Node.js Design Patterns/

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Parallel Execution: The Pattern

Parallel Execution: The Pattern

Learn about patterns in parallel execution and how to fix race conditions with concurrent tasks.

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The pattern

Finally, we can extract our nice little pattern for the parallel execution flow. Let’s represent a generic version of the pattern with the following code:

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function makeSampleTask (name) {
  return (cb) => {
    console.log(`${name} started`)
    setTimeout(() => {
      console.log(`${name} completed`)
      cb()
    }, Math.random() * 2000)
  }
}
const tasks = [
  makeSampleTask('Task 1'),
  makeSampleTask('Task 2'),
  makeSampleTask('Task 3'),
  makeSampleTask('Task 4')
]
let completed = 0
tasks.forEach(task => {
  task(() => {
    if (++completed === tasks.length) {
      finish()
    }
  })
})
function finish () {
  // all the tasks completed
  console.log('All tasks executed!')
}

With small modifications, we can adapt the pattern to accumulate the results of each task into a collection, to filter or map the elements of an array, or to invoke the finish() callback as soon as one or a given number of tasks are completed (this last situation in particular is called competitive race). ...