Micro service architectures gain a lot of mindshare recently for good reasons. Let's assume you did the hard work and you have switched your system to use micro services. What are the benefits to an agile team? What would an agile micro service looks like?
The best thing about a micro service is that it is micro. It has a very well scoped functionality and it's easy to wrap your head around what it's doing. That means it is very easy to deliver significant features or improvements to a micro service within a single sprint. It also means that it is easy for a new engineer that has never worked on the micro service to pair with another engineer and quickly get up to speed.
The interactions with other services are through well-defined interfaces, so it is easy to use dependency injection to mock them and create automated unit tests. The narrow scope of the micro service helps to also to ensure that the test coverage is adequate. Refactoring anything inside the micro service is very simple because there is no need to coordinate with other services. It also aids in scaling a system incrementally.
At some point, every assumption you made when you designed the smaller system will come back to bite you when you scale — that data fits in memory, the data fits on the hard disk, the data can be stored reliably in one data center, the server can return all the data without timing out, etc. One approach is to try to think about all these eventualities and engineer them into the system from the beginning. This is typically a very bad decision. Your system might not scale to the level of engineering you built in for years, the time to market will be significantly higher if you try to build infinitely scalable system from the start, operations and maintenance will be more difficult. But, the worse thing is that when your system actually scales to the levels that require such engineering effort you'll discover that your well-crafted super-engineering from three years ago completely missed the mark and you have to start from scratch.
Micro services can help with that too. You can often isolate scalability, fault tolerance and high-availability into dedicated micro services that you build later and weave with your existing business logic micro services. The loosely coupled nature of micro services lends itself very well to agile evolution of the system.
Unit Testing, scalability, Micro Server, agile development