Asynchronous Interactions in Java

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They are more loosely coupled They tend to be more scalable Fewer network resources are held for long periods of time
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Although there is no hard-and-fastrule, non-blocking operations should be used for remotable services when possible
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Although there is no hard-and-fast-rule, non-blocking operations should be used for remotable services when possible To see how this is done, we will modify the CreditServicecheckCredit() operation to be non-blocking Calculating a credit rating may be time-consuming, and by performing this operation asynchronously, the LoanComponent component can continue with other tasks In addition, BigBank may decide in the future to use multiple CreditService implementations that rely on different credit bureaus Making the service asynchronous will allow the LoanComponent to issue multiple calls in succession without having to wait for each to complete Specifying a non-blocking operation using Java is straightforward: Mark a method on an interface with the @OneWay annotation Listing 36 demonstrates a service contract with a non-blocking operation:
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Listing 36
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De ning a Non-Blocking Operation
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Specifying a nonblocking operation using Java is straightforward: Mark a method on an interface with the @OneWay annotation
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import orgosoascaannotationsOneWay; @Remotable public interface CreditService { @OneWay void checkCredit(String id); }
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It is important to note that SCA places two restrictions on nonblocking operations First, they must have a void return type (We cover how to return responses using callbacks in the next section) Non-blocking operations must also not throw exceptions Listing 37 shows how the CreditService is called from a client:
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Listing 37
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SCA places two restrictions on nonblocking operations: They must have a void return type, and they must not throw exceptions
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Calling a Non-Blocking Operation
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public class LoanComponent implements LoanService { private CreditService creditService;
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Service-Based Development Using Java public void LoanComponent (@Reference CreditService service){ thisservice = service; } public LoanResult apply(LoanRequest request) { creditServicecheckCredit(requestgetId()); // continue without waiting for the call to complete }
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In the previous example, when the call to CreditService checkCredit() is made, the runtime will return control immediately to the LoanComponent without waiting for the call to the CreditService to complete
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Why @OneWay Instead of @NonBlocking
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You may wonder why the SCA authors chose to name the annotation that de nes non-blocking operations @OneWay instead of @NonBlocking The reason has to do with the vocabulary used in the world of protocol standards Message exchange patterns, or MEPs, de ne the interaction pattern a protocol uses to communicate between two participants There are two basic MEP types: request-response and one-way TCP, for example, uses the former, whereas UDP uses the latter Because remote service calls are ultimately sent via a protocol, the SCA authors wanted to be precise in their terminology
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How does an SCA runtime implement non-blocking behavior This depends in part on whether the client and service provider are colocated or hosted in different processes For a local call (that is, between a co-located client and provider), the runtime will execute the invocation on a different thread If the call is remote, the runtime will ensure that the underlying communications infrastructure uses asynchronous (one-way) delivery This can be trivial with some communications mechanisms such as JMS, which are inherently asynchronous However, it may be more involved with others that
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Callbacks
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are synchronous, such as RMI/IIOP In these cases, the runtime may need to take extra steps (such as using a different thread) to ensure that the calls are sent in a non-blocking manner Reliability One issue that often comes up with non-blocking invocations is reliability Namely, given that the invocation is performed asynchronously and there is no return value, how does a client know if the target service successfully received the invocation Reliable delivery is often achieved via the underlying communications channel For example, an SCA runtime could use JMS or messaging middleware to send an invocation to a target service As we cover at length in 7, Wires, a client can place requirements such as reliable delivery on its communications with other services through the use of policy Exception Handling Because one-way invocations return immediately without waiting for the service provider to complete processing an invocation, exceptions cannot be thrown and returned to the client Instead, exceptions should be passed back to a client via a callback, the subject of the next section After we have covered callbacks, we will return to a discussion on how to use them for error handling with non-blocking invocations
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