Know more about ADAPTABILITY AND SCALABILITY OF IT SYSTEMS and Clustering for load distribution, Web architecture

Know more about  ADAPTABILITY AND SCALABILITY OF IT SYSTEMS and Clustering for load distribution, Web architecture

A further requirement of IT systems is that of adaptability and scalability: successful companies have to adapt their business processes to new market conditions in ever shorter cycles. Along the same lines, IT systems must be adapted to new business processes so that they can provide optimal support for these processes. Storage networks are also required to be scalable: on average the storage capacity required by a company doubles in the course of each year. This means that anyone who has 1 terabyte of data to manage today will have 32 terabytes in five years time. A company with only 250 gigabytes today

will reach 32 terabytes in seven years time. This section discusses the adaptability and scalability of IT systems on the basis of clusters for load distribution (Section 6.4.1), the five-tier architecture for web application servers (Section 6.4.2) and the case study 'structure of a travel portal' (Section 6.4.3).

 Clustering for load distribution

The term 'cluster' is very frequently used in information technology, but which is not clearly defined. The meaning of the term 'cluster' varies greatly depending upon context. As the greatest common denominator we can only state that a cluster is a combination of components or servers that perform a common function in one form or another. This section expands the cluster concept for protection against the failure of a server introduced in Section 6.3.2 to include clustering for load distribution. We discuss three different forms of clusters based upon the example of a file server. The three different forms of cluster are comparable to the modes of multipathing software. The starting point is the so-called shared-null configuration (Figure 6.24). The components are not designed with built-in redundancy. If a server fails, the file system itself is no longer available, even if the data is mirrored on two different disk subsystems and

redundant I/O buses are installed between server and disk subsystems (Figure 6.25) In contrast to the shared-null configuration, shared-nothing clusters protect against the failure of a server. The basic form of the shared-nothing cluster was discussed in Section 6.3.2 in relation to the protection of a file server against the failure of a server. Figure 6.26 once again shows two shared-nothing clusters each with two servers. Shared-nothing clusters can be differentiated into active/active and active/passive con- figurations. In the active/active configuration, applications run on both computers; for example, the computers 'server 1' and 'server 2' in Figure 6.26 each export a file sys- tem. If one of the two computers fails, the other computer takes over the tasks of the failed computer in addition to its own (Figure 6.27, top). This taking over of the applications of the failed server can lead to performance bottlenecks in active/active configurations. The active/passive configuration can help in this situation. In this approach the application runs only on the primary server, the second computer in the cluster (stand-by server) does nothing in normal operation. It is exclusively there to take over the applications of the primary server if this fails. If the primary server fails, the stand-by server takes over its tasks (Figure 6.27, bottom). The examples in Figures 6.26 and 6.27 show that shared-nothing clusters with only two servers are relatively inflexible. More flexibility is offered by shared-nothing clusters with more than two servers, so-called enhanced shared-nothing clusters. Current shared-nothing cluster software supports shared-nothing clusters with several dozens of computers. Figures 6.28 and 6.29 show the use of an enhanced shared-nothing cluster for static load distribution: during the daytime when the system is busy, three different servers each export two file systems (Figure 6.28). At night, access to the data is still needed; however, a single server can manage the load for the six file systems (Figure 6.28). The two otherservers are freed up for other tasks in this period (data mining, batch processes, back-up, maintenance). One disadvantage of the enhanced shared-nothing cluster is that it can only react to load peaks very slowly. Appropriate load balancing software can, for example, move the file system '/fs2' to one of the other two servers even during the day if the load on the file system '/fs1' is higher. However, this takes some time, which means that this process

is only worthwhile for extended load peaks. A so-called shared-everything cluster offers more flexibility in comparison to enhanced shared-nothing clusters. For file servers, shared disk file systems are used as local file systems here, so that all servers can access the data efficiently over the storage network. Figure 6.30 shows a file server that is configured as a shared-everything cluster with three servers. The shared disk file system is distributed over several disk subsystems. All three servers export this file system to the clients in the LAN over the same virtual IP address by means of a conventional network file system such as NFS or CIFS. Suitable load balancing software distributes new incoming accesses on the network file system equally amongst all three servers. If the three servers are not powerful enough, a fourth server can simply be linked to the cluster. The shared-everything cluster also offers advantages in the event of the failure of a

single server. For example, the file server in Figure 6.30 is realized in the form of a distributed application. If one server fails, as in Figure 6.31, recovery measures are only necessary for those clients that have just been served by the failed computer. Likewise, recovery measures are necessary for the parts of the shared disk file system and the network file system have just been managed by the failed computer. None of the other clients of the file server notice the failure of a computer apart from a possible reduction  in performance. Despite their advantages, shared-everything clusters are very seldom used. The reason

for this is quite simply that this form of cluster is the most difficult to realize, so most cluster products and applications only support the more simply realized variants of shared- nothing or enhanced shared-nothing.

 Web architecture

In the 1990s the so-called three-tier architecture established itself as a flexible architecture for IT systems (Figure 6.32). The three-tier architecture isolates the tasks of data management, applications and representation into three separate layers. Figure 6.33 shows  a possible implementation of the three-tier architecture. Individually the three layers have the following tasks:

• Data Information in the form of data forms the basis for the three-tier architecture. Databases and file systems store the data of the applications on block-oriented disks or disk subsystems. In addition, the data layer can provide interfaces to external systems and legacy applications.

• Applications Applications generate and process data. Several applications can work on the same databases or file systems. Depending upon changes to the business processes, existing applications are modified and new applications added. The separation of applications and databases makes it possible for no changes, or only minimal changes, to have to be made to the underlying databases or file systems in the event of changes to applications.

• Representation The representation layer provides the user interface for the end user. In the 1990s the user interface was normally realized in the form of the graphical interface on a PC.

The corresponding function calls of the application are integrated into the graphical interface so that the application can be controlled from there. Currently, the two outer layers can be broken down into sublayers so that the three-tier architecture is further developed into a five-tier architecture Figure 6.34 and Figure 5: • Splitting of the representation layer

In recent years the representation layer has been split up by the World Wide Web into web servers and web browsers. The web servers provide statically or dynamically generated websites that are represented in the browsers. Websites with a functional scope comparable to that of conventional user interfaces can currently be generated using Java and various script languages. The arrival of mobile end devices such as mobile phones and PDAs has meant that

web servers had to make huge modifications to websites to bring them into line with the properties of the end devices. In future there will be user interfaces that are exclusively controlled by means of the spoken word – for example navigation systems for use in the car, that are connected to the Internet for requesting up-to-date traffic data. • Splitting of the data layer In the 1990s, storage devices for data were closely coupled to the data servers (storage centric IT architecture). In the previous chapters storage networks were discussed in detail, so at this point of the book it should be no surprise to learn that the data layer

is split into the organization of the data (databases, file servers) and the storage space for data (disk subsystems).