DynamoBD/Amplify non-negative field and field validation on mutations

Question

I am new to AWS in general, I am building a relatively simple application with Amplify, but I've used Google Firebase before. My question is: Is there a way to set a constrain for a field to be non-negative? I have an application that does transactions and I don't want my balance to be negative. I just need a simple error/exception. Is it possible to set a field constraint in DynamoDB that says "This field should be >= 0"?.

I also checked if it was possible to do it in the VTL amplify generated resolver of my graphql mutation, and indeed it is possible to set some constraints, But somehow it allows the operation and crashes on the next one (when the balance on the DB is already < 0, like if it checks it before the update). I tried saying something like "current_balance - transaction >= 0" but I couldn't get it to work.

So it seems that the only way is to create a custom lambda resolver that does the various checks before submitting the mutation to DynamoDB. I haven't tried it yet but I don't understand how I can do a check on the current balance (stored in the DB) without doing a query.

More in general is it even possible to validate fields (even with simple assertions like non-negative) on amplify/dynamoDB? Moving to another DB like Aurora would help?

Thanks for you help

Answer 1

DynamoDb supports conditional updates which allow an update to be applied when the given condition is met. You can set the condition current_balance >= cost for your update.

However, the negative balance is not the main problem. What you should address is how to prevent other requests from updating the same current_balance at the same time, or in short, race conditions on current_balance. In order to deal with that, you also need a conditional update whose condition is "current_balance = initial_balance". The initial_balance is, I guess, what you get from DynamoDB at the very beginning of the purchase process.

Sample VTL code

#set( $remaining_balance = $initial_balance - $transaction_cost )
#if( $remaining_balance < 0 )
  $util.error("Insufficient balance")
#end

{
  "version" : "2018-05-29",
  "operation" : "UpdateItem",
  "key": { <your-dynamodb-key>  },
  "update" : {
    "expression" : "SET current_balance = :remaining_balance",
    "expressionValues" : {
      ":remaining_balance" : $util.dynamodb.toNumberJson($remaining_balance)
    }
  },
  "condition": {
    "expression": "current_balance = :initial_balance",
    "expressionValues" : {
      ":initial_balance" : $util.dynamodb.toNumberJson($initial_balance)
    }
  }
}