Amazon DynamoDB on-demand is a serverless, pay-per-request billing option that can serve thousands of requests per second without capacity planning. Previously, the on-demand request rate was only limited by the default throughput quota (40K read request units and 40K write request units), which uniformly applied to all tables within the account, and could not be customized or tailored for diverse workloads and differing requirements. Since on-demand mode scales instantly to accommodate varying traffic patterns, a piece of hastily written or unoptimized code could rapidly scale up and consume resources, making it difficult to keep costs and usage bounded.
Source:: Amazon AWS