A method and system for operating a data center. The method includes, in response to a map task distributed by a job tracker, a map task tracker executes the map task to generate a map output including version information. The map task tracker stores the generated map outputs. The map task tracker informs the job tracker of related information of the map output. In response to a reduce task distributed by the job tracker, the reduce task tracker acquires the map outputs for key names including given version information from the map task trackers, wherein the acquired map outputs include the map outputs with the given version information and historical map outputs with the version information prior to the given version information. The reduce task tracker executes the reduce task on the acquired map outputs.
