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Introduction to distributed systems. Mullender Based on a Lecture by Michael D. Schroeder The first four decades of computer technology are each characterized by a different approach to the way computers were used. In the s, programmers would reserve time on the computer and have the computer all to themselves while they were using it In the s, batch processing came about People would submit their jobs which were queued for processing.

They would be run one at a time and the owners would pick up their output later. Time-sharing became the way people used computers in the s so that users could share a computer under the illusion that they had it to themselves. The s are the decade of personal computing: people have their own dedicated machine on their desks.

The evolution that took place in operating systems has been made possible by a combination of economic considerations and technological developments. Batch systems could be developed because computer memories became large enough to hold an operating system as well as an application program; they were developed because they made it possible to make better use of expensive computer cycles. Time-sharing systems were desirable because they allow programmers to be more productive.

They could be developed because computer cycles became cheaper and computers more powerful. Very large scale integration and the advent of local networks has made workstations an affordable alternative to time sharing, with the same guaranteed computer capacity at all times.

Today, a processor of sufficient power to serve most needs of a single person costs less than one tenth of a processor powerful enough to serve ten. Time sharing, in fact, is no longer always a satisfactory way to use a computer system: the arrival of bit-mapped displays with graphical interfaces demands instant visual feedback from the graphics subsystem, feedback which can only be provided by a dedicated, thus personal, processor.

The workstations of the s will be even more powerful than those today. The text of this paper was taken from Distributed Systems, SJ. Mullender ed. Network interfaces will allow communication at rates matching the requirements of several channels of real-time video transmission.

A time-sharing system provides the users with a single, shared environment which allows resources, such as printers, storage space, software and data to be shared. To provide users access to the services, workstations are often connected by a network and workstation operating-system software allows copying files and remote login over the network from one workstation to another. Users must know the difference between local and remote objects, and they must know at what machine remote objects are held.

In large systems with many workstations this can become a serious problem. The problem of system management is an enormous one. In the time-sharing days, the operators could back up the file system every night; the system adminis- trators could allocate the available processor cycles where they were most needed, and the systems programmers could simply install new or improved software.

In the workstation environment, however, each user must be an operator, a system administrator and a systems programmer. In a building with a hundred autonomous workstations, the operators can no longer go round making backups, and the sys- tems programmers can no longer install new software by simply putting it on the file system.

Some solutions to these problems have been attempted, but none of the current solutions are as satisfactory as the shared environment of a time-sharing system. For example, a common and popular approach consists of the addition of network-copy commands with which files can be transferred from one workstation to another, or — as a slightly better alternative — of a network file system, which allows some real sharing of files.

In all but very few solutions, however, the user is aware of the difference between local and remote operations. The real problem is that the traditional operating systems which still form the basis of today's workstation software were never designed for an environment with many processors and many file systems. For such environments a distributed operating system is required.

The s will be the decade of distributed systems. In distributed systems, the user makes no distinction between local and remote operations. Programs do not necessarily execute on the workstation where the command to run them was given. There is one file system, shared by all the users. Peripherals can be shared. Processors can be allocated dynamically where the resource is needed most. A distributed system is a system with many processing elements and many stor- age devices, connected together by a network.

Potentially, this makes a distributed 30 system more powerful than a conventional, centralized one in two ways. First, it can be more reliable, because every function is replicated several times. When one processor fails, another can take over the work. Each file can be stored on several disks, so a disk crash does not destroy any information. Second, a distributed system can do more work in the same amount of time, because many computations can be carried out in parallel.

These two properties, fault tolerance and parallelism give a distributed system the potential to be much more powerful than a traditional operating system. In distributed systems projects around the world, researchers attempt to build systems that are fault tolerant and exploit parallelism.

What does a system have to have in order to be 'distributed'? How does one recognize a distributed system? Definitions are hard to give. Lamport was once heard to say: 'A distributed system is one that stops you from getting any work done when a machine you've never even heard of crashes. The key concept here is transparency, in other words, the use of multiple processors should be invisible transparent to the user.

Another way of expressing the same idea is to say that the user views the system as a 'virtual uniprocessor', not as a collection of distinct machines. According to this definition, a multiprocessor operating system, such as versions of Unix for Encore or Sequent multiprocessors would be distributed operating systems. Even dual-processor configurations of the IBM , or CDC number crunchers of many years ago would satisfy the definition, since one cannot tell whether a program runs on the master or slave processor.

Tanenbaum and van Renesse's definition gives a necessary condition for a distributed operating system, but I believe it is not a sufficient one. A distributed operating system also must not have any single points of failure — no single pan failing should should bring the whole system down. This is not an easy condition to fulfill in practice. Just for starters, it means a distributed system should have many power supplies; if it had only one, and it 31 1 Symptoms of a distributed system failed, the whole system would stop.

If you count a fire in the computer room as a failure, it should not even be in one physical place, but it should be geographically distributed. But one can carry failure transparency too far.

If the power fails in our building, I can tolerate my system failing, provided it fails cleanly and I don't lose the work I have just been doing — one can't work very well in the dark anyway. It is dangerous to attempt an exact definition of a distributed system. Instead, Schroeder gave a list of symptoms of a distributed system. If your system has all of the symptoms listed below, it is probably a distributed system.

If it does not exhibit one or more of the symptoms, it probably isn't one. Therefore, each processing element, or node must contain at least a CPU and memory. A distributed system cannot be fault tolerant if all nodes always fail simultaneously.

In practice, this implies that the interconnections are unreliable as well. When a node fails, it is likely that messages will be lost. If this were not done, a node failure would cause some part of the system's state to be lost. To see more clearly what constitutes a distributed system, we shall look at some examples of systems.

Multiprocessor computer with shared memory A shared-memory multiprocessor has several of the characteristics of a distributed system. It has multiple processing elements, and an interconnect via shared mem- ory, interprocessor interrupt mechanisms and a memory bus. The communication between processing elements is reliable, but this does not in itself mean that a multiprocessor cannot be considered as a distributed system. What disqualifies multiprocessors is that there is no independent failure: when one processor crashes, the whole system stops working.

However, it may well be that manufacturers, in- spired by distributed systems research, will design multiprocessors that are capable 32 of coping with partial failure; to my knowledge, only Tandem manufactures such machines, currently. Ethernet with packet-filtering bridges A bridge is a processor with local memory that can send and receive packets on two Ethernet segments.

The bridges are interconnected via these segments and they share state which is necessary for routing packets over the internet formed by the bridges and the cable segments. When a bridge fails, or when one is added to or removed from the network, the other bridges detect this and modify their routing tables to take the changed circumstances into account Therefore, an Ethernet with packet-filtering bridges can be viewed as a distributed system.

Diskless workstations with NFSftle servers Each workstation and file server has a processor and memory, and a network interconnects the machines. Workstations and servers fail independendy: when a workstation crashes, the other workstations and the file servers continue to work.

When a file server crashes, its client workstations do not crash although client processes may hang until the server comes back up.

But there is no shared state: when a server crashes, the information in it is inaccessible until the server comes back up; and when a client crashes, all of its internal state is lost A network of diskless workstations using NFS file servers, therefore, is not a distributed system. People are distributed, information is distributed Distributed systems often evolve from networks of workstations. The owners of the workstations connect their systems together because of a desire to communicate and to share informations and resources.

Information generated in one place is often needed in another. This book illustrates the point: it was written by a number of authors in different countries who needed to interact by reading each other's material and discussing issues in order to achieve some coherence. The costs of both processors and of memories are generally going down.

Each year, the same money, buys a more powerful workstation. That is, in real terms, computers are getting cheaper and cheaper. The cost of communication depends on the bandwidth of the communication channel and the length of the channel. Bandwidth increases, but not beyond the limits set by the cables and interfaces used. Wide area network cables have to be used for decades, because exchanging them is extremely expensive.


Distributed Systems

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Distributed Systems (2nd edition)






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