This is the home page for the client/server messaging library (CSlib). It is a lightweight implementation of the client/server model suitable for coupling two scientific applications runnning in serial or parallel.

The client/server model is a messaging paradigm, used ubiquitously by web-based applications, where one or more clients (e.g. your phone) communicate with a server (e.g. a website). In the context of the CSlib, it can used to couple two scientific applications (apps) together where one app acts as the "client" and the other as the "server". The client sends messages to the server requesting it perform specific computations and return the results.

Advantages of this mode of coupling, supported by the CSlib, are as follows:

• Messaging can be done by files, sockets (using the ZMQ library), or MPI.
• The CSlib can be used from C++, C, Fortran, or Python. Or any language which can make calls to a C-style API.
• The two apps operate as standalone executables; neither needs a library interface since they are not linked to each other.
• Each app can run in serial or parallel, or on different numbers of processors. They can be launched on the same or different physical processors of a parallel machine. Socket communication means the two apps can run on geographically distinct machines.
• Modifying an app to work as a client or server via the CSlib can be done in one of two ways. Calls to the CSlib can be added to the app, or a wrapper program (e.g. a Python script) can be written which calls the CSlib, creates input for the app, invokes it, and parses its output.
• Multiple instances of the CSlib can be used to couple more than 2 apps to each other in different patterns.
• A communication prototcol can be defined for a particular style of coupling. In principle this allows any client code of type A to be coupled to any server code of type B, so long as both codes implement the AB protocol via the CSlib. An example is explained below for ab initio molecular dynamics.

A limitation of the CSlib, is that it's meant for loose coupling of two apps when the cost of the data exchange is not performance-critical. It is not designed for tight-coupling scenarios where large volumes of data need to be exchanged rapidly between large numbers of processors. Other kinds of messaging paradigms and toolkits exist for that purpose. See the intro page of the CSlib manaul for some links.

The CSlib is used in LAMMPS, a parallel molecular dynamics (MD) code, via its MESSAGE package. These example use cases are included in the LAMMPS distribution:

• A Monte Carlo (MC) client code uses LAMMPS as a server for energy evaluations and dynamics runs between MC moves.
• LAMMPS runs in client mode to perform ab initio MD using the VASP quantum DFT code as a server for energy/force/virial computations. If the protocol (what messages to send and receive) were implemented by another quantum code, it could be coupled to LAMMPS with no additional changes (to LAMMPS). And vice versa for another MD code coupling to VASP (or another quantum code).

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.

• (8/18) Initial open-source release of the CSlib.