A workflow is expressed as an arrangement of a set of executable tile. In some embodiments, the tiles include operations expressed in different programming languages. A tile can include a declarative specification of datatypes so that external agents, e.g., workflow services, can identify the datatypes and schemas required as arguments and produced as result; this simplifies the defining of a workflow, e.g., by a workflow coder. A tile can have zero, one, or plural user interfaces; selection of a user interface from plural user interfaces can be based on a user selection, on user roles, or on tile lifecycle stage. Workflow services can communicate with each other so that workflows can be distributed and shifted among processor nodes.