Only build projects if something has changed - node.js

Question

We want to split up our project into smaller pieces. Our current CI process goes through a short test phase and then runs a deployment script. However, if nothing has changed in one of the sub project, we do not want to go through the build for this.
Jenkins without pipelines supports exclusions in the SCM configuration (we use git) and based on this, you can configure a specific job to run. However, when using a pipeline, how can I know, if I should build this part or not? How do I get access to the paths that were affected by the last push?
At the moment our script is very simple, and we would like to keep it as simple as possible.
We were playing around with the scripted and the declarative syntax, but could not find a good solution.
Declarative:
#!groovy​
pipeline {
agent any
tools {
nodejs '8.1'
}
stages {
stage('Checkout') {
steps {
checkout scm
}
}
# Only continue, if something has changed
stage('Install') {
steps {
sh 'npm install'
}
}
stage('Test') {
steps {
sh 'npm run test-jenkins'
}
post {
always {
junit "artifacts/test/report.xml"
}
}
}
}
}
Scripted:
#!groovy​
node {
def nodejs = tool name: '8.1', type: 'jenkins.plugins.nodejs.tools.NodeJSInstallation'
env.PATH = "${nodejs}/bin:${env.PATH}"
stage('Checkout') {
checkout scm
}
# Only continue, if something has changed
stage('Install') {
sh 'npm install'
}
stage('Test') {
try {
sh 'npm run test-jenkins'
} finally {
junit "artifacts/test/report.xml"
}
}
}

Answer 1

Thanks to ElpieKay's fast comment on my question, we now have an elegant solution:
Make a tag to the current commit on a successful build
In the next build compare the new commit and the tag for changes
We are using a multi-branch pipeline and a parallel build for the multiple projects we have under the same source root. We iterate through the projects (serviceX) and check in the corresponding directory for a change:
def projects = ['service1', 'service2']
def builders = [:]
for (p in projects) {
def label = p
builders[label] = {
def tag = "${BRANCH_NAME}_last"
node {
echo "Checking for changes compared to ${tag} in directory ${label}"
try {
sh "./check-for-changes ${tag} ${label}"
} catch (ignored) {
echo "Nothing to do"
return
}
dir (label) {
stage(label + ": Install") {
sh "npm install"
}
stage(label + ": Test") {
try {
sh "npm run test-jenkins"
} finally {
junit 'artifacts/test/report.xml'
}
}
echo "Setting tag for the last build on this branch"
sh "git tag -f ${tag}"
}
}
}
}
parallel builders
... and the script to check for changes:
#!/bin/bash
SHA_PREV=$1
if [ -z ${SHA_PREV} ]; then
echo "Usage: `basename $0` <tag> <path>"
exit 1
fi
CHECK_PATH=$2
if [ -z ${CHECK_PATH} ]; then
echo "Usage: `basename $0` <tag> <path>"
exit 1
fi
if `git rev-parse ${SHA_PREV} >/dev/null 2>&1`; then
echo "Found previous tag: ${SHA_PREV}"
else
SHA_PREV=`git rev-list --max-parents=0 HEAD`
echo "Using initial commit: ${SHA_PREV}"
fi
changes=`git diff --name-only ${SHA_PREV} HEAD | grep ${CHECK_PATH}/`
if [ ! -n "${changes}" ]; then
echo "No changes found"
exit 2 # no changes found
fi