IT 222 (Module 1)

• A distributed system is one in which components (hardware or software) located at a networked environment communicate and coordinate their actions only by passing messages.
• Certain characteristics of distributed systems, namely:

  • Concurrency of components

  • No global clock

  • Independent failures
• In a network environment, it is normal to operate concurrently.
• Concurrency: In a network of computers, tasks are simultaneously done as well as resources are shared.
• Concurrency: The capacity of the system grows as more resources are introduced (i.e. computers) to the network.
• No global clock: When programs need to be coordinated through the exchange of messages, close coordination is dependent on the notion of time at which program’s actions occur. Unfortunately, there is no single global “correct time”.
• Independent Failure: It is a responsibility of system designers to prepare for possible failures in a computer system. When this happens in a distributed system, failure in one does not necessarily lead to the failure of the entire system.
• Pervasive networking and the modern internet: With the development of network technology over the years, services also increased as a result of this growth. To name a few, we already have such a thing as cloud computing as well as grid computing.
• Mobile computing refers to the performance of computing tasks while the user is on the move or visiting places other than their usual environment.
• The capability of these portable devices to connect to the network also paved the way for ubiquitous computing where it is defined as the harnessing of many small, cheap computational devices that are present in users’ physical environments, including the home, office and even natural settings.
• Distributed multimedia systems: Support for multimedia services have also become available thus leading to the possibility of audio and video conferences, webcasting, real-time streaming, etc.
• Client-server computing is a setup wherein a service is requested by a client to the server.
• A service is a distinct part of the computer system that manages a collection of related resources and presents their functionality to users and applications.
• The term server refers to a running program (or process) on a networked computer that accepts requests from programs running on other computers to perform a service and responds appropriately.
• Servers run continuously while clients last as long as the applications of which the form a part.
• Clients can be referred to as the initiator of requests.
• Middleware – software layer that provides a programming abstraction as well as masking the heterogeneity of the underlying networks, hardware, operating systems and programming languages.
• Mobile code – program code that can be transferred from one computer to another and run at the destination.
• The openness of distributed systems is determined primarily by the degree to which a new resource-sharing services can be added and be made available for use by a variety of client programs.
• One way of resolving openness is by providing specification and documentation of the key software components to be made available to software developers.
• With the information resources that distributed systems maintain, it is intrinsic that security will have to be in place.
• When dealing with the security of information resources, three components will have to be taken into consideration: confidentiality, integrity, and availability.
• Confidentiality is the protection against disclosure to unauthorized individuals).
• Integrity is the protection against alteration and corruption.
• Availability is the protection against interference with the means to access the resources.
• To date, there are still two security challenges that are not fully met: DOS and security of mobile code.
• DOS stands for denial of service.
• Security of mobile code is running email attachments that may have disruptive/destructive effects.
• We can say that a system is scalable when it remains to be effective even when there is a significant increase in the number of resources and users of the system.
• The challenges that will have to be taken into consideration in scalability include: controlling the cost of physical resources, controlling the performance loss, preventing software resources running out, avoiding performance bottlenecks.
• Checksums: detect corrupted data
• Masking failures: detected failures can be hidden or made less severe, i.e. retransmitting messages that failed to arrive at the destination
• Tolerating failures: Services can inform users of failed service that way the user does not need to keep trying, i.e. web browser cannot establish connection to a web server
• Recovery from failures: “roll back mechanisms”
• Redundancy: Components can be made redundant, such as providing

  multiple routes to a certain destination
• The main nonfunctional properties of systems that affect quality of service: reliability, security and performance (and adaptability).
• EAI stands for Enterprise Application Integration.
• EAI is the combination of processes, software, standards, and hardware resulting in the seamless integration of two or more enterprise systems allowing them to operate as one.
• PIM is a model that allows the integration of new software through the existing presentation of the legacy software.
• PIM is typically used to create a new user interface but may be used to integrate with other applications.