Explore the challenges of building distributed systems through the lens of Hazelcast, an in-memory distributed computation and storage platform. Delve into the intricacies of implementing raw Java, custom RPC and concurrency stacks, distributed primitives, replication, and Raft-based consensus protocols. Learn from real-world examples of known and unknown issues encountered in Hazelcast's development. Gain valuable insights into critical considerations for designing and maintaining robust distributed systems, drawing from the speaker's experiences and lessons learned at Hazelcast.