Our application is not a standalone Microservice but a combination of multiple Microservices, which are supposed to work together to make sure the end user gets the results for which the application is created. With the end-to-end test, we try to think from the end user's perspective and make sure the application works as a whole. This is the most tricky and time-consuming test.

In a web application, the end-to-end testing would mean that all features, services, database, queues, and external dependencies in the application work fine. So, if we have a simple form to be submitted through an interface, and we have a Microservice or a bunch of Microservices to receive, validate, and manipulate the data provided, end-to-end testing would mean making sure that everything is working fine as a whole system.

At times, we will focus on negative testing. Say one of many services is down, how will it impact our application as a whole? Will our application handle such a scenario gracefully, or will it bring our whole application down?

At times, we will rely upon manual testing to make sure our application works from end to end. But again, there are tools that help us test an application from end to end. A popular one is Selenium (https://www.seleniumhq.org). Tools such as Selenium help us automate the browser testing, that is, we can define the user flow right from entering the URL into the browser, fill in the forms, click the submit buttons, and validate resultant responses. Selenium has gained a lot of popularity recently because of it's open source and has strong community support. You can also go for licensed solutions, such as Unified Functional Testing (which used to be popularly known as QTP).

The bottom line is that you can go for an open source or licensed solution, or even for manual end-to-end testing, but it is very important to make sure that we look at the solution from an end user's perspective.