每个区域的单例实例

Question

I have a Dynamo DB DAO class, which takes the region like us-east-1, us-east-2 etc., to instantiate the object and interacts with DDB. Now I am processing a stream of messages, which contains this region value along with other payload to be written to DDB. I want to ensure a single instance of DAO object is created per region. Currently I have created a map holding all the Dao instances per region and using it for each request to achieve this.

Sample code that I'm using looks like below.

public class DDBDao {

        private DynamoDBMapper dynamoDBMapper;

        public DDBDao(final string region) {
            AmazonDynamoDB dynamoDBClient = AmazonDynamoDBClientBuilder.standard()
                    .withRegion(Regions.fromName(region))
                    .build();
           this.dynamoDBMapper = new DynamoDBMapper(dynamoDBClient);

        }

        public save(..) {
          dynamoDBMapper.save(...)
        }
       ....
    } 


    @Singleton
    public class DaoContainer {
        Map<String, DDBDao> daoContainer = new HashMap<>();

        DaoContainer() {
            daoContainer.put("us-east-1", new DDBDao("us-east-1"));
            daoContainer.put("us-east-2", new DDBDao("us-east-2"));
            .....

        }

    }

I create a instance of DaoContainer and get the DDBDao for the given region to interact with DynamoDB. What is the best way to create singleton instances of DDBDao per region?

Answer 1

I would suggest implementing custom Region scoped bean, this works exactly same as request/ session scope beans except spring will maintain bean object per Region.

org.springframework.beans.factory.config.Scope is an interface and by implementing it one can create a custom scope in the spring container

public class RegionScope implements Scope {
    private final ThreadLocal regionScope = new ThreadLocal() {
        protected Object initialValue() {
            return new HashMap();
        }
    };

    public Object get(String name, ObjectFactory objectFactory) {
        Map scope = (Map) regionScope.get();
        Object object = scope.get(name);
        if (object == null) {
            object = objectFactory.getObject();
            scope.put(name, object);
        }
        return object;
    }

    public Object remove(String name) {
        Map scope = (Map) regionScope.get();
        return scope.remove(name);
    }



}