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12.4.2 Service-oriented system arch
12.4.2 Service-oriented system architecture
The development of the WWW meant that client computers had access to remote
servers outside their own organisations. If these organisations converted their information
to HTML, then this could be accessed by these computers. However, access
was solely through a web browse^, and direct access to the information stores by
other programs was not practical. This meant that opportunistic connections
between servers where, for example, a program queried a number of catalogues,
was not possible.
To get around this problem, the notion of a web service was proposed. Using a
web service, organisations that want to make their information accessible to other
programs can do so by defining and publishing a web service interface. This interface
defines the data available and how it can be accessed. More generally, a web
I service is a standard representation for some computational or information resource
that can be used by other programs. Therefore, you could define a tax filing service
where users could fill in their tax forms and have these automatically checked
and submitted to the tax authorities.
A web service is an instance of a more general notion of a service, whlch is
defined by (Lovelock, et al., 1996) as:
an act or perSormance offered by one party to another. Although the process
may be tied to a physical product, the pe$ormance is essentially intangible
and does not normally result in ownership of any of the factors of production.
The essence of a service, therefore, is that the provision of the service is independent
of the application using the service (Turner, et al., 2003). Service providers
can develop specialised services and offer these to a range of service users from
different organisations. Applications may be constructed by linlung services from
various providers using either a standard programming language or a specialised
service orchestration language such as BPEUWS.
There are various service models, from the JINI model (Kumaran, 2001)
through web services (Stal, 2002) and grid services (Foster, et al., 2002).
Conceptually, all of these operate according to the model shown in Figure 12.17,
which is a generalisation of the conceptual web service model described by Kreger
(Kreger, 2001). A service provider offers a service by defining its interface and implementing
the service functionality. A service requestor binds that service into its application.
This means that the requestor's application includes code to call that service
and process the results of the service call. To ensure that the service can be
accessed by external service users, the service provider makes an entry in a service registry that includes information about the service and what it does.
The differences between this service model and the distributed object approach
to distributed systems architectures are:
Services can be offered by any service provider inside or outside of an organisation.
Assuming these conform to certain standards (discussed below), organisations
can create applications by integrating services from a range of
, providers. For example, a manufacturing company can link directly to services
provided by its suppliers.
The service provider makes information about the service public so that any
authorised user can use it. The service provider and the service user do not need
to negotiate about what the service does before it can be incorporated in an
application program.
Applications can delay the binding of services until they are deployed or until
execution. Therefore, an application using a stock price service (say) could dynamically
change service providers while the system was executing.
Opportunistic construction of new services is possible. A service provider may
recognise new services that can be created by linlung existing services in innovative
ways.
Service users can pay for services according to their use rather than their provision.
Therefore, instead of buying an expensive component that is rarely used,
the application writer can use an external service that will be paid for only when
required.
Applications can be made smaller (which is important if they are to be embedded
in other devices) because they can implement exception handling as external
services.
Applications can be reactive and adapt their operation according to their environment
by binding to different services as their environment changes.
At the time of this writing, these advantages have sparked immense interest in
web services as a basis for constructing loosely coupled, distributed applications.
However, there is still limited practical experience with service-oriented architectures
so we do not yet know that practical implications of this approach.
Software reuse has been a topic of research for many years; yet, as I discuss in
Chapters 18 and 19, there remain many practical difficulties in reusing software.
One of the major problems has been that standards for reusable components have
been developed only relatively recently, and several standards are in use. However,
the web services initiative has been driven by standards from its inception, and standards
covering many aspects of web services are under development. The three fundamental
standards that enable communications between web services are:
1. SOAP (Simple Object Access Protocol) This protocol defines an organisation
for structured data exchange between web services.
2. WSDL (Web Services Description Language) This protocol defines how the interfaces
of web services can be represented.
3. UDDI (Universal Description, Discovely and Integration) This is a discovery standard that defines how service description information, used by service
requestors to discover services, can be organised.
All of these standards are based on XML-a human- and machine-readable markup
language (Skonnard and Gudgin, 2002). You don't, however, need to know details
of these standards to understand the web services concept.
Web service application architectures are loosely coupled architectures where service
bindings can change during execution. Some systems will be solely built using web services and others will mix web services with locally developed components.
To illustrate how applications may be organised, consider the following scenario:
An in-car information system provides drivers with information on weather,
road trafJic conditions, local information and so forth. This is linked to the
car radio so that information is delivered as a signal on a specific radio channel.
The car is equipped with GPS receiver to discover its position and, based
on that position, the system accesses a range of information services.
Information may be delivered in the driver's specified language.
Figure 12.18 illustrates a possible organisation for such a system. The in-car soft-
' ware includes five modules. These handle communications with the driver, with a
GPS receiver that reports the car's position and with the car radio. The Transmitter
and Receiver modules handle all communications with external services.
The car communicates with an externally provided mobile information service
which aggregates information from a range of other services that provide information
on weather, traffic information and local facilities. Different providers in different places provide this service, and the in-car system uses a discovery service
to locate the appropriate information service and bind to it. The discovery service
is also used by the mobile information service to bind to the appropriate weather,
traffic and facilities services. Services exchange SOAP messages that include GPS
position information used, by the services, to select the appropriate information. The
aggregated information is passed back to the car through a service that translates
the information language into the driver's language.
KEY PQIMTS
Distributed systems can support resource sharing, openness, concurrency, scalability, fault
tolerance and transparency.
Client-server systems are distributed systems where the system is modelled as a set of
services provided by servers to client processes.
In a client-server system, the user interface always runs on a client, and data management
is always provided by a shared server. Application functionality may be implemented on the
client computer or on the server.
In a distributed object architecture, there is no distinction between clients and servers.
Objects provide general services that may be called on by other objects. This approach may
be used for implementing client-server systems.
Distributed object systems require middleware to handle object communications and to
allow objects to be added to and removed from the system.
The CORBA standards are a set of standards for middleware that supports distributed
object architectures. They include object model definitions, definitions of an object request
broker and common service definitions. Various implementations of the CORBA standards
are available.
Peer-to-peer architectures are decentralised architectures where there are no distinguished
clients and servers. Computations can be distributed over many systems in different
organisations.
Service-oriented systems are created by linking software services provided by various
service suppliers. An important aspect of service-oriented architectures is that binding of
services to the architectural components can be delayed until the system is deployed or is
executing.
This example illustrates one of the key advantages of the service-oriented
approach. It is not necessary to decide when the system is programmed or deployed
what service provider should be used and what specific services could be accessed.
As the car moves around, the in-car software uses the service discovery service to
find the most appropriate information service 'and binds to that. Because of the use
of a translation service, it can move across borders and therefore make local information
availabl
The development of the WWW meant that client computers had access to remote
servers outside their own organisations. If these organisations converted their information
to HTML, then this could be accessed by these computers. However, access
was solely through a web browse^, and direct access to the information stores by
other programs was not practical. This meant that opportunistic connections
between servers where, for example, a program queried a number of catalogues,
was not possible.
To get around this problem, the notion of a web service was proposed. Using a
web service, organisations that want to make their information accessible to other
programs can do so by defining and publishing a web service interface. This interface
defines the data available and how it can be accessed. More generally, a web
I service is a standard representation for some computational or information resource
that can be used by other programs. Therefore, you could define a tax filing service
where users could fill in their tax forms and have these automatically checked
and submitted to the tax authorities.
A web service is an instance of a more general notion of a service, whlch is
defined by (Lovelock, et al., 1996) as:
an act or perSormance offered by one party to another. Although the process
may be tied to a physical product, the pe$ormance is essentially intangible
and does not normally result in ownership of any of the factors of production.
The essence of a service, therefore, is that the provision of the service is independent
of the application using the service (Turner, et al., 2003). Service providers
can develop specialised services and offer these to a range of service users from
different organisations. Applications may be constructed by linlung services from
various providers using either a standard programming language or a specialised
service orchestration language such as BPEUWS.
There are various service models, from the JINI model (Kumaran, 2001)
through web services (Stal, 2002) and grid services (Foster, et al., 2002).
Conceptually, all of these operate according to the model shown in Figure 12.17,
which is a generalisation of the conceptual web service model described by Kreger
(Kreger, 2001). A service provider offers a service by defining its interface and implementing
the service functionality. A service requestor binds that service into its application.
This means that the requestor's application includes code to call that service
and process the results of the service call. To ensure that the service can be
accessed by external service users, the service provider makes an entry in a service registry that includes information about the service and what it does.
The differences between this service model and the distributed object approach
to distributed systems architectures are:
Services can be offered by any service provider inside or outside of an organisation.
Assuming these conform to certain standards (discussed below), organisations
can create applications by integrating services from a range of
, providers. For example, a manufacturing company can link directly to services
provided by its suppliers.
The service provider makes information about the service public so that any
authorised user can use it. The service provider and the service user do not need
to negotiate about what the service does before it can be incorporated in an
application program.
Applications can delay the binding of services until they are deployed or until
execution. Therefore, an application using a stock price service (say) could dynamically
change service providers while the system was executing.
Opportunistic construction of new services is possible. A service provider may
recognise new services that can be created by linlung existing services in innovative
ways.
Service users can pay for services according to their use rather than their provision.
Therefore, instead of buying an expensive component that is rarely used,
the application writer can use an external service that will be paid for only when
required.
Applications can be made smaller (which is important if they are to be embedded
in other devices) because they can implement exception handling as external
services.
Applications can be reactive and adapt their operation according to their environment
by binding to different services as their environment changes.
At the time of this writing, these advantages have sparked immense interest in
web services as a basis for constructing loosely coupled, distributed applications.
However, there is still limited practical experience with service-oriented architectures
so we do not yet know that practical implications of this approach.
Software reuse has been a topic of research for many years; yet, as I discuss in
Chapters 18 and 19, there remain many practical difficulties in reusing software.
One of the major problems has been that standards for reusable components have
been developed only relatively recently, and several standards are in use. However,
the web services initiative has been driven by standards from its inception, and standards
covering many aspects of web services are under development. The three fundamental
standards that enable communications between web services are:
1. SOAP (Simple Object Access Protocol) This protocol defines an organisation
for structured data exchange between web services.
2. WSDL (Web Services Description Language) This protocol defines how the interfaces
of web services can be represented.
3. UDDI (Universal Description, Discovely and Integration) This is a discovery standard that defines how service description information, used by service
requestors to discover services, can be organised.
All of these standards are based on XML-a human- and machine-readable markup
language (Skonnard and Gudgin, 2002). You don't, however, need to know details
of these standards to understand the web services concept.
Web service application architectures are loosely coupled architectures where service
bindings can change during execution. Some systems will be solely built using web services and others will mix web services with locally developed components.
To illustrate how applications may be organised, consider the following scenario:
An in-car information system provides drivers with information on weather,
road trafJic conditions, local information and so forth. This is linked to the
car radio so that information is delivered as a signal on a specific radio channel.
The car is equipped with GPS receiver to discover its position and, based
on that position, the system accesses a range of information services.
Information may be delivered in the driver's specified language.
Figure 12.18 illustrates a possible organisation for such a system. The in-car soft-
' ware includes five modules. These handle communications with the driver, with a
GPS receiver that reports the car's position and with the car radio. The Transmitter
and Receiver modules handle all communications with external services.
The car communicates with an externally provided mobile information service
which aggregates information from a range of other services that provide information
on weather, traffic information and local facilities. Different providers in different places provide this service, and the in-car system uses a discovery service
to locate the appropriate information service and bind to it. The discovery service
is also used by the mobile information service to bind to the appropriate weather,
traffic and facilities services. Services exchange SOAP messages that include GPS
position information used, by the services, to select the appropriate information. The
aggregated information is passed back to the car through a service that translates
the information language into the driver's language.
KEY PQIMTS
Distributed systems can support resource sharing, openness, concurrency, scalability, fault
tolerance and transparency.
Client-server systems are distributed systems where the system is modelled as a set of
services provided by servers to client processes.
In a client-server system, the user interface always runs on a client, and data management
is always provided by a shared server. Application functionality may be implemented on the
client computer or on the server.
In a distributed object architecture, there is no distinction between clients and servers.
Objects provide general services that may be called on by other objects. This approach may
be used for implementing client-server systems.
Distributed object systems require middleware to handle object communications and to
allow objects to be added to and removed from the system.
The CORBA standards are a set of standards for middleware that supports distributed
object architectures. They include object model definitions, definitions of an object request
broker and common service definitions. Various implementations of the CORBA standards
are available.
Peer-to-peer architectures are decentralised architectures where there are no distinguished
clients and servers. Computations can be distributed over many systems in different
organisations.
Service-oriented systems are created by linking software services provided by various
service suppliers. An important aspect of service-oriented architectures is that binding of
services to the architectural components can be delayed until the system is deployed or is
executing.
This example illustrates one of the key advantages of the service-oriented
approach. It is not necessary to decide when the system is programmed or deployed
what service provider should be used and what specific services could be accessed.
As the car moves around, the in-car software uses the service discovery service to
find the most appropriate information service 'and binds to that. Because of the use
of a translation service, it can move across borders and therefore make local information
availabl
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12.4.2 dịch vụ theo định hướng hệ thống kiến trúcSự phát triển của WWW có nghĩa là máy khách đã truy cập từ xamáy chủ bên ngoài tổ chức của riêng họ. Nếu chuyển đổi các tổ chức thông tin của họsang HTML, sau đó điều này có thể được truy cập bởi các máy tính này. Tuy nhiên, truy cậpđã chỉ thông qua một trình duyệt web ^, và trực tiếp truy cập vào các thông tin lưu trữ bởiCác chương trình khác đã không được thực tế. Điều này có nghĩa rằng cơ hội kết nốigiữa các máy chủ ở đâu, ví dụ, một chương trình truy vấn một số danh mục sản phẩm,là không thể.Để có được xung quanh vấn đề này, khái niệm về một dịch vụ web được đề xuất. Bằng cách sử dụng mộtDịch vụ web, tổ chức mà muốn làm cho thông tin của họ có thể truy cập để khácchương trình có thể làm như vậy bằng cách xác định và xuất bản một giao diện dịch vụ web. Giao diện nàyxác định dữ liệu sẵn có và làm thế nào nó có thể được truy cập. Nói chung, một trang webTôi dịch vụ là một đại diện tiêu chuẩn đối với một số tính toán hoặc thông tin tài nguyênmà có thể được sử dụng bởi các chương trình khác. Vì vậy, bạn có thể xác định một thuế nộp hồ sơ dịch vụnơi người dùng có thể điền vào biểu mẫu thuế của họ và có những tự động kiểm travà gửi đến các cơ quan thuế.Một dịch vụ web là một thể hiện của một khái niệm tổng quát hơn về một dịch vụ, whlchđịnh nghĩa bởi (Lovelock, và ctv., 1996) là:một hành động hoặc perSormance được cung cấp bởi một bên khác. Mặc dù quá trìnhcó thể được gắn vào một sản phẩm vật lý, pe$ ormance là về cơ bản vô hìnhvà không không bình thường kết quả trong quyền sở hữu của bất kỳ của các yếu tố sản xuất.Bản chất của một dịch vụ, do đó, là việc cung cấp các dịch vụ này là độc lậpcủa ứng dụng bằng cách sử dụng dịch vụ (Turner, et al., 2003). Nhà cung cấp dịch vụcó thể phát triển các dịch vụ đặc biệt và cung cấp chúng với một loạt các dịch vụ người dùng từtổ chức khác nhau. Ứng dụng có thể được xây dựng bởi linlung dịch vụ từnhà cung cấp khác nhau bằng cách sử dụng hoặc là một ngôn ngữ lập trình tiêu chuẩn hoặc một chuyên mônDịch vụ dàn nhạc ngôn ngữ chẳng hạn như BPEUWS.Có các mô hình dịch vụ khác nhau, từ các mô hình JINI (Kumaran, 2001)thông qua dịch vụ web (Stal, 2002) và mạng lưới dịch vụ (Foster, et al., 2002).Khái niệm, tất cả các hoạt động theo mô hình hiển thị trong hình 12,17,đó là một generalisation của mô hình dịch vụ web khái niệm mô tả bởi Kreger(Kreger, 2001). Một nhà cung cấp dịch vụ cung cấp một dịch vụ bằng cách xác định giao diện của nó và thực hiệnCác chức năng dịch vụ. Requestor dịch vụ liên kết dịch vụ đó vào ứng dụng của nó.Điều này có nghĩa rằng requestor của ứng dụng bao gồm mã gọi Dịch vụ đóvà xử lý các kết quả của các cuộc gọi Dịch vụ. Để đảm bảo rằng các dịch vụ có thểtruy cập bởi người dùng bên ngoài Dịch vụ, các nhà cung cấp dịch vụ làm cho một mục nhập trong một đăng ký dịch vụ bao gồm các thông tin về dịch vụ và những gì nó làm.Sự khác biệt giữa các mô hình dịch vụ này và cách tiếp cận đối tượng phân phốiđể phân phối hệ thống kiến trúc là:Dịch vụ có thể được cung cấp bởi bất kỳ nhà cung cấp dịch vụ bên trong hoặc bên ngoài của một tổ chức.Giả sử chúng phù hợp với tiêu chuẩn nhất định (được thảo luận dưới đây), tổ chứccó thể tạo các ứng dụng bằng cách tích hợp các dịch vụ từ một loạt các, nhà cung cấp. Ví dụ, một công ty sản xuất có thể liên kết trực tiếp đến dịch vụcung cấp bởi các nhà cung cấp.Các nhà cung cấp dịch vụ làm cho thông tin về dịch vụ công cộng như vậy rằng bất cứngười dùng được ủy quyền có thể sử dụng nó. Các nhà cung cấp dịch vụ và người sử dụng dịch vụ không cầnđàm phán về những gì các dịch vụ nào trước khi nó có thể được kết hợp trong mộtchương trình ứng dụng.Ứng dụng có thể trì hoãn các ràng buộc của dịch vụ cho đến khi họ đã được triển khai hoặc cho đến khithực hiện. Vì vậy, một ứng dụng bằng cách sử dụng một dịch vụ giá cổ phiếu (nói) có thể tự độngthay đổi nhà cung cấp dịch vụ trong khi hệ thống thực hiện.Cơ hội xây dựng Dịch vụ mới có thể. Một nhà cung cấp dịch vụ có thểnhận ra dịch vụ mới có thể được tạo ra bởi linlung dịch vụ sẵn có trong sáng tạocách.Dịch vụ người dùng có thể trả tiền cho các dịch vụ theo sử dụng của họ chứ không phải là cung cấp của họ.Vì vậy, thay vì mua một thành phần đắt tiền mà hiếm khi được sử dụng,Các nhà văn ứng dụng có thể sử dụng một dịch vụ bên ngoài mà sẽ được trả tiền cho chỉ khiyêu cầu.Ứng dụng có thể được làm nhỏ (mà là quan trọng nếu họ muốn được nhúngtrong các thiết bị khác) bởi vì họ có thể thực hiện ngoại lệ xử lý như bên ngoàiDịch vụ.Ứng dụng có thể được phản ứng và thích ứng của họ hoạt động theo môi trường của họbởi ràng buộc để dịch vụ khác nhau như thay đổi môi trường của họ.Tại thời điểm văn bản này, những lợi ích này đã gây ra quan tâm đến bao laDịch vụ web làm cơ sở để xây dựng một cách lỏng lẻo cùng, phân phối ứng dụng.Tuy nhiên, đó là vẫn còn hạn chế kinh nghiệm thực tế với kiến trúc hướng dịch vụVì vậy chúng tôi không được biết thực tế rằng tác động của cách tiếp cận này.Tái sử dụng phần mềm đã là một chủ đề của nghiên cứu trong nhiều năm; Tuy vậy, khi tôi thảo luậnChương 18 và 19, có vẫn còn nhiều thực tế khó khăn trong việc tái sử dụng phần mềm.Một trong những vấn đề lớn đã là các tiêu chuẩn cho các thành phần tái sử dụng cóphát triển chỉ tương đối mới, và một số tiêu chuẩn sử dụng. Tuy nhiên,web dịch vụ sáng kiến đã được thúc đẩy bởi các tiêu chuẩn từ khi ra đời, và tiêu chuẩnbao gồm nhiều khía cạnh của dịch vụ web đang được triển khai. Ba cơ bảntiêu chuẩn cho phép truyền thông giữa các dịch vụ web là:1. xà phòng (đơn giản đối tượng Access Protocol) giao thức này định nghĩa một tổ chứcđể trao đổi dữ liệu có cấu trúc giữa dịch vụ web.2. WSDL (ngôn ngữ mô tả Dịch vụ Web) giao thức này xác định như thế nào các giao diệnweb dịch vụ có thể được biểu diễn.3. UDDI (Universal mô tả, Discovely và hội nhập) đây là một tiêu chuẩn phát hiện xác định như thế nào dịch vụ thông tin mô tả, được sử dụng bởi dịch vụrequestors để khám phá dịch vụ, có thể được tổ chức.Tất cả các tiêu chuẩn này được dựa trên XML-một con người - và machine - readable đánh dấungôn ngữ (Skonnard và Gudgin, 2002). Bạn không, Tuy nhiên, cần phải biết chi tiếtCác tiêu chuẩn để hiểu khái niệm dịch vụ web.Web dịch vụ ứng dụng kiến trúc là lỏng lẻo cùng kiến trúc trong trường hợp dịch vụbindings có thể thay đổi trong thực hiện. Một số hệ thống sẽ được chỉ duy nhất được xây dựng bằng cách sử dụng dịch vụ web và những người khác sẽ kết hợp dịch vụ web với thành phần phát triển tại địa phương.Để minh họa cho ứng dụng có thể được tổ chức như thế nào, hãy xem xét kịch bản sau đây:Một hệ thống trong xe hơi thông tin cung cấp trình điều khiển với thông tin về thời tiết,điều kiện đường trafJic, thông tin địa phương và vv. Đây liên kết với cácxe đài phát thanh để thông tin đó được phân phối như là một tín hiệu trên một kênh radio cụ thể.Chiếc xe được trang bị với máy thu GPS để khám phá các vị trí của nó và, dựavào vị trí đó, Hệ thống truy cập một loạt các dịch vụ thông tin.Thông tin có thể được phân phát trong trình điều khiển được chỉ định ngôn ngữ.12.18 hình minh hoạ một tổ chức có thể cho một hệ thống. Trong xe hơi mềm-' sản phẩm bao gồm 5 mô-đun. Các xử lý thông tin liên lạc với trình điều khiển, với mộtBộ tiếp nhận GPS báo cáo vị trí của xe hơi và với các đài phát thanh xe hơi. Bộ phátvà xử lý mô-đun nhận tất cả liên lạc với bên ngoài Dịch vụ.Xe liên lạc với một dịch vụ thông tin bên ngoài cung cấp điện thoại di độngmà tập hợp thông tin từ các dịch vụ cung cấp thông tintrên thời tiết, thông tin giao thông và các cơ sở địa phương. Nhà cung cấp khác nhau ở những nơi khác nhau cung cấp dịch vụ này, và hệ thống trong xe hơi sử dụng một dịch vụ khám pháđể xác định vị trí các dịch vụ thông tin phù hợp và liên kết với nó. Các dịch vụ khám phácũng được sử dụng bởi các dịch vụ điện thoại di động thông tin liên kết với thời tiết thích hợp,lưu lượng truy cập và các cơ sở dịch vụ. Dịch vụ trao đổi tin nhắn xà phòng bao gồm GPSthông tin vị trí được sử dụng, bởi các dịch vụ, chọn các thông tin thích hợp. Cácthông tin tổng hợp được truyền lại cho xe thông qua một dịch vụ mà dịchngôn ngữ thông tin vào các trình điều khiển ngôn ngữ.CHÍNH PQIMTSHệ thống phân phối có thể hỗ trợ chia sẻ tài nguyên, cởi mở, concurrency, khả năng mở rộng, lỗikhoan dung và minh bạch.Hệ thống máy khách-máy chủ là phân phối hệ thống mà hệ thống mô hình như là một tập hợp cácDịch vụ cung cấp bởi các máy chủ để khách hàng quá trình.Trong một hệ thống máy khách-máy chủ, giao diện người dùng luôn luôn chạy trên một khách hàng, và quản lý dữ liệuluôn luôn được cung cấp bởi một máy chủ chia sẻ. Chức năng ứng dụng có thể được thực hiện trên cáckhách hàng máy tính hoặc trên máy chủ.Một kiến trúc phân phối đối tượng, có là không có sự phân biệt giữa khách hàng và máy chủ.Các đối tượng cung cấp dịch vụ tổng hợp có thể được gọi là các đối tượng khác. Cách tiếp cận này có thểđược sử dụng cho việc thực hiện hệ thống máy khách-máy chủ.Hệ thống phân phối đối tượng yêu cầu trung gian để xử lý thông tin liên lạc đối tượng và đếncho phép các đối tượng để được thêm vào và gỡ bỏ khỏi hệ thống.Các tiêu chuẩn CORBA là một tập hợp các tiêu chuẩn cho middleware hỗ trợ phân phốiđối tượng kiến trúc. Chúng bao gồm đối tượng mô hình định nghĩa, định nghĩa của một yêu cầu đối tượngmôi giới và phổ biến dịch vụ định nghĩa. Việc triển khai khác nhau của các tiêu chuẩn CORBAcó sẵn.Peer-to-peer kiến trúc có kiến trúc phân trong trường hợp không có không phân biệtkhách hàng và máy chủ. Tính toán có thể được phân phối trên nhiều hệ thống khác nhautổ chức.Theo định hướng dịch vụ hệ thống được tạo ra bằng cách liên kết dịch vụ phần mềm cung cấp của nhiều nghệ sĩnhà cung cấp dịch vụ. Một khía cạnh quan trọng của kiến trúc hướng dịch vụ là đó ràng buộc củaDịch vụ cho các thành phần kiến trúc có thể được trì hoãn cho đến khi hệ thống được triển khai hoặc làthực hiện.Ví dụ này minh hoạ một trong những lợi thế quan trọng của các dịch vụ theo định hướngcách tiếp cận. Nó không phải là cần thiết để quyết định khi hệ thống được lập trình hoặc triển khainhà cung cấp dịch vụ những gì nên được sử dụng và những dịch vụ cụ thể có thể được truy cập.Khi xe di chuyển xung quanh, các phần mềm trong xe hơi sử dụng các dịch vụ khám phá dịch vụtìm thấy dịch vụ thông tin thích hợp nhất ' và liên kết với đó. Bởi vì việc sử dụngmột dịch vụ, nó có thể di chuyển qua biên giới và do đó làm cho thông tin địa phươngthể thay đổi
đang được dịch, vui lòng đợi..
12.4.2 Service-oriented system architecture
The development of the WWW meant that client computers had access to remote
servers outside their own organisations. If these organisations converted their information
to HTML, then this could be accessed by these computers. However, access
was solely through a web browse^, and direct access to the information stores by
other programs was not practical. This meant that opportunistic connections
between servers where, for example, a program queried a number of catalogues,
was not possible.
To get around this problem, the notion of a web service was proposed. Using a
web service, organisations that want to make their information accessible to other
programs can do so by defining and publishing a web service interface. This interface
defines the data available and how it can be accessed. More generally, a web
I service is a standard representation for some computational or information resource
that can be used by other programs. Therefore, you could define a tax filing service
where users could fill in their tax forms and have these automatically checked
and submitted to the tax authorities.
A web service is an instance of a more general notion of a service, whlch is
defined by (Lovelock, et al., 1996) as:
an act or perSormance offered by one party to another. Although the process
may be tied to a physical product, the pe$ormance is essentially intangible
and does not normally result in ownership of any of the factors of production.
The essence of a service, therefore, is that the provision of the service is independent
of the application using the service (Turner, et al., 2003). Service providers
can develop specialised services and offer these to a range of service users from
different organisations. Applications may be constructed by linlung services from
various providers using either a standard programming language or a specialised
service orchestration language such as BPEUWS.
There are various service models, from the JINI model (Kumaran, 2001)
through web services (Stal, 2002) and grid services (Foster, et al., 2002).
Conceptually, all of these operate according to the model shown in Figure 12.17,
which is a generalisation of the conceptual web service model described by Kreger
(Kreger, 2001). A service provider offers a service by defining its interface and implementing
the service functionality. A service requestor binds that service into its application.
This means that the requestor's application includes code to call that service
and process the results of the service call. To ensure that the service can be
accessed by external service users, the service provider makes an entry in a service registry that includes information about the service and what it does.
The differences between this service model and the distributed object approach
to distributed systems architectures are:
Services can be offered by any service provider inside or outside of an organisation.
Assuming these conform to certain standards (discussed below), organisations
can create applications by integrating services from a range of
, providers. For example, a manufacturing company can link directly to services
provided by its suppliers.
The service provider makes information about the service public so that any
authorised user can use it. The service provider and the service user do not need
to negotiate about what the service does before it can be incorporated in an
application program.
Applications can delay the binding of services until they are deployed or until
execution. Therefore, an application using a stock price service (say) could dynamically
change service providers while the system was executing.
Opportunistic construction of new services is possible. A service provider may
recognise new services that can be created by linlung existing services in innovative
ways.
Service users can pay for services according to their use rather than their provision.
Therefore, instead of buying an expensive component that is rarely used,
the application writer can use an external service that will be paid for only when
required.
Applications can be made smaller (which is important if they are to be embedded
in other devices) because they can implement exception handling as external
services.
Applications can be reactive and adapt their operation according to their environment
by binding to different services as their environment changes.
At the time of this writing, these advantages have sparked immense interest in
web services as a basis for constructing loosely coupled, distributed applications.
However, there is still limited practical experience with service-oriented architectures
so we do not yet know that practical implications of this approach.
Software reuse has been a topic of research for many years; yet, as I discuss in
Chapters 18 and 19, there remain many practical difficulties in reusing software.
One of the major problems has been that standards for reusable components have
been developed only relatively recently, and several standards are in use. However,
the web services initiative has been driven by standards from its inception, and standards
covering many aspects of web services are under development. The three fundamental
standards that enable communications between web services are:
1. SOAP (Simple Object Access Protocol) This protocol defines an organisation
for structured data exchange between web services.
2. WSDL (Web Services Description Language) This protocol defines how the interfaces
of web services can be represented.
3. UDDI (Universal Description, Discovely and Integration) This is a discovery standard that defines how service description information, used by service
requestors to discover services, can be organised.
All of these standards are based on XML-a human- and machine-readable markup
language (Skonnard and Gudgin, 2002). You don't, however, need to know details
of these standards to understand the web services concept.
Web service application architectures are loosely coupled architectures where service
bindings can change during execution. Some systems will be solely built using web services and others will mix web services with locally developed components.
To illustrate how applications may be organised, consider the following scenario:
An in-car information system provides drivers with information on weather,
road trafJic conditions, local information and so forth. This is linked to the
car radio so that information is delivered as a signal on a specific radio channel.
The car is equipped with GPS receiver to discover its position and, based
on that position, the system accesses a range of information services.
Information may be delivered in the driver's specified language.
Figure 12.18 illustrates a possible organisation for such a system. The in-car soft-
' ware includes five modules. These handle communications with the driver, with a
GPS receiver that reports the car's position and with the car radio. The Transmitter
and Receiver modules handle all communications with external services.
The car communicates with an externally provided mobile information service
which aggregates information from a range of other services that provide information
on weather, traffic information and local facilities. Different providers in different places provide this service, and the in-car system uses a discovery service
to locate the appropriate information service and bind to it. The discovery service
is also used by the mobile information service to bind to the appropriate weather,
traffic and facilities services. Services exchange SOAP messages that include GPS
position information used, by the services, to select the appropriate information. The
aggregated information is passed back to the car through a service that translates
the information language into the driver's language.
KEY PQIMTS
Distributed systems can support resource sharing, openness, concurrency, scalability, fault
tolerance and transparency.
Client-server systems are distributed systems where the system is modelled as a set of
services provided by servers to client processes.
In a client-server system, the user interface always runs on a client, and data management
is always provided by a shared server. Application functionality may be implemented on the
client computer or on the server.
In a distributed object architecture, there is no distinction between clients and servers.
Objects provide general services that may be called on by other objects. This approach may
be used for implementing client-server systems.
Distributed object systems require middleware to handle object communications and to
allow objects to be added to and removed from the system.
The CORBA standards are a set of standards for middleware that supports distributed
object architectures. They include object model definitions, definitions of an object request
broker and common service definitions. Various implementations of the CORBA standards
are available.
Peer-to-peer architectures are decentralised architectures where there are no distinguished
clients and servers. Computations can be distributed over many systems in different
organisations.
Service-oriented systems are created by linking software services provided by various
service suppliers. An important aspect of service-oriented architectures is that binding of
services to the architectural components can be delayed until the system is deployed or is
executing.
This example illustrates one of the key advantages of the service-oriented
approach. It is not necessary to decide when the system is programmed or deployed
what service provider should be used and what specific services could be accessed.
As the car moves around, the in-car software uses the service discovery service to
find the most appropriate information service 'and binds to that. Because of the use
of a translation service, it can move across borders and therefore make local information
availabl
The development of the WWW meant that client computers had access to remote
servers outside their own organisations. If these organisations converted their information
to HTML, then this could be accessed by these computers. However, access
was solely through a web browse^, and direct access to the information stores by
other programs was not practical. This meant that opportunistic connections
between servers where, for example, a program queried a number of catalogues,
was not possible.
To get around this problem, the notion of a web service was proposed. Using a
web service, organisations that want to make their information accessible to other
programs can do so by defining and publishing a web service interface. This interface
defines the data available and how it can be accessed. More generally, a web
I service is a standard representation for some computational or information resource
that can be used by other programs. Therefore, you could define a tax filing service
where users could fill in their tax forms and have these automatically checked
and submitted to the tax authorities.
A web service is an instance of a more general notion of a service, whlch is
defined by (Lovelock, et al., 1996) as:
an act or perSormance offered by one party to another. Although the process
may be tied to a physical product, the pe$ormance is essentially intangible
and does not normally result in ownership of any of the factors of production.
The essence of a service, therefore, is that the provision of the service is independent
of the application using the service (Turner, et al., 2003). Service providers
can develop specialised services and offer these to a range of service users from
different organisations. Applications may be constructed by linlung services from
various providers using either a standard programming language or a specialised
service orchestration language such as BPEUWS.
There are various service models, from the JINI model (Kumaran, 2001)
through web services (Stal, 2002) and grid services (Foster, et al., 2002).
Conceptually, all of these operate according to the model shown in Figure 12.17,
which is a generalisation of the conceptual web service model described by Kreger
(Kreger, 2001). A service provider offers a service by defining its interface and implementing
the service functionality. A service requestor binds that service into its application.
This means that the requestor's application includes code to call that service
and process the results of the service call. To ensure that the service can be
accessed by external service users, the service provider makes an entry in a service registry that includes information about the service and what it does.
The differences between this service model and the distributed object approach
to distributed systems architectures are:
Services can be offered by any service provider inside or outside of an organisation.
Assuming these conform to certain standards (discussed below), organisations
can create applications by integrating services from a range of
, providers. For example, a manufacturing company can link directly to services
provided by its suppliers.
The service provider makes information about the service public so that any
authorised user can use it. The service provider and the service user do not need
to negotiate about what the service does before it can be incorporated in an
application program.
Applications can delay the binding of services until they are deployed or until
execution. Therefore, an application using a stock price service (say) could dynamically
change service providers while the system was executing.
Opportunistic construction of new services is possible. A service provider may
recognise new services that can be created by linlung existing services in innovative
ways.
Service users can pay for services according to their use rather than their provision.
Therefore, instead of buying an expensive component that is rarely used,
the application writer can use an external service that will be paid for only when
required.
Applications can be made smaller (which is important if they are to be embedded
in other devices) because they can implement exception handling as external
services.
Applications can be reactive and adapt their operation according to their environment
by binding to different services as their environment changes.
At the time of this writing, these advantages have sparked immense interest in
web services as a basis for constructing loosely coupled, distributed applications.
However, there is still limited practical experience with service-oriented architectures
so we do not yet know that practical implications of this approach.
Software reuse has been a topic of research for many years; yet, as I discuss in
Chapters 18 and 19, there remain many practical difficulties in reusing software.
One of the major problems has been that standards for reusable components have
been developed only relatively recently, and several standards are in use. However,
the web services initiative has been driven by standards from its inception, and standards
covering many aspects of web services are under development. The three fundamental
standards that enable communications between web services are:
1. SOAP (Simple Object Access Protocol) This protocol defines an organisation
for structured data exchange between web services.
2. WSDL (Web Services Description Language) This protocol defines how the interfaces
of web services can be represented.
3. UDDI (Universal Description, Discovely and Integration) This is a discovery standard that defines how service description information, used by service
requestors to discover services, can be organised.
All of these standards are based on XML-a human- and machine-readable markup
language (Skonnard and Gudgin, 2002). You don't, however, need to know details
of these standards to understand the web services concept.
Web service application architectures are loosely coupled architectures where service
bindings can change during execution. Some systems will be solely built using web services and others will mix web services with locally developed components.
To illustrate how applications may be organised, consider the following scenario:
An in-car information system provides drivers with information on weather,
road trafJic conditions, local information and so forth. This is linked to the
car radio so that information is delivered as a signal on a specific radio channel.
The car is equipped with GPS receiver to discover its position and, based
on that position, the system accesses a range of information services.
Information may be delivered in the driver's specified language.
Figure 12.18 illustrates a possible organisation for such a system. The in-car soft-
' ware includes five modules. These handle communications with the driver, with a
GPS receiver that reports the car's position and with the car radio. The Transmitter
and Receiver modules handle all communications with external services.
The car communicates with an externally provided mobile information service
which aggregates information from a range of other services that provide information
on weather, traffic information and local facilities. Different providers in different places provide this service, and the in-car system uses a discovery service
to locate the appropriate information service and bind to it. The discovery service
is also used by the mobile information service to bind to the appropriate weather,
traffic and facilities services. Services exchange SOAP messages that include GPS
position information used, by the services, to select the appropriate information. The
aggregated information is passed back to the car through a service that translates
the information language into the driver's language.
KEY PQIMTS
Distributed systems can support resource sharing, openness, concurrency, scalability, fault
tolerance and transparency.
Client-server systems are distributed systems where the system is modelled as a set of
services provided by servers to client processes.
In a client-server system, the user interface always runs on a client, and data management
is always provided by a shared server. Application functionality may be implemented on the
client computer or on the server.
In a distributed object architecture, there is no distinction between clients and servers.
Objects provide general services that may be called on by other objects. This approach may
be used for implementing client-server systems.
Distributed object systems require middleware to handle object communications and to
allow objects to be added to and removed from the system.
The CORBA standards are a set of standards for middleware that supports distributed
object architectures. They include object model definitions, definitions of an object request
broker and common service definitions. Various implementations of the CORBA standards
are available.
Peer-to-peer architectures are decentralised architectures where there are no distinguished
clients and servers. Computations can be distributed over many systems in different
organisations.
Service-oriented systems are created by linking software services provided by various
service suppliers. An important aspect of service-oriented architectures is that binding of
services to the architectural components can be delayed until the system is deployed or is
executing.
This example illustrates one of the key advantages of the service-oriented
approach. It is not necessary to decide when the system is programmed or deployed
what service provider should be used and what specific services could be accessed.
As the car moves around, the in-car software uses the service discovery service to
find the most appropriate information service 'and binds to that. Because of the use
of a translation service, it can move across borders and therefore make local information
availabl
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