Designing Remotable Services in Java

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In SCA, remotable services are made available to multiple clients across process boundaries These clients may be components in the same domain or, if the service is exposed over a binding, another system altogether Although the speci c qualities of well-architected SCA applications will vary, a common indicator of good design is that an application will have only a few remotable services that expose a set of general operations Examples of general operations include apply for loan, get credit rating, inventory check, and place backorder Think of remotable services as an API As with a good API,
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The SCA programming model is based on the goal of protocol abstraction rather than location transparency
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remotable services should be concise, easy to understand, and limited Moreover, they should have the following attributes:
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Remotable services account for the network Remotable services should account for the realities of the physical network they are called over, particularly latency and connectivity interruptions In particular, they should limit the number of exchanges required between a client and service provider Remotable service contracts take versioning into account Remotable service contracts should be evolvable Rarely do APIs get it right in the rst iteration Furthermore, new requirements often arise after an application has gone into production Barring fundamental changes, it should be possible to version services without breaking compatibility with existing clients Remotable services limit the assumptions made about clients Remotable services should limit the assumptions they make about clients Most important, they should not assume that clients will be written in the same language they are written in
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SCA relies on techniques of loose coupling developed by integration technologies in particular, messageoriented middleware (MOM)
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To achieve these qualities, SCA relies on techniques of loose coupling developed by integration technologies in particular, message-oriented middleware (MOM) Loose coupling can take a variety of forms The two most important forms of loose coupling in SCA are coarse-grained services and asynchronous communications We deal with designing coarse-grained service contracts in the next section, followed by a detailed discussion of how SCA allows for asynchronous communications via non-blocking operations and callbacks in subsequent sections
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Perspective: How Loosely Coupled Should Services Be
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A common question that arises when designing service-based applications is how loosely coupled remote communications should be One school of thought says that services should be as loosely coupled as possible and, in order to achieve this, an
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Enterprise Service Bus (ESB) should be used to route messages between all clients and providers ESBs offer the following forms of loose coupling: Target abstraction The capability to dynamically route service requests to service providers based on message content or type Protocol translation The capability to transform a service request from a client over one protocol into the protocol supported by the service provider These capabilities are provided through a service bus that is placed between the client and service provider (see Figure 31)
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Figure 31 provider
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A service bus mediates the communications between a client and service
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When clients are targeted at the bus instead of the actual service provider, it is possible to change the provider by simply changing the bus con guration This can usually be done without any programming, typically through an administration console This gives the added exibility of allowing the bus to use a different protocol to call the service provider than is used to communicate with the client SCA takes a different approach to loose coupling by asserting that services should be no more loosely coupled than necessary Loosely coupled systems are generally more dif cult to write and complex to manage Moreover, introducing mediation can result in an unnecessary and potentially expensive invocation hop As shown in Figure 32, in an ESB, a message is sent from the client to an intermediary and on to the target service, creating three hops
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