$29
Read one the following papers:
Leslie Lamport, “The Part-Time Parliament”, ACM Transactions on Computer Systems (TOCS), 16(2), pp. 133-169, May 1998.
Diego Ongaro, John Ousterhout, “In search of an understandable consensus algorithm”, USENIX Annual Technical Conference, 305-320, 2014.
Briefly summarize:
What Paxos/Raft algorithm is
How it works (at a high level)
What real-world problems can be solved using it