As indicated at 620, checkpoint metadata for the partition may be updated to indicate a pending checkpoint for the data records. Checkpoint metadata may be updated to indicate a pending checkpoint in many ways. For example, a checkpoint for the data records may be stored as a location or point in the sequence of data records (e.g., a sequence number) which indicates the point in the sequence of data records in the partition of the data stream at which log records have been retrieved and processed. A marker or other indication may also be included in the checkpoint metadata which may indicate that the checkpoint is pending or complete. In some embodiments, the checkpoint metadata may include data to ensure that any processing or emission of data records is overwritten or not repeated. For instance, the delivery destination may be a data store and the checkpoint metadata for the partition may include a location in the data store at which the data records are to be written. Using this location, partially emitted data records may be over written in the same location so that the result of processing the data records utilizing the checkpoint metadata is idempotent (because the state of the data records in the data store is the same based on the first attempt to perform or the later successful attempt to process the records).