Techniques of data flow control are disclosed herein. One example technique includes upon receiving a notification indicating a change to a content item in a source shard, parsing the content item to extract data representing attributes of the content item and identifying a partition of the system-wide index based on the extracted data representing the attributes of the content item. The example technique can also include transmitting, to a token issuer, a request for a token that represents a permission to write to the identified partition of the system-wide index and upon receiving a token issued by the token issuer in response to the transmitted request, transmitting the extracted data representing the attributes of the content item along with the received token to write the extracted data in the partition of the system-wide index.