DynamoDB how to Update a Map if an attribute exists, else silently ignore - amazon-dynamodb

Question

I have a table called Products, whose Key is a Range : orgzviceid + productid. It has a map attribute called "checkout" and a quantity storing attribute called "prod_stk_qty_i_i".
Say initially, for a product with Product ID 34, total available quantity is 10. As soon as a Cart checkout happens, assuming the Checkout ID is 5, and it has checked 2 quantities out a product id 34, then the product's (for productid 34) "checkout" map entry and "prod_stk_qty_i_i" in DynamoDB would be something like this:
"checkout" : { "5" : 2 },
"prod_stk_qty_i_i" : 8
If another checkout happens for the same product (say 1 quantity), and if that checkout ID is 7, then the checkout ooks like this:
"checkout" : { "5" : 2, "7" : 1 },
"prod_stk_qty_i_i" : 7
If payment is made, the checkout entry is removed, and quantity is increased.
Now, my requirement is to periodically after some timeout (30 minutes), release the Product Quantities which have been checked out, but not released. I do this by
Increasing the Quantity by "checkout."'s value
Removing the checkout. map entry
It is important that this operation not fail even if this operation is attempted multiple times, (idempotent), so its necessary that it only update if the checkout.checkoutID field exists. If not, it should simply ignore.
I tried the following:
[
"UpdateItem",
[
{
"TableName": "Products",
"Key": {
"orgzviceid": {
"N": "3000161710"
},
"productid": {
"N": "11"
}
},
"UpdateExpression": "REMOVE #checkout.#checkoutID SET #prod_stk_qty_i_i = #prod_stk_qty_i_i + #checkout.#checkoutID",
"ExpressionAttributeNames": {
"#checkout": "checkout",
"#checkoutID": "235",
"#prod_stk_qty_i_i": "prod_stk_qty_i_i"
},
"ConditionExpression": "attribute_exists(#checkout.#checkoutID)",
"ReturnValues": "ALL_NEW"
}
]
]
However, it gives me an error in case the checkout entry is not found for checkout id 235. Note that I've written ConditionExpression to do the update only if attribute "condition.235" exists.
Error Logs:
com.amazonaws.dynamodb.v20120810#ConditionalCheckFailedException","message":"The
conditional request failed ..."
So, how do I write a query such that if the map entry exist, then do the above operation, other wise not fail?
Obviously, one bad hack is to first check in a GetItem query if the checkout entry exists for the provided CheckoutID, and then only do this, however, it just does not seem right

Answer 1

I believe your using conditional expressions incorrectly. The point of the conditional is to fail if certain criteria is not met. WHy do you have the conditional at all? Without the conditional it would just execute the update expression and if the item does not exist I would not expect you to get an error. Like querying for an item that does not exist. You should simply get an empty set back. Not an error.

Answer 2

Your approach will not work because you are mixing "Attribute" and "AttributeValue" together in your conditional expression. Let me explain:
"ConditionExpression": "attribute_exists(#checkout.#checkoutID)"
In your table, checkout is an attribute in dynamo db, whereas checkoutID is in no way related to the table schema. So for dynamo DB, checkoutID is part of the attribute's value and not the attribute itself.
Therefore, to having the condition that you do will not work.
A conditional expression for your use case would be something which says attribute checkout exists and it's value is . However, in order to do that, you'd need to pass the expected map which boils down to reading the record before updating.
I do think that reading the record, updating the value and persisting it should be the way to go ahead in this case (and is not necessarily a bad idea)
Do consider using some kind of optimistic locking in this case to prevent against dirty reads and writes.