Database System Architectures:Network Types

Network Types

Distributed databases and client – server systems are built around communication networks. There are basically two types of networks: local-area networks and wide- area networks. The main difference between the two is the way in which they are distributed geographically. In local-area networks, processors are distributed over small geographical areas, such as a single building or a number of adjacent buildings. In wide-area networks, on the other hand, a number of autonomous processors are distributed over a large geographical area (such as the United States or the en- tire world). These differences imply major variations in the speed and reliability of the communication network, and are reflected in the distributed operating-system design.

Local-Area Networks

Local-area networks (LANs) (Figure 18.10) emerged in the early 1970s as a way for computers to communicate and to share data with one another. People recognized that, for many enterprises, numerous small computers, each with its own self-

contained applications, are more economical than a single large system. Because each small computer is likely to need access to a full complement of peripheral devices (such as disks and printers), and because some form of data sharing is likely to oc- cur in a single enterprise, it was a natural step to connect these small systems into a network.

LANs are generally used in an office environment. All the sites in such systems are close to one another, so the communication links tend to have a higher speed and lower error rate than do their counterparts in wide-area networks. The most common links in a local-area network are twisted pair, coaxial cable, fiber optics, and, increasingly, wireless connections. Communication speeds range from a few megabits per second (for wireless local-area networks), to 1 gigabit per second for Gigabit Ether- net. Standard Ethernet runs at 10 megabits per second, while Fast Ethernet run at 100 megabits per second.

A storage-area network (SAN) is a special type of high-speed local-area network designed to connect large banks of storage devices (disks) to computers that use the data. Thus storage-area networks help build large-scale shared-disk systems. The motivation for using storage-area networks to connect multiple computers to large banks of storage devices is essentially the same as that for shared-disk databases, namely

• Scalability by adding more computers

• High availability, since data is still accessible even if a computer fails

RAID organizations are used in the storage devices to ensure high availability of the data, permitting processing to continue even if individual disks fail. Storage area networks are usually built with redundancy, such as multiple paths between nodes, so if a component such as a link or a connection to the network fails, the network continues to function.

Wide-area networks (WANs) emerged in the late 1960s, mainly as an academic re- search project to provide efficient communication among sites, allowing hardware and software to be shared conveniently and economically by a wide community of users. Systems that allowed remote terminals to be connected to a central computer via telephone lines were developed in the early 1960s, but they were not true WANs. The first WAN to be designed and developed was the Arpanet. Work on the Arpanet began in 1968. The Arpanet has grown from a four-site experimental network to a worldwide network of networks, the Internet, comprising hundreds of millions of computer systems. Typical links on the Internet are fiber-optic lines and, sometimes, satellite channels. Data rates for wide-area links typically range from a few megabits per second to hundreds of gigabits per second. The last link, to end user sites, is of- ten based on digital subscriber loop (DSL) technology supporting a few megabits per second), or cable modem (supporting 10 megabits per second), or dial-up modem connections over phone lines (supporting up to 56 kilobits per second).

WANs can be classified into two types:

• In discontinuous connection WANs, such as those based on wireless connections, hosts are connected to the network only part of the time.

• In continuous connection WANs, such as the wired Internet, hosts are connected to the network at all times.

Networks that are not continuously connected typically do not allow transactions across sites, but may keep local copies of remote data, and refresh the copies periodically (every night, for instance). For applications where consistency is not critical, such as sharing of documents, groupware systems such as Lotus Notes allow up- dates of remote data to be made locally, and the updates are then propagated back to the remote site periodically. There is a potential for conflicting updates at differ- ent sites, conflicts that have to be detected and resolved. A mechanism for detecting conflicting updates is described later, in Section 23.5.4; the resolution mechanism for conflicting updates is, however, application dependent.