One or more requests from a client for data representation items can be received by an items manager. The data representation items can represent data items in a data source. In response, the items manager can perform the following: generating placeholders corresponding to the data representation items; returning the placeholders to the client; fetching the data items from the data source; providing the data representation items to the client; and informing the client that the placeholders are to be replaced with the data representation items. The items manager may determine whether each data representation item can be synchronously returned to the client. If so, the items manager may return that data representation item synchronously, and may forego providing the client with a placeholder for that data representation item. This may allow a client to interact with a single interface for both synchronous and asynchronous data sources.