In modern applications, services communicate often through APIs: application programming interfaces. Instead of going at an higher level to test the communication, you can directly explore the API. It will be more accurate and faster. There are some resources about API testing strategies.

• 2015 – API, Web Services & Mibycroservices Testing Pathway – Katrina Clokie
• 2016 – Getting Started With API Testing For Fun & Profit – Dennis Gurock
• 2016 – Exploratory Testing an API – Maaret Pyhäjärvi
• 2018 – API Testing is Exploratory Testing – offbeattesting

• 2017 – Methods and Tools for Data-Driven API Testing – Albert Gareev

• 2018 –What API Tests to Automate, and When to Automate Them – Kristin Jackvony

Resources about Postman:

• 2018 – An introduction to Postman API testing – Viv Richards
• 2018 – Adding Postman Assertions – Kristin Jackvony
• 2018 – Using variables in Postman – Kristin Jackvony
• 2018 – Organizing Your API Tests with Postman – Kristin Jackvony
• 2018 – Automating Your API Tests with Newman – Kristin Jackvony

• 2017 – How to use Postman with Newman in your CI – MelTheTester

• 2017 – Postman cheat sheet – Quick guide to automating APIs – Mark Winteringham
• All-Things-Postman – Danny Dainton

Some other API testing tools:

• 2017 – Get rid of your external dependencies with WireMock – Slawomir Radzyminski

• 2017 – API Testing Tools: Restlet and Stoplight