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Let s take a stab at the primary interface for this client Listing 10-1 shows ISimpleDB, a Java interface with abstract methods for all of the key functionsThis interface is not intended as a necessary part of the client, although there is no reason why it could not be Its purpose is to present the API of the client in a way that allows the initial discussion of the concepts without the cognitive load of the full implementation
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Listing 10-1 ISimpleDBjava The Public Methods of the Client Interface
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package comsimpledbbook; import javautilList; import javautilMap; public interface ISimpleDB {
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void createDomain(String domainName); void deleteDomain(String domainName); List<String> listDomains(); Map<String, Long> domainMetadata(String domain); void batchPut(String domain, List<Item> items); void put(String domain, Item item); void putIfNotExists(String domain, Item item, String expectedName); void putIfExists(String domain, Item item, String expectedName, String expectedValue); void delete(String domain, Item item); void deleteIfNotExists(String domain, Item item, String expected); void deleteIfExists(String domain, Item item, String expectedName, String expectedValue); Item get(String domain, String itemName, String atts); Item getConsistent(String domain, String itemName, String atts); List<Item> select(String select); List<Item> selectConsistent(String select); boolean hasNextPage(); List<Item> getNextPage(); List<Item> getNextPageConsistent(); }
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One thing you can see in this class is that built-in Java collections and types are returned from the methods in combination with the container class Item and its attribute childrenThis keeps the usage simple and minimizes the dependencies One example of this is the listDomains() method that invokes the ListDomains operationThis method returns the list of SimpleDB domains, each in the form of a Java String A second example is the domainMetadata() method, which returns values in the form of a Java Map of String names to Long values Custom classes could be added here to add
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10 Writing a SimpleDB Client: A Language-Independent Guide
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another layer of API abstraction and allow more data to be returned for instance, the box usage value contained in the SimpleDB responseThat approach results in a more capable client by way of empowering the user with advanced features like the ability to track box usage However, advanced features must be weighed against the extra complexity and boilerplate code it might require Providing a way for users to access everything that comes back in a response is important, but it doesn t need to be done at the level of the individual call Logging to a file is one option; embedding the full response within an exception for error conditions is another Continuing to look down through the class, the next four methods make use of abbreviated naming put(), batchPut(), get(), and delete() correspond to the SimpleDB operations PutAttributes, BatchPutAttributes, GetAttributes, and DeleteAttributesThis is done here to keep the line length down for the constraints of the printed page It would be better to use names that match the operations identically, to avoid any confusion The remaining methods in this class expose the query functionality Unlike the other operations, for which there is a one-to-one correspondence between operations and methods, this sample client defines four methods for SelectThe first has a single parameter for the query expression, whereas the second adds an additional parameter for passing the NextTokenYou will notice that the return type for both methods is an Item list and that no NextToken is ever actually returned to the callerThis design choice is justified in basic use cases, if not in advance situationsThe caller has no use for a NextToken other than to pass it back to a subsequent call It is purely a bookkeeping construct, and requiring callers to manage it explicitly is inconvenient The inconvenience can be understood considering any query in SimpleDB that runs too long can return with a NextToken along with as few as zero resultsThis means that proper handling of a query will require the user to code NextToken handling at the site of every select() callThe following chapter will present a more substantial solution to this problemThis chapter resolves it in a basic way, by implementing some rudimentary NextToken handling and providing a method to check for its presence called hasNextPage() and a method to resubmit the same query with the NextToken called getNextPage()To support these methods, the class declares the private members nextToken and selectThese store the values from the immediately prior call to select()
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