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When the data is sharded across multiple domains, each call PutAttributes, GetAttributes, and DeleteAttributes will need to be routed to the proper domain Each of these operations requires an item name and exactly one of the domains will be the proper location To find that location, the item name must be run though the hash function and then mapped to the domain responsible for that hash The implementation of the domain routing requires the hash function, the number of bits to examine, and the name of the domains in useThe domain names can use a naming convention as in the example of user_0 and so on However, it is better not to depend on a naming convention and instead use configurationThe domain names may
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need to change for administrative reasons like the restoration from a backup Explicitly configured domain naming is more flexible The code that implements the routing will need to stand between the application and the normal SimpleDB client It could be a wrapper class for the client, selecting the domain before passing the data onto the client codeThis wrapper code requires access to the application configuration that specifies the parameters of the hashing and domain mapping Listing 11-3 shows what a class might look like that holds this configuration data
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Listing 11-3 AppConfigjava A Class to Hold the Application Configuration
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package comsimpledbbookmetadata; import javautil*; public class AppConfig { private private private private List<String> domainList; int bitCount; int bitMask; String serviceURL;
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private AppConfig(String appName, int bits, String domains) { bitCount = bits; bitMask = (int) (Mathpow(2, bitCount) - 1); domainList = ArraysasList(domains); } public String getDomain(int hash) { return domainListget(hash & bitMask); } public String getServiceURL() { return serviceURL; } }
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The configuration would need to be stored somewhere accessible to the application and be loaded upon initializationThe data is passed into the constructor and consists of an application name, the number of hash bits to use, and an ordered list of domains for the mapping A derived field named bitMask is created in the constructorThe purpose of the bit mask is to encode the number of bits currently in use into a mask that easily allows a hash value to be converted into a list index The getDomain() method demonstrates the use of the bit maskThe hash value is passed into the method as an argumentWithin the method, the hash value is combined
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11 Improving the SimpleDB Client
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with the bit mask using the Java bitwise AND (&) operatorThe result of this operation is a numeric index corresponding to a domain in the listThe index is used to look up and return the name of the domain Listing 11-4 shows the implementation of a sharding wrapper class for the client developed in the last chapterThe wrapper class uses the AppConfig from Listing 11-3 to route all the gets, puts, and deletes to the proper domain
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Listing 11-4 ShardingClientjavaA SimpleDB Client Wrapper Class for Sharding
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package comsimpledbbookmetadata; import comsimpledbbook*; public class ShardingClient { private final SimpleDB sdb; private AppConfig config; public ShardingClient(AppConfig appConfig) { config = appConfig; sdb = new SimpleDB(configgetServiceURL()); } public void put(Item item) { sdbput(domainForItemName(itemgetName()), item); } public void putIfNotExists(Item item, String expectedName) { String domain = domainForItemName(itemgetName()); sdbputIfNotExists(domain, item, expectedName); } public void putIfExists(Item item, String expectedName, String expectedValue) { String domain = domainForItemName(itemgetName()); sdbputIfExists(domain, item, expectedName, expectedValue); } public void delete(Item item) { sdbdelete(domainForItemName(itemgetName()), item); } public void deleteIfNotExists(Item item, String expectedName) { String domain = domainForItemName(itemgetName()); sdbdeleteIfNotExists(domain, item, expectedName); }
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public void deleteIfExists(Item item, String expectedName, String expectedValue) { String domain = domainForItemName(itemgetName()); sdbdeleteIfExists(domain, item, expectedName, expectedValue); } public Item get(String itemName, String atts) { return sdbget(domainForItemName(itemName), itemName, atts); } public Item getConsistent(String itemName, String atts) { return sdbget(domainForItemName(itemName), itemName, atts); } private String domainForItemName(String itemName) { int hash = ConsistentHashhash(itemName); return configgetDomain(hash); } }
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This sharding client takes an AppConfig instance in the constructor and initializes a SimpleDB object for use in request forwarding Notice that each put, get, and delete method from the original client is present and that the method signatures are identical except that the domain name parameter is absent Each method makes an initial call to domainForItemName()The bottom of Listing 11-4 shows the implementation of that method First, a consistent hash value is computed for the item name and then the corresponding domain name is looked up and returned using the saved AppConfig instanceThe referenced class ConsistentHash is fictitious and not providedAll that is needed is the implementation of a hash function that returns values that are well distributed With the appropriate domain name in place, each of the public methods has all the necessary parameters to call the underlying SimpleDB clientAn instance of this class would be initialized and used instead of the vanilla SimpleDB client
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