# Distributed system | Notes (Part 1) Def : A distributed system consists of **hardware** and **software** components located in a network of computers that communicate and coordinate their actions only by passing messages. ### Consequences of distributed systems * Concurrency * 多台電腦同時工作 * 網路延遲不是不變的所以同步化困難(Network delays are not constant make synchronization difficult.) * No Global Clock * When programs need to cooperate they coordinate their actions by exchanging messages. * Close coordination often depends on a shared idea of the time at which events occur. * There are limits to the accuracy with which components in a network can synchronize their clocks. * Direct consequence of the fact that messages are sent through a network and network delays are not constant(or even bounded) * Independent failures * All computer systems can fail it is good design to build robustness in. * Distributed systems fail in new ways. * Network faults might result in components being isolated but not stopping.