Jenkins pipeline nested parallel

Jenkins pipeline, declarative.

Simple question, how do I create a parallel stages inside a stage which is parallel by itself?

```
pipeline {
agent { label 'master' }
options {
timestamps()
timeout(time: 15, unit: 'MINUTES')
ansiColor('xterm')
}

stages {
stage('Main parallel') {
parallel {
stage('Start containers') {
agent { label 'my-agent' }
stages {
stage('Start Linux containers') {
steps {
script {
dir (""){
println "running"
}
}
}
}

}
}
stage('Build UI') {
stages {
stage('Cleanup main workspace'){
steps {
dir("") {
cleanWs()
}
}
}
stage('Git checkout') {
steps {
dir("") {
checkout()
}
}
}

stages {
stage('Gradle build') {
parallel{
stage('gradle1') {
steps {
dir("") {
script {
buildInfo = rtGradle.run switches: gradleParams , tasks: 'clean deploy'

Sure, you can do this with Scripted Pipelines. I've never tried to do it with Declarative so I'm not sure whether it's possible to do with Declarative or, if it is, if it's easy or straightforward.

Here's an example of what this might look like with a Scripted Pipeline. I haven't tested it so there may be syntax errors or other problems.

def parallelTopLevelSteps = [:]

parallelTopLevelSteps['A and B'] = {
  def parallelNestedSteps = [:]

  parallelNestedSteps['step A'] = { echo('A') }
  parallelNestedSteps['step B'] = { echo('B') }

  parallel(parallelNestedSteps)
}

parallelTopLevelSteps['C and D'] = {
  def parallelNestedSteps = [:]

  parallelNestedSteps['step C'] = { echo('C') }
  parallelNestedSteps['step D'] = { echo('D') }

  parallel(parallelNestedSteps)
}

parallel(parallelTopLevelSteps)

StackExchange DevOps Q#5240, answer score: 5