Need for Middleware

FUNDAMENTALS OF DISTRIBUTED SYSTEMS

Motivation: Too many networked applications were hard or difficult to integrate:
_ Departments are running different NOSs
_ Integration and interoperability only at level of primitive NOS services
_ Need for federated information systems:
– Combining different databases, but providing a single view to applications
– Setting up enterprise-wide Internet services, making use of existing information systems
– Allow transactions across different databases
– Allow extensibility for future services (e.g., mobility, teleworking, collaborative applications)
_ Constraint: use the existing operating systems, and treat them as the underlying environment (they provided the basic functionality anyway)
Communication services: Abandon primitive socket based message passing in favor of:
Procedure calls across networks
Remote-object method invocation
Message-queuing systems
Advanced communication streams
_ Event notification service
Information system services: Services that help manage data in a distributed system:
Large-scale, system wide naming services
Advanced directory services (search engines)
Location services for tracking mobile objects
Persistent storage facilities

Data caching and replication
Control services: Services giving applications control over when, where, and how they access data:
Distributed transaction processing
Code migration
Security services: Services for secure processing and communication:
Authentication and authorization services
Simple encryption services
Auditing service

Networks of computers are everywhere. The Internet is one, as are the many networks of which it is composed. Mobile phone networks, corporate networks, factory networks, campus networks, home networks, in-car networks – all of these, both separately and in combination, share the essential characteristics that make them relevant subjects for study under the heading distributed systems.

Characteristics/Issues in Distributed systems:
Concurrency: In a network of computers, concurrent program execution is the norm. I can do my work on my computer while you do your work on yours, sharing resources such as web pages or files when necessary. The capacity of the system to handle shared resources can be increased by adding more resources (for example. computers) to the network. The coordination of concurrently executing programs that share resources is also an important and recurring topic.
No global clock: When programs need to cooperate they coordinate their actions by exchanging messages. Close coordination often depends on a shared idea of the time at which the programs’ actions occur. But it turns out that there are limits to the accuracy with which the computers in a network can synchronize their clocks – there is no single global notion of the correct time. This is a direct consequence of the fact that the only communication is by sending messages through a network.

Independent failures: All computer systems can fail, and it is the responsibility of system designers to plan for the consequences of possible failures. Distributed systems can fail in new ways. Faults in the network result in the isolation of the computers that are connected to it, but that doesn’t mean that they stop running. In fact, the programs on them may not be able to detect whether the network has failed or has become unusually slow. Similarly, the failure of a computer, or the unexpected termination of a program somewhere in the system (a crash), is not immediately made known to the other components with which it communicates.

Each component of the system can fail independently, leaving the others still running.

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