What is the best way to measure software reuse?   We are developing a domain API catalog that our application teams can use (so they don't have to reinvent the wheel several times over). I would like to develop 2-3 KPIs that accurately measure the value of this initiative, but I am not sure how to best measure API reusability itself.  

Nice idea! Maybe you can track the number of requests made to these APIs, the number of hours your Engineering team has done to these APIs vs the ones they used to created new APIs, the number of tasks that went to technical debt, and the number of hours of refactor you’ll do to these API catalog.

Developing a centralised catalog for APIs and encouraging developers to utilise them is an excellent way to promote best practice and efficiency within your team.

I can suggest a few KPIs that I have used myself;

a) Reduction in Development Time -> using APIs in a structured way via a central catalog should speed up your development lifecycle. I would benchmark the outputs and development time prior to implementing your API catalog to see if there is a marked improvement.

b) API Adoption Rate -> a central catalog should increase user awareness and with access to accurate and centralised documentation, using APIs should become easier as confidence grows within the organisation. The only point of creating your catalog is to ensure it is used and this can be measured in a number of ways but one suggestion is the number of transactions passing through each API.

c) Stability and Quality of Service (QoS) -> the APIs within the catalog must be high quality, have high availability, and have minimal failure rates. Perhaps a combination of KPIs to measure QoS would be a good idea.

d) Costs savings -> there are many areas to save money whether through a reduction in maintenance, increased availability, or faster transaction speeds. Be careful of external API vendors who charge an increased price per transaction for providing an API gateway service.

Hopefully, from these suggestions, you can come up with 2 or 3 KPIs.

Good luck and let me know which ones you decide upon and how you get on with your catalog creation!