What do we mean by distributed systems?

What are the characteristics of Distributed Systems?

These characteristics are taken from the book Distributed Systems - Concepts and Design:

• Networks - they are all networked one way or another
• Communication - often message passing
• Resource sharing - files, data, processors
• Openness - UNIX, Java, CORBA
• Concurrency - components of the system may execute simultaneously
• Scalability - increasing the size of a system should have no effect other than an increase in performance or capacity.
• High Availability and fault tolerance - acheived through the natural redundancy  of multiple replicated components.
• Transparency - so that the DS is perceived as a whole rather than a collection of individual components by users and programs.
  

• Access
• Location
• Concurrency
• Replication
• Failure
• Migration
• Performance
• Scaling

An Example

An Air Ticket Booking System (Hypothetical)

The requirements of such a system includes

a high level of reliability,
security against external interference and
privacy of the information that the system holds.

The main problems
Taken from the same source.

• Naming
  We need some means of referring to objects uniquely, wherever they are in our system. In the worst case the system could be the entire InterNet.
• Communication
  We need a  high speed communication infrastructure which supports the high level softwrae structures we use.
• Software Structure - for us this will almost always be object based, though we will look at some non object based systems - NFS, RPC and Andrew
• Load Balancing - we need to ensure that the processing capability of our system is not overloaded and as fully utilised as possible.as it workload changes over time.
• Maintaining Consistency
  As soon as we introduce concurrency we need tostart worrying about consistency - ultimately we will need some form of transaction based system to ensure consistency.

We will not be looking in depth at distributed (microkernel) operating systems although they can form the basis for distributed applications. The following links may be of interest :

In my opinion the near future will see a lot of work based on distribution through replicated VM's on heterogeneous hardware systems.
 

The Course Structure
Initially we will be looking at some of the infrastructure needed for distributed systems in particular Remote Procedure Call (RPC) and its object oriented relative Remote Method Invocation (RMI).
We will also look briefly at communication between processes mainly using BSD sockets.
With this infrastructure we will look at the provision of distributed services e.g. Sun's Network File system (NFS).
A major component of the course will be an investigation of Jini as a distributed application infrastructure and the use of this to support a high level framework such as JavaSpaces. These examples will allow us to look at the use of transactions and at design patterns or distributed systems.
Other distribution mechanisms (CORBA and DCOM) will be considered in the light of the facilities provided by Jini.
Finally we will discuss security issues and the provision of distributed database systems.