Key-based logging for processing of structured data items with executable logic

A data processing system for identifying one or more portions of executable logic that are executed in processing one or more data items that are each associated with a given value of a key by accessing the specification that represents the executable logic; and generating a log record for the given value of the key, the generating including: for each of the one or more of the data items associated with the given value of the key: identifying a given state of the specification that is maintained for the given value of the key associated with that data item; processing that data item including executing, on that data item, executable logic that are specified in the identified given state of the specification; and generating the log record specifying the execution of those one or more portions of the executable logic for that given value of the key.