5.9. Network services

For the user, the network is not computers, cables and hubs or even information flows, but is primarily a set of network services that allow you to view a list of computers on the network or a remote file, print a document on a foreign printer, or send an email message. It is the combination of the listed possibilities - how wide their choice is, how convenient, reliable and safe they are - it sets the look of each network for the user.

In addition to the data exchange itself, network services are called upon to solve other, more specific, tasks, in particular, those generated by distributed data processing. These are tasks aimed at ensuring the consistency of several copies of data hosted on different machines (replication service), or organizing the execution of a single task simultaneously on several machines on the network (the remote procedure call service). It is possible to distinguish administrative services from network services, that is, those oriented not at a simple user, but at the administrator and intended for the organization of the correct operation of the network as a whole. These include: the user account administration service, which allows the administrator to maintain a common database of network users; network monitoring system, whose functions include capturing and analyzing network traffic; a security service that, among other things, performs log-in procedures with a subsequent password check, etc.

The work of network services is done by software. The main services are the file and print services, which are usually provided by the network OS, and the secondary services are the database, fax, or voice services, performed by system network applications or utilities that work closely with the network OS. The distribution of services between the OS and utilities is quite arbitrary and varies in specific implementations of this system.

When developing network services, it is necessary to solve problems inherent in any distributed applications, including the definition of the interaction protocol between the client and server parts, the distribution of functions between them, the choice of the application addressing scheme, etc.

One of the main indicators of the quality of the network service is its convenience. For the same resource, you can develop several services that solve one and the same task in different ways. The main problems are the performance or level of convenience of the services provided. For example, a file service may be based on the use of the command to transfer a file from one computer to another by the file name, and for this it is necessary that the user knows the name of the desired file. The same file service can be organized in such a way that the user mounts the remote file system to a local directory, and then accesses the remote files as his own, which is much more convenient. The quality of the network service is determined by the quality of the user interface - intuitive clarity, clarity, rationality.

In the case of determining the degree of convenience of a shared resource, the term “transparency” is often used. Transparent is such access in which the user does not notice where the necessary resource is located - on his computer or on a remote one. After the remote file system is mounted in its directory tree, access to remote files becomes completely transparent to it. The mount operation itself may also have varying degrees of transparency. In networks with less transparency, the user needs to know and specify in the command the name of the computer storing the remote file system, in networks with a greater degree of transparency, the corresponding network software component searches for shared file volumes regardless of their storage locations, and then shows them to the user in a convenient way. , for example in the form of a list or set of icons.

To achieve transparency, the way of addressing (naming) shared network resources is important. The names of such resources should not depend on their physical location on one computer or another. At best, the user should not change anything in his work if the network administrator has moved a volume or directory between computers. The administrator and the network operating system have information about the location of file systems, but it is hidden from the user. This degree of transparency is rarely found in networks. Most often, in order to gain access to the resources of a particular computer, a logical connection should be established with it. This approach is used, in particular, in Windows NT networks.