## Why Use A Continuous Integration Service?

Continuous integration services allow code updates to be built and tested on a variety of platforms. This saves the developer a lot of time by not having to manually test the code. As Aither grows larger it becomes more and more beneficial to use a continuous integration service. For example, say a more efficient way to calculate the inviscid flux was found, and a new branch was created to refactor the invisicid flux code to use this new method. This change should result in the same solution, but should take less time to complete. To be thorough, before merging the code back into the develop branch, unit tests covering all of the code’s various functionality should be completed. These tests should still show that the solution is the same as it was prior to the refactor. It can be tedious and time consuming to manually run these tests, not to mention the tests should be run on different operating systems, and with different compilers as well. This is where continuous integration saves the day! A continuous integration service will automatically build the most updated code on a variety of operating systems with a variety of compilers, and can be made to run regression tests. This way it can easily be determined if the refactor introduced any bugs.

## Aither’s Requirements For Continuous Integration

Ok, so it is clear that continuous integration is a good thing, but which service should be used? Ideally, a continuous integration service would provide the following:

• Free (Aither is not a money making venture after all)
• Testing on multiple operating systems (Aither is cross platform)
• Testing with multiple compilers
• Ability to use modern C++ (Aither uses C++14)
• Support for required dependencies (Aither requires an MPI implementation and Cmake)
• Ability to run regression tests in parallel
• Easy to use within Github

After a brief survery of available options, Aither recently started using Travis CI for continuous integration. Travis CI meets all of the above requirements. It is free for open source projects, widely used in the Github community (i.e. SU2), and supports builds on Ubuntu and macOS.

## Using Travis CI

Once an account has been created with Travis CI it is easy to integrate with Github. All that is required is to add a .travis.yml file to the repository. This file instructs Travis CI on how to build the code and run any regression tests. For Aither, a matrix of five builds is setup (Ubuntu/gcc-5, Ubuntu/gcc-6, Ubuntu/clang, macOS/gcc-6, macOS/clang). These builds are setup under the matrix data field of the .travis.yml file. An abbreviated build matrix is shown below; each of the builds is marked by the - os: line.

# set up build matrix
matrix:
include:
# build for Ubuntu/gcc-6
- os: linux
dist: trusty
sudo: required
compiler: gcc
apt:
sources:
- ubuntu-toolchain-r-test
packages:
- g++-6 gcc-6 libstdc++-6-dev
# change default compiler to newer gcc-6
env:
- CXX_COMPILER=g++-6
- C_COMPILER=gcc-6
# build for macOS/clang
- os: osx
osx_image: xcode8
compiler: clang
# change defualt and homebrew compilers to clang
env:
- CXX_COMPILER=clang++
- C_COMPILER=clang
- HOMEBREW_CC=clang
- HOMEBREW_CXX=clang++


### Installing MPI

During the build matrix setup, environment variables for the C/C++ compilers are changed to reflect the newer C++14 supporting compiler to be used in the build. Travis CI only offers Ubuntu 14.04 as its newest linux offering. Since this version of the operating systems is a few years old, updated compilers are needed for the latest C++ standard. However, when the Ubuntu package manager is used, it installs binaries that were created with the system C/C++ compilers. For compatability purposes, it would be best if Aither used a version of MPI that was compiled with the same compiler that will be used to compile Aither itself. For this reason OpenMPI is compiled from source using the updated compilers.

For macOS, things are a little different. The macOS virtual machines from Travis CI come preinstalled with the homebrew package manager. With homebrew the HOMEBREW_CC and HOMEBREW_CXX environment variables control the compiler that new packages are built with. This means that installing MPI is easier because the package manager can do it automatically.

This means that the .travis.yml script has to tell Travis CI to install MPI in a different way depending on which operating system the build is happening on. This can easily be done with a simple bash script as shown below.

#!/bin/bash

# for macOS builds use OpenMPI from homebrew
if [ "$TRAVIS_OS_NAME" == "osx" ]; then cd openmpi # check to see if OpenMPI is cached from previous build if [ -f "bin/mpirun" ]; then echo "Using cached OpenMPI" else echo "Installing OpenMPI with homebrew" HOMEBREW_TEMP=$TRAVIS_BUILD_DIR/openmpi
brew install open-mpi
fi
else
# for Ubuntu builds install OpenMPI from source
# check to see if OpenMPI is cached from previous build
if [ -f "openmpi/bin/mpirun" ] && [ -f "openmpi-2.0.1/config.log" ]; then
echo "Using cached OpenMPI"
echo "Configuring OpenMPI"
cd openmpi-2.0.1
./configure --prefix=$TRAVIS_BUILD_DIR/openmpi CC=$C_COMPILER CXX=$CXX_COMPILER &> openmpi.configure else # install OpenMPI from source echo "Downloading OpenMPI Source" wget https://www.open-mpi.org/software/ompi/v2.0/downloads/openmpi-2.0.1.tar.gz tar zxf openmpi-2.0.1.tar.gz echo "Configuring and building OpenMPI" cd openmpi-2.0.1 ./configure --prefix=$TRAVIS_BUILD_DIR/openmpi CC=$C_COMPILER CXX=$CXX_COMPILER &> openmpi.configure
make -j4 &> openmpi.make
make install &> openmpi.install
cd ..
fi
# recommended by Travis CI documentation to unset these for MPI builds
test -n $CC && unset CC test -n$CXX && unset CXX
fi


### Caching Dependencies

Builds can be sped up on Travis CI by caching dependencies. Aither depends on MPI which can take a while to build from source. However, this really only needs to be done once if the MPI installation can be cached and retrieved from build to build. Fortunately, Travis CI allows this capability even for their free tier of services. To cache the MPI install directory is simple. Only the following few lines need to be added to the .travis.yml file. This caches the OpenMPI source code directory, as well as the installation directory.

cache:
directories:
- openmpi
- openmpi-2.0.1


## Regression Tests

Once the build completes Travis CI will run the Aither regression tests. The regression tests are located in the testCases directory of the repository. Travis CI will run each case for 100 iterations and compare the residuals to some “truth” values. If the residuals differ by less than a given ammount (1% for Aither), the test passes. The idea is that the regression tests cover most or all of the code’s functionality. On Ubuntu builds there are two processors available, so the tests are run in parallel. On macOS there is only one processor available, so the tests are run in serial. The Aither repository includes a python script to automate the running of these regression tests. After Travis CI builds the code, the script is invoked to run the tests.

## Conclusion

Travis CI is now used by Aither to test builds on Ubuntu and macOS using gcc-5, gcc-6, and clang. Regression tests are run for all of Aither’s test cases to ensure that no existing functionality is broken with changes to the code. For more information on how the whole thing is set up, visit the repository and check out the .travis.yml and travis/installMPI files.