How to run for loop in batch of 4 for 20 items at once in jenkinsfile

Question

I have to run for loop in groovy for 40 items, but do wish to run it for 4 items in parallel then next batch and so on. I know of parallel deployments in jenkinsfile but it triggers all 40 at once.

          def i = 0
          mslist.collate(4).each{
               build job: 'deploy', parameters: [string(name: 'PROJECT', value: "${it[i]}"), string(name: 'ENVIRONMENT', value: params.ENVIRONMENT)]
               i=i+1
}

My Updated code:

stages {
    stage ('Parallel Deployments') {
      steps {
        script {
          def m = rc.ic()
          m = m.collect { "${it}" }
          println "$m"
          m.collate(4).each{
              def deployments = [:]
              batch.each {
                deployments[it] = {
                   build job: 'jb', parameters: [string(name: 'pt', value: it), string(name: 'pl', value: params.gh), string(name: 'dc', value: params.nb)]
                }   
              
            }  
            parallel deployments
          }
        deployments["failFast"] = false
        }
    }
  } 
 }

Answer 1

It can be done like this :

node {
    def items = (1..40).collect { "item-${it}" }
    items.collate(4).each { List batch -> 
        def n=[:]
        
        batch.each {
            n[it] = {
                stage(it) {
                    build job: 'x', parameters: [ string( name: "it", value : it) ]      
                }    
            }
        }
        
      parallel n
    }
}

job: x Jenkinsfile content

node {
    echo "Hello from Pipeline x"
    print params
}

This will invoke 4 jobs at a time and run parallelly. Make sure you have more than 4 executors configured on Jenkins.