Function apply_command 

async fn apply_command(
    corrections: &mut OkErr<Correction<Row>, Correction<DataflowErrorSer>>,
    writer: &mut WriteHandle<SourceData, (), Timestamp, StorageDiff>,
    cmd: WriteCommand,
    resp_tx: &UnboundedSender<WriteResponse>,
)

Apply a single command to the task state.

desired updates enter corrections as positive contributions and persist updates as negative contributions, so the buffer contains desired - persist, i.e. the updates that need to be written to bring the shard in line with desired.