Introduction

The big idea is “messaging” <..> The key in making great and growable systems is much more to design how its modules communicate rather than what their internal properties and behaviors should be. Alan Kay, creator of Smalltalk¹

What is distributed?

Distributed system is any system in which a number of independent interconnected computers can cooperate in order to achieve a common goal. Oxford Dictionary of Computer Science

If we have two virtual machines (VM) with a web server on the first VM and a database server on the second one, do we consider such a system distributed? If VMs are on the same physical node, is it a distributed system? If the web server and the database are running in separate containers and these containers are on different physical nodes, then this system is distributed. But a container orchestrator then moves both containers on the same node. Does the system of two connected containers lose its distributed flavor?

So there are many questions like above and for the purpose of the book we will follow the next definition of a distributed system.

Distributed system is a software which has at least two components interacting with each other only by a network.

If not otherwise specified, we will call components nodes and a group of connected nodes is named cluster.

Even if a network used for communication of the components for the present moment is a loopback interface (“localhost”), it can become a real network for the next deployment.

Why we need make our software distributed?

At least, to make it highly available at the cost of an increased complexity and managing effort.

Overview of the book

TODO