Distributed systems

Introduction

A distributed system is a collection of computers that present themselves to the user as a single, coherent system. The components of the system are spread over multiple computers that communicate over a network. The benefit of distributed systems is that they allow us to achieve levels of reliability and performance that are unobtainable on a single machine. The problem is that it is very difficult to design and run distributed systems correctly. They introduce whole new classes of sometimes very subtle bugs that can make it hard to reason about how the system operates. Leslie Lamport, a computer scientist renowned for his work in distributed computing, wryly describes them thus: “A distributed system is one in which the failure of a computer you didn’t even know existed can render your own computer unusable.”

By their very nature, distributed systems force us to think about the physical limitations of time and space. Latency is the time it takes for a result to become apparent to an observer. When you click a hyperlink, it takes some time for your request to reach the server. During that period of latency, the request has been initiated but it is not yet visible to the server. Communication, even at the speed of light, can never be completely instantaneous and so some latency is inevitable. We know from the architecture chapter that latency within a single machine needs to be handled carefully but for the most part we can leave it to the computer architects to worry about. In a distributed system, spread over multiple machines possibly at great physical distances from each other, the effects of latency are too obvious to ignore. Much of the challenge of distributed systems comes from understanding and managing the effects of latency. As we’ll see, the more a distributed system tries to behave like a single-node system, the greater the latency cost.

Distributed systems are increasingly common and some familiarity with them is essential for web developers. Simply put, all web work involves distributed systems to some degree. A web application running in the client’s browser and communicating with a back end service forms a distributed system. Front end developers need to understand the problems latency introduces and how to mitigate them. The back end service itself is probably made up of at least an application server and a separate database server. Even that simple design is a distributed system. A production-ready architecture will probably