The Client/Server messaging library (CSlib) is a lightweight implementation of the client/server model suitable for coupling two scientific applications runnning in serial or parallel.
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The CSlib is an open-source code, distributed freely under the terms of the modified Berkeley Software Distribution (BSD) License. See this page for more details. It was developed at Sandia National Laboratories, a US Department of Energy facility, for use in coupling scientific applications running on desktop machines to large supercomputers. Funding for the CSlib came from DOE's Office of Advanced Scientific Computing Program (OASCR) under the auspices of its Exascale Computing Program (ECP).
The author of the CSlib is Steve Plimpton, who can be contacted at sjplimp at sandia.gov.
Additional info is available at these links:
IMPORTANT NOTE: the GitHub site will not be stood up until ~Aug 2018.