Decaf: Decoupled Dataflows for In Situ Workflows

In situ workflows manage the coordination and communication in a directed graph of heterogeneous tasks executing simultaneously in an high-performance computing system. The communication through the graph can be modeled as a dataflow, and Decaf is a software library for managing the dataflow for in situ workflows. Decaf includes a Python API to define a workflow, creating a complete stand-alone system, but the dataflow design also allows Decaf to support the communication needs of other workflow management systems, because a science campaign may be composed of several workflow tools. Decaf creates efficient parallel communication channels over MPI, including arbitrary data transformations ranging from simple data forwarding to complex data redistribution. Decaf provides three building blocks: (i) a lightweight data model that enables users to define the policies needed to preserve semantic integrity during data redistribution, (ii) flow control designed to prevent overflows in the communication channels between tasks, and (iii) a data contract mechanism that allows users to specify the required data in the parallel communication of the workflow tasks. Decaf has been used in a variety of applications. Two examples are highlighted. The first case is from materials science, where the science campaign consists of several workflow tools that cooperate, and Decaf supports these tools as the dataflow layer. The second problem is motivated by computational cosmology, where the in situ workflow consists of three parallel tasks: synthetic particle generation, Voronoi tessellation, and density estimation.