Nice! The use of S3 ListObjects is an immediate deal breaker though, its eventual consistency can cause silent data corruption. To avoid the List, you'd need

Nice! The use of S3 ListObjects is an immediate deal breaker though, its eventual consistency can cause silent data corruption. To avoid the List, you'd need to write a file manifest somewhere that contains a list of all S3 objects. If it were me, I'd use DynamoDB and append keys to a StringSet on a single item (if you use S3 for the manifest, it needs to be a single object, which means you need to aggregate the keys first, which sounds tricky with Lambda). You'll hit a scaling limit with DDB's item size limit, if you want to avoid that, perhaps writing an item per mapper with the same hash key and a different range key might be better, then you'd do a strongly consistent query to reconstruct the manifest.