Ask HN: Practical/open-source distributed systems expertise

I work at a enterprise software company that has built scale-out storage system employing distributed system concepts like two-phase commit, load balancing, dealing with node failures etc. All of this is being developed in-house in C++ on Linux system and not using off-the-shelf open source projects. I like my job and people at the company. However I see that there has been lot of buzz about open-source distributed systems like Apache Zookeeper, Kafka, Samza, Spark etc. and most companies these days seem to list them in their job requirements. Reading about these open-source projects gives me high-level overview but not good understanding on how they are being used. Are there online courses with hands on development that'll help me gain better understanding of these systems? What are other ways to gain expertise?