Concurrent Pushes and How Parallel and Matrix Builds Work

# Rajesh pushes to main
git push origin main
# Success: commit abc1234

# Priya pushes to main at the same time
git push origin main
# Error: non-fast-forward. Priya must pull and rebase.
git pull --rebase origin main
git push origin main
# Success: commit def5678

# GitHub Actions: each push triggers its own run
on:
  push:
    branches: [main]
jobs:
  build:
    runs-on: ubuntu-latest   # Each run gets its own fresh runner
    steps:
      - uses: actions/checkout@v4
      - run: mvn clean verify

# GitHub Actions: concurrency group ensures only one deploy at a time
jobs:
  deploy-qa:
    concurrency:
      group: deploy-qa          # Only one run in this group at a time
      cancel-in-progress: true  # Cancel older run if a newer one starts
    steps:
      - run: kubectl apply -f qa/

// Jenkins: lock step serialises access to a shared resource
pipeline {
    agent any
    stages {
        stage('Build') { steps { sh 'mvn package' } }   // Runs in parallel
        stage('Deploy QA') {
            steps {
                lock('qa-deploy-slot') {   // Only one job at a time
                    sh 'kubectl apply -f qa/'
                }
            }
        }
    }
}

// Jenkins: parallel stages cut wall-clock time
pipeline {
    agent any
    stages {
        stage('Build') { steps { sh 'mvn clean compile' } }
        stage('Parallel Checks') {
            parallel {
                stage('Unit Tests')        { steps { sh 'mvn test' } }
                stage('Integration Tests') { steps { sh 'mvn verify -Pintegration' } }
                stage('Security Scan')     { steps { sh 'mvn sonar:sonar' } }
            }
        }
        stage('Deploy') { steps { sh 'kubectl apply -f deploy/' } }
    }
}

# GitHub Actions: parallel jobs with no needs dependency
jobs:
  unit-tests:
    runs-on: ubuntu-latest
    steps: [{run: 'mvn test'}]
  integration-tests:
    runs-on: ubuntu-latest
    steps: [{run: 'mvn verify -Pintegration'}]
  security-scan:
    runs-on: ubuntu-latest
    steps: [{run: 'mvn sonar:sonar'}]
  deploy:
    needs: [unit-tests, integration-tests, security-scan]  # Fan-in
    runs-on: ubuntu-latest
    steps: [{run: 'kubectl apply -f deploy/'}]

# GitHub Actions: matrix across Java versions and services
jobs:
  build:
    strategy:
      matrix:
        java: [17, 21]
        service: [transfer-service, fraud-service]
      fail-fast: false      # Do not cancel others on one failure
      max-parallel: 4       # Cap concurrent jobs
    runs-on: ubuntu-latest
    steps:
      - uses: actions/setup-java@v4
        with: { java-version: '${{ matrix.java }}', distribution: temurin }
      - run: mvn clean verify -pl ${{ matrix.service }}

# WRONG: no concurrency group on deploy.
# Two merges to main both try to deploy at the same time.
# The environment may end up on the older commit if it finishes last.
jobs:
  deploy:
    steps:
      - run: kubectl apply -f prod/

# CORRECT: concurrency group ensures the newer commit deploys last.
jobs:
  deploy:
    concurrency:
      group: deploy-prod
      cancel-in-progress: false   # Let the newer one finish
    steps:
      - run: kubectl apply -f prod/

# WRONG: unbounded matrix swamps the runner fleet.
strategy:
  matrix:
    service: [s1, s2, s3, s4, s5, s6, s7, s8, s9, s10]  # 10 services
    java: [17, 21]                                        # x 2 = 20 jobs!

# CORRECT: cap with max-parallel.
strategy:
  matrix:
    service: [s1, s2, s3, s4, s5, s6, s7, s8, s9, s10]
    java: [17, 21]
  max-parallel: 6   # Only 6 run at once

// WRONG: tests share a static counter.
public class TransferServiceTest {
    private static int transferCount = 0;

    @Test
    void testTransferA() {
        transferCount++;
        assertEquals(1, transferCount); // Fails if another test ran first!
    }
}

// CORRECT: each test has its own state.
public class TransferServiceTest {
    @Test
    void testTransferA() {
        int localCount = 0;
        localCount++;
        assertEquals(1, localCount); // Always passes.
    }
}