace the icons for the custom activity in public/images.
Deploy the application to the service you selected in step 1.
Make sure the server hosting the backend is allowed to access to your Service Cloud instance (Setup -> Administration Setup -> Security Controls -> Network Access). Otherwise you will get an error as soon as the backend tries to login.