In a previous post we used DynamoDBMapper in order to map DynamoDB Tables into Java objects.

When it comes to insert, our actions are pretty much the same but with a more convenient way. In order to insert an item all you have to do is to persist an object using the object mapper

In our case, we will create a User repository that does a simple insert.

package com.gkatzioura.dynamodb.mapper.repository;

import com.amazonaws.services.dynamodbv2.AmazonDynamoDB;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMapper;
import com.gkatzioura.dynamodb.mapper.entities.User;

import java.util.ArrayList;
import java.util.Date;
import java.util.List;

/**
 * Created by gkatzioura on 9/22/16.
 */
public class UserMapperRepository {

    private DynamoDBMapper dynamoDBMapper;

    public UserMapperRepository(AmazonDynamoDB amazonDynamoDB) {
        dynamoDBMapper = new DynamoDBMapper(amazonDynamoDB);
    }

    public void insert(User user) {

        dynamoDBMapper.save(user);
    }

}

To persist we just have to create a simple object.

    @Test
    public void testInsertUser() {

        User user = new User();
        user.setRegisterDate(new Date().getTime());
        user.setFullName("John Doe");
        user.setEmail("john@doe.com");

        userMapperRepository.insert(user);
    }

Also using DynamoDBMapper we can do batch inserts or batch deletes. Therefore we will add two extra methods to the repository.

    public void insert(List<User> users) {

        dynamoDBMapper.batchWrite(users,new ArrayList<>());
    }

    public void delete(List<User> users) {
        dynamoDBMapper.batchDelete(users);
    }

Adding items in batch (or deleting) them, simply requires to pass a list of objects that contains values for the keys defined.

    @Test
    public void testBatchUserInsert() {

        List<User> users = new ArrayList<>();

        for(int i=0;i<10;i++) {

            String email = emailPrefix+i+"@doe.com";
            User user = new User();
            user.setRegisterDate(new Date().getTime());
            user.setFullName("John Doe");
            user.setEmail("john@doe.com");
            users.add(user);
        }

        userMapperRepository.insert(users);
    }

    @Test
    public void testBatchDelete() {

        testBatchUserInsert();

        List<User> users = new ArrayList<>();

        for(int i=0;i<10;i++) {

            String email = emailPrefix+i+"@doe.com";
            User user = new User();
            user.setRegisterDate(new Date().getTime());
            user.setFullName("John Doe");
            user.setEmail("john@doe.com");
            users.add(user);
        }
        
        userMapperRepository.delete(users);
    }

You can find the sourcecode on github

Previously we created DynamoDB Tables using Java.

For various databases such sql databases or nosql there is a set of tools that help to access, persist, and manage data between objects/classes and the underlying database. For example for SQL databases we use JPA, for Cassandra we use MappingManager.

DynamoDBMapper is a tool that enables you to access your data in various tables, perform various CRUD operations on items, and execute queries and scans against tables.

We will try to map the Users, Logins, Supervisors and companies tables from the previous example.
Users is a simple table using the users email as a Hash key.

package com.gkatzioura.dynamodb.mapper.entities;

import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBAttribute;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBHashKey;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBTable;

/**
 * Created by gkatzioura on 9/20/16.
 */
@DynamoDBTable(tableName="Users")
public class User {

    private String email;
    private String fullName;

    @DynamoDBHashKey(attributeName="email")
    public String getEmail() {
        return email;
    }

    @DynamoDBAttribute(attributeName="fullname")
    public void setEmail(String email) {
        this.email = email;
    }

    public String getFullName() {
        return fullName;
    }

    public void setFullName(String fullName) {
        this.fullName = fullName;
    }
}

However in various cases our DynamoDB Table uses a hash and a range key. The Logins table keeps track of the login attempts of a user. The email is the hash key and the timestamp the range key.

package com.gkatzioura.dynamodb.mapper.entities;

import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBHashKey;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBRangeKey;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBTable;

/**
 * Created by gkatzioura on 9/20/16.
 */
@DynamoDBTable(tableName="Logins")
public class Login {

    private String email;
    private Long timestamp;

    @DynamoDBHashKey(attributeName="email")
    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    @DynamoDBRangeKey(attributeName="timestamp")
    public Long getTimestamp() {
        return timestamp;
    }

    public void setTimestamp(Long timestamp) {
        this.timestamp = timestamp;
    }
}

Another popular case are tables with global secondary indexes (GSI). For example the Supervisors table is used to retrieve a supervisor by his name. However we also use this table in order to retrieve all the supervisors from a specific company or the supervisors who work on specific factory of a company.
The supervisor name is our hash key, the company name is the hash key and the factory name is the range key of the global secondary index.

package com.gkatzioura.dynamodb.mapper.entities;

import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBHashKey;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBIndexHashKey;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBIndexRangeKey;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBTable;

/**
 * Created by gkatzioura on 9/21/16.
 */
@DynamoDBTable(tableName="Supervisors")
public class Supervisor {

    private String name;
    private String company;
    private String factory;

    @DynamoDBHashKey(attributeName="name")
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @DynamoDBIndexHashKey(globalSecondaryIndexName = "FactoryIndex",attributeName = "company")
    public String getCompany() {
        return company;
    }

    public void setCompany(String company) {
        this.company = company;
    }

    @DynamoDBIndexRangeKey(globalSecondaryIndexName = "FactoryIndex",attributeName = "factory")
    public String getFactory() {
        return factory;
    }

    public void setFactory(String factory) {
        this.factory = factory;
    }
}

Last but not least we can use Local Secondary Indexes. The Companies table uses the company name as a hash key and the subsidiary name as a range key. Since we want to issue queries based on a company’s CEOs a local secondary index is used with a range key based on the name of the CEO.

package com.gkatzioura.dynamodb.mapper.entities;

import com.amazonaws.services.dynamodbv2.datamodeling.*;

/**
 * Created by gkatzioura on 9/21/16.
 */
@DynamoDBTable(tableName="Companies")
public class Company {

    private String name;
    private String subsidiary;
    private String ceo;

    @DynamoDBHashKey(attributeName="name")
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @DynamoDBRangeKey(attributeName = "subsidiary")
    public String getSubsidiary() {
        return subsidiary;
    }

    public void setSubsidiary(String subsidiary) {
        this.subsidiary = subsidiary;
    }

    @DynamoDBIndexRangeKey(localSecondaryIndexName = "CeoIndex",attributeName = "ceo")
    public String getCeo() {
        return ceo;
    }

    public void setCeo(String ceo) {
        this.ceo = ceo;
    }
}

You can find the source code on github.

On a previous post we proceeded into inserting items to DynamoDB using Java. DynamoDB also supports updating items.

We will use the Login table for the update examples.
When issuing an update you must specify the primary key of the item you want to update.

    public void updateName(String email,String fullName) {

        Map<String,AttributeValue> attributeValues = new HashMap<>();
        attributeValues.put("email",new AttributeValue().withS(email));
        attributeValues.put("fullname",new AttributeValue().withS(fullName));

        UpdateItemRequest updateItemRequest = new UpdateItemRequest()
                .withTableName(TABLE_NAME)
                .addKeyEntry("email",new AttributeValue().withS(email))
                .addAttributeUpdatesEntry("fullname",
                        new AttributeValueUpdate().withValue(new AttributeValue().withS(fullName)));

        UpdateItemResult updateItemResult = amazonDynamoDB.updateItem(updateItemRequest);
    }

We can proceed on more advanced statements using conditional updates. Conditional updates can help us in many cases such as handling concurrent updates.

We can achieve so by using plain expressions.

    public void updateConditionallyWithExpression(String email,String fullName,String prefix) {

        Map<String, AttributeValue> key = new HashMap<>();
        key.put("email", new AttributeValue().withS(email));

        Map<String, AttributeValue> attributeValues = new HashMap<>();
        attributeValues.put(":prefix", new AttributeValue().withS(prefix));
        attributeValues.put(":fullname", new AttributeValue().withS(fullName));

        UpdateItemRequest updateItemRequest = new UpdateItemRequest()
                .withTableName(TABLE_NAME)
                .withKey(key)
                .withUpdateExpression("set fullname = :fullname")
                .withConditionExpression("begins_with(fullname,:prefix)")
                .withExpressionAttributeValues(attributeValues);
        UpdateItemResult updateItemResult = amazonDynamoDB.updateItem(updateItemRequest);
    }

Or through by specifying attributes.

    public void updateConditionallyWithAttributeEntries(String email, String fullName, String prefix){

        Map<String,AttributeValue> key = new HashMap<>();
        key.put("email",new AttributeValue().withS(email));

        UpdateItemRequest updateItemRequest = new UpdateItemRequest()
                .withTableName(TABLE_NAME)
                .withKey(key)
                .addAttributeUpdatesEntry("fullname",new AttributeValueUpdate().withValue(new AttributeValue().withS(fullName)).withAction(AttributeAction.PUT))
                .addExpectedEntry("fullname",new ExpectedAttributeValue().withValue(new AttributeValue().withS(prefix)).withComparisonOperator(ComparisonOperator.BEGINS_WITH));

        UpdateItemResult updateItemResult = amazonDynamoDB.updateItem(updateItemRequest);
    }

Another feature is atomic counters. We can issue updates to a DynamoDB item and increase the attribute values. We will add an extra field called count. Also we will add another update function, which once called will update the field specified, but will also increase the counter attribute. Thus the counter attribute will represent how many times an update was performed on a specific item.

    public void addUpdateCounter(String email) {

        Map<String,AttributeValue> key = new HashMap<>();
        key.put("email",new AttributeValue().withS(email));

        UpdateItemRequest updateItemRequest = new UpdateItemRequest()
                .withTableName(TABLE_NAME)
                .withKey(key)
                .addAttributeUpdatesEntry("counter",new AttributeValueUpdate().withValue(new AttributeValue().withN("0")).withAction(AttributeAction.PUT));

        UpdateItemResult updateItemResult = amazonDynamoDB.updateItem(updateItemRequest);
    }

    public void updateAndIncreaseCounter(String email,String fullname) {

        Map<String,AttributeValue> key = new HashMap<>();
        key.put("email",new AttributeValue().withS(email));

        UpdateItemRequest updateItemRequest = new UpdateItemRequest()
                .withTableName(TABLE_NAME)
                .withKey(key)
                .addAttributeUpdatesEntry("fullname",new AttributeValueUpdate().withValue(new AttributeValue().withS(fullname)).withAction(AttributeAction.PUT))
                .addAttributeUpdatesEntry("counter",new AttributeValueUpdate().withValue(new AttributeValue().withN("1")).withAction(AttributeAction.ADD));

        UpdateItemResult updateItemResult = amazonDynamoDB.updateItem(updateItemRequest);
    }

You can find the sourcecode on github.

On previous posts we covered how to query a DynamoDB database
Query DynamoDB Part 1
Query DynamoDB Part2.

Apart from issuing queries DynamoDB also offers Scan functionality.
What scan does is fetching all the Items you might have on your DynamoDB Table.
Therefore scan does not require any rules based on our partition key or your global/local secondary indexes.
What scan offers is filtering based on the items already fetched and return specific attributes from the items fetched.

The snippet below issues a scan on the Logins table by adding filtering and selecting only the email field.

public List<String> scanLogins(Date date) {

        List<String> emails = new ArrayList<>();

        Map<String, String> attributeNames = new HashMap<String, String >();
        attributeNames.put("#timestamp", "timestamp");

        Map<String, AttributeValue> attributeValues = new HashMap<String, AttributeValue>();
        attributeValues.put(":from", new AttributeValue().withN(Long.toString(date.getTime())));

        ScanRequest scanRequest = new ScanRequest()
                .withTableName(TABLE_NAME)
                .withFilterExpression("#timestamp < :from")
                .withExpressionAttributeNames(attributeNames)
                .withExpressionAttributeValues(attributeValues)
                .withProjectionExpression("email");

        Map<String,AttributeValue> lastKey = null;

        do {

            ScanResult scanResult = amazonDynamoDB.scan(scanRequest);

            List<Map<String,AttributeValue>> results = scanResult.getItems();
            results.forEach(r->emails.add(r.get("email").getS()));
            lastKey = scanResult.getLastEvaluatedKey();
            scanRequest.setExclusiveStartKey(lastKey);
        } while (lastKey!=null);

        return emails;
    }

Before using scan to an application we have to take into consideration that scan fetches all table items. Therefore It has a high cost both on charges and performance. Also it might consume your provision capacity.
It is better to stick to queries and avoid scans.

You can find the sourcecode on github.

I’ve compiled a cheat sheet that lists dynamodb functions in Java
Sign up in the link to receive it.