8 CHAPTER 1 CHARACTERIZATION OF DIS

8 CHAPTER 1 CHARACTERIZATION OF DISTRIBUTED SYSTEMS

This script is based on the functionality provided by Apama [www.progress.com], a
commercial product in the financial world originally developed out of research carried out at the University of Cambridge. The script detects a complex temporal sequence based on the share prices of Microsoft, HP and a basket of other share prices, resulting in decisions to buy or sell particular shares.
This style of technology is increasingly being used in other areas of financial
systems including the monitoring of trading activity to manage risk (in particular,
tracking exposure), to ensure compliance with regulations and to monitor for patterns of
activity that might indicate fraudulent transactions. In such systems, events are typically
intercepted and passed through what is equivalent to a compliance and risk firewall
before being processed (see also the discussion of firewalls in Section 1.3.1 below).

1.3 Trends in distributed systems

Distributed systems are undergoing a period of significant change and this can be traced back to a number of influential trends:
• the emergence of pervasive networking technology;
• the emergence of ubiquitous computing coupled with the desire to support user
mobility in distributed systems;
• the increasing demand for multimedia services;
• the view of distributed systems as a utility.

1.3.1 Pervasive networking and the modern Internet
The modern Internet is a vast interconnected collection of computer networks of many
different types, with the range of types increasing all the time and now including, for
example, a wide range of wireless communication technologies such as WiFi, WiMAX,
Bluetooth (see Chapter 3) and third-generation mobile phone networks. The net result is
that networking has become a pervasive resource and devices can be connected (if
desired) at any time and in any place.
Figure 1.3 illustrates a typical portion of the Internet. Programs running on the computers connected to it interact by passing messages, employing a common means of communication. The design and construction of the Internet communication mechanisms (the Internet protocols) is a major technical achievement, enabling a program running anywhere to address messages to programs anywhere else and abstracting over the myriad of technologies mentioned above.
The Internet is also a very large distributed system. It enables users, wherever they
are, to make use of services such as the World Wide Web, email and file transfer.
(Indeed, the Web is sometimes incorrectly equated with the Internet.) The set of services
is open-ended - it can be extended by the addition of server computers and new types of
service. The figure shows a collection of intranets - subnetworks operated by companies
and other organizations and typically protected by firewalls. The role of a firewall is to
protect an intranet by preventing unauthorized messages from leaving or entering. A

SECTION 1.3 TRENDS IN DISTRIBUTED SYSTEMS 9

Figure 1.3 A typical portion of the Internet




intranet
ISP








desktop computer:
server:
network link:





firewall is implemented by filtering incoming and outgoing messages. Filtering might
be done by source or destination, or a firewall might allow only those messages related
to email and web access to pass into or out of the intranet that it protects. Internet Service
Providers (ISPs) are companies that provide broadband links and other types of
connection to individual users and small organizations, enabling them to access services
anywhere in the Internet as well as providing local services such as email and web
hosting. The intranets are linked together by backbones. A backbone is a network link
with a high transmission capacity, employing satellite connections, fibre optic cables
and other high-bandwidth circuits.
Note that some organizations may not wish to connect their internal networks to the Internet at all. For example, police and other security and law enforcement agencies are likely to have at least some internal intranets that are isolated from the outside world (the most effective firewall possible - the absence of any physical connections to the Internet). Firewalls can also be problematic in distributed systems by impeding legitimate access to services when resource sharing between internal and external users is required. Hence, firewalls must often be complemented by more fine-grained mechanisms and policies, as discussed in Chapter 11.
The implementation of the Internet and the services that it supports has entailed the development of practical solutions to many distributed system issues (including most of those defined in Section 1.5). We shall highlight those solutions throughout the book, pointing out their scope and their limitations where appropriate.

10 CHAPTER 1 CHARACTERIZATION OF DISTRIBUTED SYSTEMS

1.3.2 Mobile and ubiquitous computing
Technological advances in device miniaturization and wireless networking have led increasingly to the integration of small and portable computing devices into distributed systems. These devices include:
• Laptop computers.
• Handheld devices, including mobile phones, smart phones, GPS-enabled devices,
pagers, personal digital assistants (PDAs), video cameras and digital cameras.
• Wearable devices, such as smart watches with functionality similar to a PDA.
• Devices embedded in appliances such as washing machines, hi-fi systems, cars
and refrigerators.
The portability of many of these devices, together with their ability to connect
conveniently to networks in different places, makes mobile computing possible. Mobile
computing is the performance of computing tasks while the user is on the move, or
visiting places other than their usual environment. In mobile computing, users who are
away from their ‘home’ intranet (the intranet at work, or their residence) are still
provided with access to resources via the devices they carry with them. They can
continue to access the Internet; they can continue to access resources in their home
intranet; and there is increasing provision for users to utilize resources such as printers
or even sales points that are conveniently nearby as they move around. The latter is also
known as location-aware or context-aware computing. Mobility introduces a number of
challenges for distributed systems, including the need to deal with variable connectivity
and indeed disconnection, and the need to maintain operation in the face of device
mobility (see the discussion on mobility transparency in Section 1.5.7).
Ubiquitous computing is the harnessing of many small, cheap computational
devices that are present in users’ physical environments, including the home, office and
even natural settings. The term ‘ubiquitous’ is intended to suggest that small computing
devices will eventually become so pervasive in everyday objects that they are scarcely
noticed. That is, their computational behaviour will be transparently and intimately tied
up with their physical function.
The presence of computers everywhere only becomes useful when they can communicate with one another. For example, it may be convenient for users to control their washing machine or their entertainment system from their phone or a ‘universal remote control’ device in the home. Equally, the washing machine could notify the user via a smart badge or phone when the washing is done.
Ubiquitous and mobile computing overlap, since the mobile user can in principle
benefit from computers that are everywhere. But they are distinct, in general. Ubiquitous
computing could benefit users while they remain in a single environment such as the
home or a hospital. Similarly, mobile computing has advantages even if it involves only
conventional, discrete computers and devices such as laptops and printers.
Figure 1.4 shows a user who is visiting a host organization. The figure shows the user’s home intranet and the host intranet at the site that the user is visiting. Both intranets are connected to the rest of the Internet.
The user has access to three forms of wireless connection. Their laptop has a
means of connecting to the host’s wireless LAN. This network provides coverage of a

SECTION 1.3 TRENDS IN DISTRIBUTED SYSTEMS 11


Figure 1.4 Portable and handheld devices in a distributed system



Internet




Host intranet
Wireless LAN


Mobile













Home intranet


GPS satellite signal

phone 3G phone network
Printer Laptop
Camera Host site



Few hundred metres (a floor of a building, say). It connects to the rest of the host intranet via a gateway or access point. The user also has a mobile (cellular) telephone, which is connected to the Internet. The phone gives access to the Web and other Internet services, constrained only by what can be presented on its small display, and may also provide location information via built-in GPS functionality. Finally, the user carries a digital camera, which can communicate over a personal area wireless network (with range up to about 10m) with a device such as a printer.
With a suitable system infrastructure, the user can perform some simple tasks in
the host site using the devices they carry. While journeying to the host site, the user can
fetch the latest stock prices from a web server using the mobile phone and can also use
the built-in GPS and route finding software to get directions to the site location. During
the meeting with their hosts, the user can show them a recent photograph by sending it
from the digital camera directly to a suitably enabled (local) printer or projector in the
meeting room (discovered using a location service). This requires only the wireless link
between the camera and printer or projector. And they can in principle send a document
from their laptop to the same printer, utilizing the wireless LAN and wired Ethern

Hasil (Bahasa Indonesia) 1:

BAB 8 1 KARAKTERISASI SISTEM TERDISTRIBUSIScript ini didasarkan pada fungsi yang disediakan oleh Apama [www.progress.com], produk komersial di dunia keuangan awalnya dikembangkan dari penelitian yang dilakukan di Universitas Cambridge. Script mendeteksi urutan sementara kompleks yang didasarkan pada harga saham Microsoft, HP dan keranjang lain harga saham, mengakibatkan keputusan untuk membeli atau menjual saham tertentu. Gaya ini teknologi semakin sering digunakan di bidang keuangan sistem termasuk pemantauan kegiatan untuk mengelola risiko (khususnya, perdagangan Pelacakan eksposur), untuk memastikan kepatuhan terhadap peraturan dan untuk memantau untuk pola kegiatan yang mungkin menunjukkan transaksi penipuan. Pada sistem seperti itu, peristiwa yang biasanya dicegat dan melewati apa setara dengan firewall kepatuhan dan resiko sebelum diproses (Lihat juga diskusi firewall di bagian 1.3.1 di bawah ini). 1.3 tren dalam sistem terdistribusi Sistem terdistribusi sedang mengalami periode perubahan signifikan dan ini dapat ditelusuri kembali ke sejumlah berpengaruh tren: • munculnya teknologi jaringan meresap; • munculnya komputasi mana-mana ditambah dengan keinginan untuk mendukung pengguna mobilitas dalam sistem terdistribusi; • meningkatnya permintaan untuk layanan multimedia; • pandangan sistem terdistribusi sebagai utilitas. 1.3.1 meresap networking dan modern Internet The modern Internet is a vast interconnected collection of computer networks of many different types, with the range of types increasing all the time and now including, for example, a wide range of wireless communication technologies such as WiFi, WiMAX, Bluetooth (see Chapter 3) and third-generation mobile phone networks. The net result is that networking has become a pervasive resource and devices can be connected (if desired) at any time and in any place. Figure 1.3 illustrates a typical portion of the Internet. Programs running on the computers connected to it interact by passing messages, employing a common means of communication. The design and construction of the Internet communication mechanisms (the Internet protocols) is a major technical achievement, enabling a program running anywhere to address messages to programs anywhere else and abstracting over the myriad of technologies mentioned above. The Internet is also a very large distributed system. It enables users, wherever they are, to make use of services such as the World Wide Web, email and file transfer. (Indeed, the Web is sometimes incorrectly equated with the Internet.) The set of services is open-ended - it can be extended by the addition of server computers and new types of service. The figure shows a collection of intranets - subnetworks operated by companies and other organizations and typically protected by firewalls. The role of a firewall is to melindungi intranet dengan mencegah pesan yang tidak sah dari meninggalkan atau memasuki. A BAGIAN 1.3 TREN DALAM SISTEM TERDISTRIBUSI 9Gambar 1.3 A khas sebagian dari InternetintranetISPdesktop komputer: Server: link jaringan: Firewall ini dilaksanakan dengan menyaring pesan masuk dan keluar. Penyaringan mungkin dilakukan oleh sumber atau tujuan, atau firewall mungkin membiarkan hanya pesan tersebut terkait Akses email dan web untuk lulus masuk atau keluar dari intranet yang melindungi. Layanan Internet Provider (ISP) adalah perusahaan yang menyediakan link broadband dan jenis lainnya koneksi ke pengguna perorangan dan organisasi kecil, memungkinkan mereka untuk mengakses Layanan di mana saja di Internet serta menyediakan layanan lokal seperti email dan web hosting. Intranet dihubungkan bersama-sama oleh backbones. Backbone adalah link jaringan dengan kapasitas tinggi transmisi, mempekerjakan koneksi satelit, kabel serat optik dan sirkuit bandwidth tinggi lainnya. Perhatikan bahwa beberapa organisasi mungkin tidak ingin untuk menghubungkan jaringan internal mereka ke Internet sama sekali. Misalnya, polisi dan keamanan dan penegak hukum lainnya yang cenderung memiliki setidaknya beberapa intranet internal yang terisolasi dari dunia luar (paling efektif firewall mungkin - tidak adanya koneksi fisik ke Internet). Firewall juga bisa menimbulkan masalah pada sistem terdistribusi dengan menghalangi sah akses ke layanan ketika berbagi antara pengguna internal dan eksternal sumber daya diperlukan. Oleh karena itu, firewall harus sering dilengkapi dengan mekanisme yang lebih halus dan kebijakan, seperti yang dibahas dalam bab 11. Pelaksanaan Internet dan layanan yang mendukung ini mengharuskan pengembangan solusi praktis untuk banyak masalah sistem terdistribusi (termasuk sebagian besar dari mereka didefinisikan dalam bagian 1.5). Kita akan menyoroti solusi mereka seluruh buku, menunjuk jangkauan dan keterbatasan mereka dimana tepat. KARAKTERISASI 10 BAB 1 SISTEM TERDISTRIBUSI1.3.2 Mobile dan komputasi mana-manaKemajuan teknologi di miniaturisasi perangkat dan jaringan nirkabel telah menyebabkan semakin integrasi perangkat komputasi yang kecil dan portabel ke sistem terdistribusi. Perangkat ini meliputi: • Komputer laptop. • Perangkat handheld, termasuk ponsel, ponsel pintar, perangkat GPS-enabled, Pager, personal digital Assistant (PDA), kamera video dan kamera digital. • Dpt dipakai perangkat, seperti smart watches dengan fungsionalitas mirip dengan PDA. • Perangkat tertanam dalam peralatan seperti mesin cuci, sistem hi-fi, mobil dan lemari es. Portabilitas banyak dari perangkat ini, bersama dengan kemampuan mereka untuk menghubungkan nyaman untuk jaringan di tempat yang berbeda, memungkinkan komputasi mobile. Mobile Computing adalah kinerja komputasi tugas sementara pengguna bergerak, atau mengunjungi tempat-tempat lain dari lingkungan mereka biasa. Mobile komputasi, pengguna yang dari intranet mereka 'rumah' (intranet di tempat kerja, atau tinggal mereka) yang masih diberi akses ke sumber daya melalui perangkat mereka membawa bersama mereka. Mereka dapat Lanjutkan untuk mengakses Internet; mereka dapat terus mengakses sumber daya di rumah mereka intranet; dan ada peningkatan penyediaan bagi pengguna untuk memanfaatkan sumber daya seperti printer atau bahkan penjualan poin yang strategis ketika mereka bergerak di sekitar. Yang kedua adalah juga dikenal sebagai lokasi-sadar atau pengenal konteks komputasi. Mobilitas memperkenalkan sejumlah tantangan bagi sistem terdistribusi, termasuk kebutuhan untuk berurusan dengan variabel konektivitas dan memang pemutusan, dan kebutuhan untuk mempertahankan operasi dalam menghadapi perangkat mobilitas (Lihat diskusi di mobilitas transparansi dalam bagian 1.5.7). Mana-mana computing adalah memanfaatkan banyak kecil, murah komputasi perangkat yang hadir dalam lingkungan fisik pengguna, termasuk rumah, kantor dan even natural settings. The term ‘ubiquitous’ is intended to suggest that small computing devices will eventually become so pervasive in everyday objects that they are scarcely noticed. That is, their computational behaviour will be transparently and intimately tied up with their physical function. The presence of computers everywhere only becomes useful when they can communicate with one another. For example, it may be convenient for users to control their washing machine or their entertainment system from their phone or a ‘universal remote control’ device in the home. Equally, the washing machine could notify the user via a smart badge or phone when the washing is done. Ubiquitous and mobile computing overlap, since the mobile user can in principle benefit from computers that are everywhere. But they are distinct, in general. Ubiquitous computing could benefit users while they remain in a single environment such as the home or a hospital. Similarly, mobile computing has advantages even if it involves only conventional, discrete computers and devices such as laptops and printers. Figure 1.4 shows a user who is visiting a host organization. The figure shows the user’s home intranet and the host intranet at the site that the user is visiting. Both intranets are connected to the rest of the Internet. The user has access to three forms of wireless connection. Their laptop has a means of connecting to the host’s wireless LAN. This network provides coverage of a SECTION 1.3 TRENDS IN DISTRIBUTED SYSTEMS 11 Figure 1.4 Portable and handheld devices in a distributed systemInternetHost intranetWireless LANMobile Home intranetGPS satellite signal phone 3G phone networkPrinter LaptopCamera Host siteFew hundred metres (a floor of a building, say). It connects to the rest of the host intranet via a gateway or access point. The user also has a mobile (cellular) telephone, which is connected to the Internet. The phone gives access to the Web and other Internet services, constrained only by what can be presented on its small display, and may also provide location information via built-in GPS functionality. Finally, the user carries a digital camera, which can communicate over a personal area wireless network (with range up to about 10m) with a device such as a printer. With a suitable system infrastructure, the user can perform some simple tasks in the host site using the devices they carry. While journeying to the host site, the user can fetch the latest stock prices from a web server using the mobile phone and can also use the built-in GPS and route finding software to get directions to the site location. During the meeting with their hosts, the user can show them a recent photograph by sending it from the digital camera directly to a suitably enabled (local) printer or projector in the meeting room (discovered using a location service). This requires only the wireless link between the camera and printer or projector. And they can in principle send a document from their laptop to the same printer, utilizing the wireless LAN and wired Ethern