How to avoid concurrent requests to a lambda

Question

I have a ReportGeneration lambda that takes request from client and adds following entries to a DDB table.

Customer ID <hash key>
ReportGenerationRequestID(UUID) <sort key>
ExecutionStartTime
ReportExecutionStatus < workflow status>

I have enabled DDB stream trigger on this table and a create entry in this table triggers the report generation workflow. This is a multi-step workflow that takes a while to complete. Where ReportExecutionStatus is the status of the report processing workflow. I am supposed to maintain the history of all report generation requests that a customer has initiated.

Now What I am trying to do is avoid concurrent processing requests by the same customer, so if a report for a customer is already getting generated don’t create another record in DDB ?

Option Considered :

• query ddb for the customerID(consistent read) : - From the list see if any entry is either InProgress or Scheduled
• If not then create a new one (consistent write)
• Otherwise return already existing

Issue: If customer clicks in a split second to generate report, two lambdas can be triggered, causing 2 entires in DDB and two parallel workflows can be initiated something that I don’t want.

Can someone recommend what will be the best approach to ensure that there are no concurrent executions (2 worklflows) for the same Report from same customer. In short when one execution is in progress another one should not start.

Answer 1

You can use ConditionExpression to only create the entry if it doesn't already exist - if you need to check different items, than you can use DynamoDB Transactions to check if another item already exists and if not, create your item.

Those would be the ways to do it with DynamoDB, getting a higher consistency.

Another option would be to use SQS FIFO queues. You can group them by the customer ID, then you wouldn't have concurrent processing of messages for the same customer. Additionally with this SQS solution you get all the advantages of using SQS - like automated retry mechanisms or a dead letter queue.