Fig. 2
(a) A schematic provenance graph representing the execution of a workflow W1 receiving three data nodes D1, D2 and D3 as input, containing the values x, y and z respectively. W1 computes the expression (x + y) · z by calling two calculations C1 (to perform the sum) and C2 (to perform the product), forwarding the correct inputs to them. C1 creates the intermediate node D4 (with the value x + y) and C2 then creates the node D5 with the final result, that is then also returned by W1. While this simplified example is purely for illustrative purposes, it demonstrates that by storing execution information as a graph, the provenance of all data is fully recorded. (b) Data-provenance layer: it includes calculation and data nodes only, showing the exact sequence of steps that led to the creation of the data nodes. (c) Logical-provenance layer: it hides the details of all intermediate results and focuses only on how the workflow produced the final results from a given set of inputs.
