DynamoDBMapper仅在唯一时保存项目

Question

I'm trying to keep items in my table unique based on a combination of two different columns.

I have an instanceId and imageId column (along with others) and based on a couple of posts on Stackoverflow and AWS Forums the below should work?

public void saveUnique(Server server) {
    DynamoDBSaveExpression saveExpression = new DynamoDBSaveExpression();
    Map<String, ExpectedAttributeValue> expectedAttributes =
        ImmutableMap.<String, ExpectedAttributeValue>builder()
            .put("instanceId", new ExpectedAttributeValue(false))
            .put("imageId", new ExpectedAttributeValue(false))
            .build();
    saveExpression.setExpected(expectedAttributes);
    saveExpression.setConditionalOperator(ConditionalOperator.AND);
    try {
        mapper.save(server, saveExpression);
    } catch (ConditionalCheckFailedException e) {
        //Handle conditional check
    }
}

However every time I try and save a duplicate item (same instanceId and imageId) it's successfully being saved into the database.

Am I missing anything else here?

EDIT RE notionquest answer

Updating to the answer below.

I have a job that runs once a minute polling an API. The response from the API is represented as a Server POJO. The Server has an attribute named instanceId .

I want to make sure that if a Server with that instanceId is already in the database, don't save it.

The Server object has another attribute of id which is set as the table primary key.

public void saveUnique(Server server) {
    DynamoDBSaveExpression saveExpression = new DynamoDBSaveExpression();

    Map<String, ExpectedAttributeValue> expected = new HashMap<>();
    expected.put("instanceId", new ExpectedAttributeValue(new AttributeValue(server.getInstanceId())).withComparisonOperator(ComparisonOperator.NE));
    saveExpression.setExpected(expected);

    try {
        mapper.save(server, saveExpression);
    } catch (ConditionalCheckFailedException e) {
        LOGGER.info("Skipped saving as not unique...");
    }
}

This code will save the Server object over and over again with the exception never being thrown.

The Server POJO

@DynamoDBTable(tableName = "Servers")
public class Server {

    @Id
    private String id = UUID.randomUUID().toString();

    @DynamoDBTypeConvertedJson
    private Task task;

    @DynamoDBAttribute(attributeName = "instanceId")
    private String instanceId;

    public Server() {
    }

    @DynamoDBHashKey
    public String getId() {
        return id;
    }

    // other standard getters and setters
}

Answer 1

If I correctly understood the question, you'd like to ensure uniqueness for a field which is not the hash key.

(I'm not sure why you do not use instanceId as the hash key for Servers table, I guess you have a reason for that).

My answer: looks like you can't do that without using an auxiliary table .

Here is your existing Servers table:

+----------------------------------------------+
|                 Servers                      |   
+----------------------------------------------+
| * id            the hash key                 |
| * instanceId    non-key field, must be unique|
|                                              |
| * ...                                        |
| * other fields                               |
| * ...                                        | 
+----------------------------------------------+

I would create an additional table with instanceId as the hash key:

+----------------------------------------------+
|                 Instance                     |   
+----------------------------------------------+
| * instanceId    the hash key                 |
+----------------------------------------------+

Having such a table, before saving a record into Servers, you have to ensure first that instanceId value is unique by adding a record (putItem) into Instance, providing a ConditionExpression like attribute_not_exists(instanceId) .

And only if the put operation completes without an ConditionalCheckFailedException error, you can proceed with adding the record into Servers.

If you'd like to ensure uniqueness of records in Servers based on combination of two fields, instanceId and imageId, instead of just instanceId use concatenated values of these fields as a single field in your aux table:

+----------------------------------------------+
|               Instance_Image                 |   
+----------------------------------------------+
| * instanceId_imageId   the hash key          |
+----------------------------------------------+

Answer 2

The below code checks whether the attribute value is NOT equal to "somevalue" and updates the item if the condition is satisfied.

Map<String, ExpectedAttributeValue> expected = new HashMap<>();     
expected.put("yourattribute", new ExpectedAttributeValue(new AttributeValue("yourvalue")).withComparisonOperator(ComparisonOperator.NE));