1. A data delivery system comprising: a plurality of data delivery apparatuses that process input data based on a predefined workflow, the data delivery apparatuses including at least a first data delivery apparatus that receives a request for executing the workflow from a user and a second data delivery apparatus that receives a request for completing a partly executed workflow, wherein the first data delivery apparatus includes a user-input receiving unit that receives target data to be processed and the request for executing the workflow, a first data processing unit that processes the target data following a workflow definition defined in the workflow, a destination obtaining unit that obtains a transfer destination data delivery apparatus described in the workflow, and a transferring unit that transfers the target data, the workflow definition, and a progress of the workflow to the transfer destination data delivery apparatus, and the second data delivery apparatus includes a transfer receiving unit that receives the target data, the workflow definition, and the progress of the workflow from other data delivery apparatus, a workflow executing unit that executes the workflow from a non-executed part based on the progress of the workflow, and a data delivery unit that delivers data to other destination.