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[[Image:Service Layers Image A.JPG|thumb|alt=Diagram A|Diagram A<br/>In the absence of any layers, services contain a mixture of different types of logic. This makes it difficult to manage these services.]]
[[Image:Service Layers Image A.JPG|thumb|alt=Diagram A|Diagram A<br/>In the absence of any layers, services contain a mixture of different types of logic. This makes it difficult to manage these services.]]
[[Image:Service Layers Image B.JPG|thumb|alt=Diagram B|Diagram B<br/>A service inventory divided into layers where each layer contains the same type of logic.]]
[[Image:Service Layers Image B.JPG|thumb|alt=Diagram B|Diagram B<br/>A service inventory divided into layers where each layer contains the same type of logic.]]
Applying this pattern requires creating a service inventory blueprint,<ref name='serviceinventoryblueprint'>
In order to apply this pattern, first it needs to be established which different types of layers are required. This requires creating a service inventory blueprint<ref name='serviceinventoryblueprint'>[http://www.soamethodology.com/p2.php service inventory blueprint]</ref>: a pool of services consisting of candidate services containing candidate functionality. By creating such an inventory, enough information is available to find out the different types of functionality within the intended service inventory. Based on this information, the required types of layers can be established. On the other hand, by applying this pattern at this stage within the service delivery process, the design of the service can be modified so that it contains the relevant type of logic as dictated by the type of the service layer under which this particular service falls.
[http://www.soamethodology.com/p2.php service inventory blueprint]</ref> a list of services with associated functionality. Next, group the services into layers according to function. Adopting a common layering strategy across the enterprise facilitates reuse in other applications, because developers don't have as much to learn (or invent) when they join a project.<ref name='service models'>[http://www.soamethodology.com/p5.php service models]</ref> One common layering uses task,<ref name='task'>[http://www.soamethodology.com/p7.php Task Service]</ref> [[Entity Abstraction Pattern|entity]] <ref name='entity'>[http://www.soamethodology.com/p6.php Entity Service]</ref> and [[Utility Abstraction Pattern|utility]] <ref name='utility'>[http://www.soamethodology.com/p8.php Utility Service]</ref> layers.<ref name='ProcessAbstraction'>
<br/>
[http://www.soapatterns.org/process_abstraction.php Process Abstraction]</ref><ref name='WK'>
Although service grouping can be performed based different types of functionalities, however, to keep the grouping standardized across the enterprise, the actual groups can be based on established service models<ref name='service models'>[http://www.soamethodology.com/p5.php service models]</ref> that depict the most common types of logic that services would normally contain. Depending upon the particular area of the business, a service inventory would usually be divided into task<ref name='task'>[http://www.soamethodology.com/p7.php Task Service]</ref>, entity<ref name='entity'>[http://www.soamethodology.com/p6.php Entity Service]</ref> and utility<ref name='utility'>[http://www.soamethodology.com/p8.php Utility Service]</ref> services. Each of these different types of service models bear specific characteristics that would be eventually demonstrated by the services that belong to the layer, which is based on a particular service model. To design a service based on the aforementioned service models the Process Abstraction<ref name='ProcessAbstraction'>[http://www.soapatterns.org/process_abstraction.php Process Abstraction]</ref>, the [[Entity Abstraction Pattern|Entity Abstraction]] and the [[Utility Abstraction Pattern|Utility Abstraction]] design patterns can be applied as these design patterns help in structuring the solution logic of the services according to specific types<ref name='WK'>Wajid Khattak [http://www.informit.com/articles/article.aspx?p=1583177 Service Layers][Online].Date accessed: 21 April 2010.</ref>.The aforementioned service types are by no means concrete and an SOA could be categorized into a different set of service layers depending upon which characteristics constitute a particular layer, for example, as argued by Bieberstein et al.<ref>Bieberstein. et al.[http://books.google.com/books?id=NISyExeJ5mAC&pg=PA88&dq=%22service+layer%22&lr=&hl=sv#PPA87,M1 Service-oriented Architecture Compass: Business Value, Planning, and Enterprise Roadmap].FT Press, 2005. ISBN 0-13-187002-5, 9780131870024</ref>, an SOA could be divided in to five different layers, namely Enterprise, Process, Service, Component and Object layers.
Wajid Khattak [http://www.informit.com/articles/article.aspx?p=1583177 Service Layers][Online].Date accessed: 21 April 2010.</ref>.


An alternative layering from Bieberstein et al.<ref>Bieberstein. et al.[http://books.google.com/books?id=NISyExeJ5mAC&pg=PA88&dq=%22service+layer%22&lr=&hl=sv#PPA87,M1 Service-oriented Architecture Compass: Business Value, Planning, and Enterprise Roadmap].FT Press, 2005. ISBN 0-13-187002-5, 9780131870024</ref>, involves five layers, namely Enterprise, Process, Service, Component and Object.
The application of the Service Layers pattern would necessitate a change in the architecture of the service and the overall architecture of the service inventory<ref name="SvcArch">[[Service-Oriented Architecture Types|SOA Types]]</ref>.

The Service Layers pattern invokes a specific [[Service-Oriented Architecture Types|service architecture]].


==Considerations==
==Considerations==

Revision as of 23:30, 14 September 2010

Service Layers is a design pattern, applied within the service-orientation design paradigm, which aims to organize the services[1], within a service inventory[2], into a set of logical layers. Services that are categorized into a particular layer share functionality. This helps to reduce the conceptual overhead related to managing the service inventory, as the services belonging to the same layer address a smaller set of activities.

Rationale

Grouping services into functional layers reduces the impact of change. Most changes affect only the layer in which they're made, with few side-effects that impact other layers. This fundamentally simplifies service maintenance.

The Service Reusability principle dictates that services should be designed to maximize reuse. Similarly, the Service Composability principle advocates designing services so that they can be composed into various ways.[3] Both principles require that a service contain only a specific type of logic e.g., either reusable or process-specific logic.[4] Restricting each layer to a particular functionality, simplifies the design of the service.[5]

Usage

Diagram A
Diagram A
In the absence of any layers, services contain a mixture of different types of logic. This makes it difficult to manage these services.
Diagram B
Diagram B
A service inventory divided into layers where each layer contains the same type of logic.

Applying this pattern requires creating a service inventory blueprint,[6] a list of services with associated functionality. Next, group the services into layers according to function. Adopting a common layering strategy across the enterprise facilitates reuse in other applications, because developers don't have as much to learn (or invent) when they join a project.[7] One common layering uses task,[8] entity [9] and utility [10] layers.[11][5].

An alternative layering from Bieberstein et al.[12], involves five layers, namely Enterprise, Process, Service, Component and Object.

The Service Layers pattern invokes a specific service architecture.

Considerations

The application of this design pattern depends upon having enough knowledge about the kind of services in a service inventory before they are actually developed. Consequently, a top-down service delivery approach[13] needs to be adopted so that a pool of candidate services exists from which the need for different service layers can be established. This will increase the time and efforts required to actually deliver a set of usable services. Secondly, the confidence with which the need for different types of service layers can be established is directly proportional to the size of the service inventory. This means that as more and more services are added to the service inventory, the already established service layers may need to be modified in case new services do not fit into these existing service layers.

References

  1. ^ services
  2. ^ service inventory
  3. ^ service compositions
  4. ^ Thomas Erl.Introducing SOA Design Pattern[Online].Date accessed:6 April 2010.
  5. ^ a b Wajid Khattak Service Layers[Online].Date accessed: 21 April 2010.
  6. ^ service inventory blueprint
  7. ^ service models
  8. ^ Task Service
  9. ^ Entity Service
  10. ^ Utility Service
  11. ^ Process Abstraction
  12. ^ Bieberstein. et al.Service-oriented Architecture Compass: Business Value, Planning, and Enterprise Roadmap.FT Press, 2005. ISBN 0-13-187002-5, 9780131870024
  13. ^ Top-Down Service Delivery Approach
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