The Basics
What is a distributed system? It's one of those things that's hard to define without first defining many other things. Here is a "cascading" definition of a distributed system:
- A program
- is the code you write.
- A process
- is what you get when you run it.
- A message
- is used to communicate between processes.
- A packet
- is a fragment of a message that might travel on a wire.
- A protocol
- is a formal description of message formats and the rules that two processes must follow in order to exchange those messages.
- A network
- is the infrastructure that links computers, workstations, terminals, servers, etc. It consists of routers which are connected by communication links.
- A component
- can be a process or any piece of hardware required to run a process, support communications between processes, store data, etc.
- A distributed system
- is an application that executes a collection of protocols to coordinate the actions of multiple processes on a network, such that all components cooperate together to perform a single or small set of related tasks.
Why build a distributed system? There are lots of advantages including the ability to connect remote users with remote resources in an open and scalable way. When we say open, we mean each component is continually open to interaction with other components. When we say scalable, we mean the system can easily be altered to accommodate changes in the number of users, resources and computing entities.
Thus, a distributed system can be much larger and more powerful given the combined capabilities of the distributed components, than combinations of stand-alone systems. But it's not easy - for a distributed system to be useful, it must be reliable. This is a difficult goal to achieve because of the complexity of the interactions between simultaneously running components.
