DynamoDBMapper saves an item only if it is unique

Question

I am trying to keep the elements in my table unique based on a combination of two different columns.

Do I have an instanceId and imageId column (along with others) and based on several posts on https://stackoverflow.com/a/21922/ ... and should the AWS forums below 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 to save a duplicate of an element (the same instanceId and imageId), it is successfully saved in the database.

Did I miss anything else here?

EDIT RE notionquest answer

Update answer below.

I have a job that runs an API poll once a minute. The API response is presented as a ServerPOJO. Serverhas an attribute with a name instanceId.

, Server instanceId , .

Server id, .

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...");
    }
}

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

2019

2019 , , , . DynamoDB. , Amazon: https://aws.amazon.com/blogs/database/simulating-amazon-dynamodb-unique-constraints-using-transactions/

, , , .

, , , instance_id = 2c5d0cc8d900 , "instance_id # 2c5d0cc8d900". , .

+--------------------------+-------------------------+--------+----------------
|           id             |       instance_id       | attr1  | other fields... 
|                          |                         |        |
|      (primary key)       |     (a non-key field,   |        |
|                          |     must be unique)     |        |
+--------------------------+-------------------------+--------+----------------
| instance_id#2c5d0cc8d900 |                         |        |
| a9fd702a                 | 2c5d0cc8d900            | qwerty | ...

, , . , , , .

---

, , -.

( , instanceId - , , ).

: , .

:

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

instanceId -:

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

, , instanceId , (putItem) Instance, ConditionExpression, attribute_not_exists(instanceId).

put ConditionalCheckFailedException, .

, instanceId imageId, instanceId :

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

Answer 2

, "somevalue" , .

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