Exercise 4
Task 1
❖ Distributed System
➢ A distributed system is a collection of autonomous computing elements that appears to its
users as a single coherent system.
➢ A distributed system is a collection of computing elements each being able to behave
independently of the other. A computing element, which we will generally refer to as a
node, can be either a hardware device or a software process.
➢ A second feature is that users (be they people or applications) believe they are dealing with
a single system.
➢ This means that one way or another the autonomous nodes need to collaborate.
Establishing this collaboration lies at the heart of developing distributed systems.
➢ Nodes are programmed to achieve common goals, which are realized by exchanging
messages with each other.
➢ A node reacts to incoming messages, which are then processed and, in turn, lead to further
communication through message passing.
➢ An important observation is that, because of dealing with independent nodes, each one will
have its notion of time. We cannot always assume that there is something like a global clock.
➢ Practice shows that a distributed system is often organized as an overlay network [Tarkoma,
2010].
➢ A node is typically a software process equipped with a list of other processes it can directly
send messages to. It may also be the case that a neighbour needs to be first looked up to.
Message passing is then done through TCP/IP or UDP channels.
➢ Each node contains a list of the node to whom to connect directly
➢ As mentioned, a distributed system should appear as a single coherent system. Achieving a
single-system view is often asking too much, for which reason, in our definition of a
distributed system, we have opted for something weaker, namely that it appears to be
coherent.
➢ Offering a single coherent view is often challenging enough.
➢ Data is stored should be of no concern, and neither should it matter that the system may be
replicating data to enhance performance. This so-called distribution transparency.
➢ Unix-like operating systems in which resources are accessed through a unifying file-system
interface, effectively hiding the differences between files, storage devices, and main
memory, but also networks.
➢ To assist the development of distributed applications, distributed systems are often
organized to have a separate layer of software that is logically placed on top of the
respective operating systems of the computers that are part of the system.
➢ Each application is offered the same interface. The distributed system provides the means
for components of a single distributed application to communicate with each other, but also
to let different applications communicate.
➢ In a sense, middleware is the same to a distributed system as what an operating system is to
a computer: a manager of resources offering its applications to efficiently share and deploy
those resources across a network. Next to resource management, it offers services that can
also be found in most operating systems
➢ The main difference with their operating-system equivalents is that middleware services are
offered in a networked environment.
➢ Middleware can also be viewed as a container of commonly used components and functions
that now no longer must be implemented by applications separately.
