An apparatus, a computer program product and a computer-implemented method performed by a computerized device, comprising: receiving a description of a workflow, the workflow comprising a plurality of blocks, wherein each block comprises one or more instructions, the plurality of blocks comprising at least a first block and a second block, wherein the first block is adapted to output information, and the second block is adapted to receive the information wherein at least one of the plurality of blocks is associated with a ratio between a number of records input into the block and a number of records output by the block; and validating that the workflow can operate properly, using the ratio, wherein during execution, each of the first block and the second block can keep an internal state and request to receive again data previously received as input.