Directi is one of the largest domain registrars on the internet with their services such as ResellerClub, and they provide wonderfully complete to-go solutions for resellers. But there are times when you would want to build upon their platforms rather than just use their entire solution. For this, they provide you the Directi API. You can build upon these as you wish to build out customized solutions of your own.
One of the first steps in registering a domain is checking the availability of the domain name you are interested in. DirectI API provides the checkAvailability function for this.