Distributed Systems — Episode 4

The fourth episode follows communication from network layers to group dissemination. OSI shows each layer's header and contract. Synchronous exchange blocks a client until the reply, asynchronous exchange separates receipt from result, and persistent communication outlives temporary unavailability.

RPC generates client and server stubs, serializes parameters, and presents a remote procedure as if it were local. The illusion breaks on timeouts and uncertain outcomes, because retrying may duplicate an effect. Sockets expose TCP or UDP more directly, while MPI organizes communication in high-performance clusters.

Message-oriented middleware separates sender from receiver with a queue. A queue manager handles addressing, storage, and routing; a broker may transform formats and protocols. RabbitMQ demonstrates the relationship between protocol and implementation, while Kafka shows why a log-based stream is not automatically a traditional queue.

The multicast section compares flooding, dissemination trees, and gossip. Flooding quickly overloads a network; probabilistic neighbor selection is cheaper but cannot guarantee delivery to every node. Push works well early in an epidemic, pull works when copies are common, and a combined scheme converges faster across both phases.