Mosel interface to NEOS solvers using AMPL nl format

This provides a wrapper for NEOS Server's XML-RPC API, for use from FICO Mosel modelling language. It is based on the NEOS Server's official PythonClient, but adjusted for Mosel. In addition, it has a support for storing of NEOS credentials.

Installation

The package is pip-installable from its gitlab repository or from a local clone.

Pip-install from gitlab

There are two options to install directly from the github repository. The first one is using HTTPs and requires a token registerd at GitHub, while the other is using SSH and requires a registered certificate instead:

pip install git+https://github.com/sintefore/neos4mosel.git

pip install git+ssh://git@github.com/sintefore/neos4mosel.git

Usage

The package creates an executable/binary file nemos, which can be used as a solver in mosel models using the nlsolv package. The minimal code for using it in Mosel, assuming that the nemos binary is in the PATH, is:

uses nlsolv
setparam("NL_solver", "neos")       ! solver identifier
setparam("NL_solverpath", "nemos")  ! name of the solver binary - must be in PATH
setparam("NL_binary", false)        ! NEOS XML does not work with the default binary format

The last parameter is required because the XML format does not support NL model files with binary format. Also note that names of Mosel parameters are not case sensitive, so "nl_solver" etc. works as well.

Solver-specific parameters can be passed to the solver using

setparam("NL_options", SOLVER_OPTIONS)

where SOLVER_OPTIONS is a space-separated list of key=value entries. In addition to the solver parameters, the list can include the following parameters for the nemos binary:

• category - NEOS problem category, defaults to milp
• solver - NEOS solver, defaults to FICO-Xpress
• priority - NEOS job priority
  • short (default) limits jobs to 5 minutes, but start immediately and reports progress
  • long allows jobs up to 24 hours, but they can be put in a queue
• email - email to use for the job
  • NEOS requires email for each running job
  • it is also possible to register the email, see below
• user - NEOS username (optional)
  • must belong to credentials registered in system keyring

Run nemos --neos-info to get a list of supported solvers and problem categories. Note that this includes only combinations usable from Mosel, i.e., supporting the AMPL NL input. Moreover, the list does not include Gurobi, since this cannot be used via the XML-RPC API.

Authentication

Each NEOS job must be provided with an email address. In addition, it is possible to make a free account on NEOS, providing access to results of jobs rund under this account.

The nemos binary allows registering both email and the NEOS credentials, to simplify the work. The username is stored in the package's config file, while the password is stored in system's keyring.

For email, it provides commands for registering an email and then reviewing it:

nemos --email name@server
nemos --show-email

The syntax is slightly different for NEOS credentials, since we don't want to show password in clear text:

nemos --set-cred
nemos --show-user
nemos --del-cred

The first command asks for username and password, while the last command removes the password from the keyring and the username from the config file.

Stand-alone usage

While nemos is meant to be called from Mosel, it can solve a standalone NL file as well. For this, simply run

nemos problem.nl

and there should appear problem.sol in the same directory as problem.nl. Solver options can be provided using environmental variable neos_options.

To avoid warning about wrong syntax of input arguments, add dummy arguments -s -e to the call.