Use Case - Contact import/consolidation from multiple external databases

As a freelancer, here are a few notes and an overview on how I solved a client's contacts consolidation when the data came from external sources:

My Use Case
- Client had ongoing events and conferences over several years which meant there were databases and mailing lists all over the place (many of which had crossover).
- I set up ongoing JSON exports of the contact data from the external sites, which could be queried by date range to get the latest changes.