Dynamodb ConditionalCheckFailedException on read / update operation - java sdk

Question

I am new to DynamoDB and have tested a sample script using Transaction support. I use the same object provided in the dynamodb-transaction library. The only difference is that I added a range key with a hash key. Here is the table definition:

• ItemId hash key, string
• ItemName range key, string

---

 @DynamoDBTable(tableName = "Item") public static class ExampleItem { private String itemId; private String value; private String itemName; private Long version; @DynamoDBHashKey(attributeName = "ItemId") public String getItemId() { return itemId; } public void setItemId(String itemId) { this.itemId = itemId; } @DynamoDBRangeKey(attributeName="ItemName") public String getItemName() { return itemName; } public void setItemName(String itemName) { this.itemName = itemName; } public String getValue() { return value; } public void setValue(String value) { this.value = value; } @DynamoDBVersionAttribute public Long getVersion() { return version; } public void setVersion(Long version) { this.version = version; } } 

As you can see, I also use the version attribute. Now I am trying to execute a simple create or update script using transaction / dbmapper. Here's a snippet of code.

 Transaction t1 = txManager.newTransaction(); ExampleItem keyItem = new ExampleItem(); keyItem.setItemId("Item1"); keyItem.setItemName("USA"); ExampleItem item = t1.load(keyItem); if (item != null) { item.setValue("Magenta"); item.setItemName("UK"); t1.save(item); } else { item = new ExampleItem(); item.setItemId(keyItem.getItemId()); item.setItemName("USA"); item.setValue("Violet"); t1.save(item); } t1.commit(); t1.delete(); 

I can add the record without any problems, but I had a problem when I try to read the record and update any attribute. I get the following exception:

 Uncaught exception:com.amazonaws.services.dynamodbv2.model.ConditionalCheckFailedException: Status Code: 0, AWS Service: null, AWS Request ID: null, AWS Error Code: null, AWS Error Message: Item {ItemId={S: Item1,}, ItemName={S: UK,}} had unexpected attributes: Status Code: 0, AWS Service: null, AWS Request ID: null, AWS Error Code: null, AWS Error Message: expected attribute(s) {version={Value: {N: 1,},Exists: true}} but found null com.amazonaws.services.dynamodbv2.model.ConditionalCheckFailedException: Status Code: 0, AWS Service: null, AWS Request ID: null, AWS Error Code: null, AWS Error Message: Item {ItemId={S: Item1,}, ItemName={S: UK,}} had unexpected attributes: Status Code: 0, AWS Service: null, AWS Request ID: null, AWS Error Code: null, AWS Error Message: expected attribute(s) {version={Value: {N: 1,},Exists: true}} but found null 

It seems like this has something to do with the version, but don't know where I am going wrong. Any pointers would be appreciated.

-Thanks

Shamik

Answer 1

It looks like you need to install the version when creating the element class. He expects 1, but found zero. The versionedAttribute version expects a value, and it seems that when an element is first created, it expects 1.

If you update this item in the future, it should take care of increasing the version inside.

Answer 2

Dynamodb has this concept. Optimistic locking is a strategy to ensure that the client-side item that you update (or delete) is the same as the item in DynamoDB.

• First save:

If this is the first update, you should follow this format

 DynamodbSaveExpression saveExpression = new DynamodbSaveExpression() .withExpectedEntry("ItemId", new ExpectedAttributeValue().withExists(false) .withConditionalOperator(AND) .withExpectedEntry("ItemName", new ExpectedAttributeValue().withExists(false)); // saving the item to dynamodb dynamodbmapper.save(itemToSave, saveExpression); 

Explanation: We are reporting that itemToSave should only be saved when the combination "ItemId", "ItemValue" is not present in dynamodb.

2. Update existing item:

To update an existing item in dynamodb you need to follow this format

 // Step1: Do a GET ExampleItem itemInDynamo = dynamoDBMapper.load(ExampleItem.class, "item_id_value", "item_name_value"); // Step2: Get the version in Dynamo Long versionInDynamo = itemInDynamo.getVersion(); DynamodbSaveExpression saveExpression = new DynamodbSaveExpression() .withExpectedEntry("version", new ExpectedAttributeValue(new AttributeValue().withN(versionInDynamodb.toString()))); // Step3: Update the item dynamodbmapper.save(itemToSave, saveExpression); 

Save will only succeed, if the version you save with is the same in Dynamodb, if you don’t save the crash with ConditionalCheckFailedException , then try again from step 1. First, Read, then write.

Answer 3

This usually happens when your keys are the same for more than one element in dynamodb. For example, if you have a Folder with id as hashKey and folder_name as rangeKey, you will not save the Folder again, where your hash (hashkey + rangeKey) will be the same.