DynamoDb. Conditional write on Create. Check by the filed if NotEqual (NE)

问题

Say, I want to save/create new item to the DynamoDb table, if and only if there is not any existent item already that that would contain the referenceId equal to the same value I set.

In my case I want to create a item with withReferenceId=123 if there is not any other withReferenceId=123 in the table.

the referenceId is not primary key! (I don not want it to be it)

So the code:

 val withReferenceIdValue = "123";
 val saveExpression = new DynamoDBSaveExpression();

    final Map<String, ExpectedAttributeValue> expectedNoReferenceIdFound = new HashMap();
    expectedNoReferenceIdFound.put(
            "referenceId",
            new ExpectedAttributeValue(new AttributeValue().withS(withReferenceIdValue)).withComparisonOperator(ComparisonOperator.NE)
    );
    saveExpression.setExpected(expectedNoReferenceIdFound);

    newItemRecord.setReferenceId(withReferenceId);

    this.mapper.save(newItemRecord, saveExpression); // do not fail..

That seems does not work.

I the table has the referenceId=123 already the save() does not fail. I expected this.mapper.save to fail with exception.

Q: How to make it fail on condition?

I also checked this one where they suggest to add auxiliary table (transaction-state table)..because seems the saveExpression works only for primary/partition key... if so:

not sure why there that limitation. in any case if it is primary key one can not create duplicated item with the same primary key.. why creating conditions on first place. 3rd table is too much.. why there is not just NE to whatever field I want to use. I may create an index for this filed. not being limited to use only primary key.. that what I mean

UPDATE:

My table mapping code:

@Data // I use [lombok][2] and it does generate getters and setters.
@DynamoDBTable(tableName = "MyTable")
public class MyTable {

    @DynamoDBHashKey(attributeName = "myTableID")
    @DynamoDBAutoGeneratedKey
    private String myTableID;
    @DynamoDBAttribute(attributeName = "referenceId")
    private String referenceId;
    @DynamoDBAttribute(attributeName = "startTime")
    private String startTime;
    @DynamoDBAttribute(attributeName = "endTime")
    private String endTime;
    ...
}

回答1:

Correct me if I'm wrong, but from the:

https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/dynamodb-dg.pdf

Conditional Writes By default, the DynamoDB write operations (PutItem, UpdateItem, DeleteItem) are unconditional: each of these operations will overwrite an existing item that has the specified primary key

the primary key - that makes me thing that the conditional write works ONLY with primary keys

--

Also there is attempt use the transactional way r/w from the db. There is a library. That event has not maven repo: https://github.com/awslabs/dynamodb-transactions

As an alternative seems is the way to use 3rd transaction table with the primary keys that are responsible to tell you whether you are ok to read or write to the table. (ugly) as we replied here: DynamoDBMapper save item only if unique

Another alternative, I guess (by design): it is to design your tables in a way so you use the primary key as your business-key, so you can use it for the conditional writes.

--

Another option: use Aurora :)

--

Another options (investigating): https://aws.amazon.com/blogs/database/building-distributed-locks-with-the-dynamodb-lock-client/ - this I do not like either. because potentially it would create timeouts for others who would want to create new items in this table.

--

Another option: Live with this let duplication happens for the item-creation (not including the primary key). And take care of it as a part of "garbage collection". Depends on the scenario.

来源：https://stackoverflow.com/questions/50005071/dynamodb-conditional-write-on-create-check-by-the-filed-if-notequal-ne