X-Git-Url: https://git.piment-noir.org/?a=blobdiff_plain;f=src%2Fpools%2Fabstract-pool.ts;h=476a523740807e14dc44a3c9d8ff5bc323d0b804;hb=8881ae32256c9a9ebe6d78c95672f610a3f46719;hp=1ccaca8b207c613359265eb757d74dffc8c47176;hpb=afbc1e280c219008e0afeddc6bf4ae2d00a85b2e;p=poolifier.git

diff --git a/src/pools/abstract-pool.ts b/src/pools/abstract-pool.ts
index 1ccaca8b..538d66c7 100644
--- a/src/pools/abstract-pool.ts
+++ b/src/pools/abstract-pool.ts
@@ -1,174 +1,778 @@
-import EventEmitter from 'events'
-import type { MessageValue } from '../utility-types'
-import type { IPool } from './pool'
-
-export type ErrorHandler<Worker> = (this: Worker, e: Error) => void
-export type OnlineHandler<Worker> = (this: Worker) => void
-export type ExitHandler<Worker> = (this: Worker, code: number) => void
-
-export interface IWorker {
-  on(event: 'error', handler: ErrorHandler<this>): void
-  on(event: 'online', handler: OnlineHandler<this>): void
-  on(event: 'exit', handler: ExitHandler<this>): void
-}
+import crypto from 'node:crypto'
+import type { MessageValue, PromiseResponseWrapper } from '../utility-types'
+import {
+  DEFAULT_WORKER_CHOICE_STRATEGY_OPTIONS,
+  EMPTY_FUNCTION,
+  isPlainObject,
+  median
+} from '../utils'
+import { KillBehaviors, isKillBehavior } from '../worker/worker-options'
+import { CircularArray } from '../circular-array'
+import { Queue } from '../queue'
+import {
+  type IPool,
+  PoolEmitter,
+  PoolEvents,
+  type PoolInfo,
+  type PoolOptions,
+  type PoolType,
+  PoolTypes,
+  type TasksQueueOptions,
+  type WorkerType
+} from './pool'
+import type { IWorker, Task, TasksUsage, WorkerNode } from './worker'
+import {
+  WorkerChoiceStrategies,
+  type WorkerChoiceStrategy,
+  type WorkerChoiceStrategyOptions
+} from './selection-strategies/selection-strategies-types'
+import { WorkerChoiceStrategyContext } from './selection-strategies/worker-choice-strategy-context'
+
+/**
+ * Base class that implements some shared logic for all poolifier pools.
+ *
+ * @typeParam Worker - Type of worker which manages this pool.
+ * @typeParam Data - Type of data sent to the worker. This can only be serializable data.
+ * @typeParam Response - Type of execution response. This can only be serializable data.
+ */
+export abstract class AbstractPool<
+  Worker extends IWorker,
+  Data = unknown,
+  Response = unknown
+> implements IPool<Worker, Data, Response> {
+  /** @inheritDoc */
+  public readonly workerNodes: Array<WorkerNode<Worker, Data>> = []
+
+  /** @inheritDoc */
+  public readonly emitter?: PoolEmitter
 
-export interface PoolOptions<Worker> {
   /**
-   * A function that will listen for error event on each worker.
+   * The execution response promise map.
+   *
+   * - `key`: The message id of each submitted task.
+   * - `value`: An object that contains the worker, the execution response promise resolve and reject callbacks.
+   *
+   * When we receive a message from the worker, we get a map entry with the promise resolve/reject bound to the message id.
    */
-  errorHandler?: ErrorHandler<Worker>
+  protected promiseResponseMap: Map<
+  string,
+  PromiseResponseWrapper<Worker, Response>
+  > = new Map<string, PromiseResponseWrapper<Worker, Response>>()
+
   /**
-   * A function that will listen for online event on each worker.
+   * Worker choice strategy context referencing a worker choice algorithm implementation.
+   *
+   * Default to a round robin algorithm.
    */
-  onlineHandler?: OnlineHandler<Worker>
+  protected workerChoiceStrategyContext: WorkerChoiceStrategyContext<
+  Worker,
+  Data,
+  Response
+  >
+
   /**
-   * A function that will listen for exit event on each worker.
+   * Constructs a new poolifier pool.
+   *
+   * @param numberOfWorkers - Number of workers that this pool should manage.
+   * @param filePath - Path to the worker file.
+   * @param opts - Options for the pool.
    */
-  exitHandler?: ExitHandler<Worker>
+  public constructor (
+    public readonly numberOfWorkers: number,
+    public readonly filePath: string,
+    public readonly opts: PoolOptions<Worker>
+  ) {
+    if (!this.isMain()) {
+      throw new Error('Cannot start a pool from a worker!')
+    }
+    this.checkNumberOfWorkers(this.numberOfWorkers)
+    this.checkFilePath(this.filePath)
+    this.checkPoolOptions(this.opts)
+
+    this.chooseWorkerNode = this.chooseWorkerNode.bind(this)
+    this.executeTask = this.executeTask.bind(this)
+    this.enqueueTask = this.enqueueTask.bind(this)
+    this.checkAndEmitEvents = this.checkAndEmitEvents.bind(this)
+
+    this.setupHook()
+
+    for (let i = 1; i <= this.numberOfWorkers; i++) {
+      this.createAndSetupWorker()
+    }
+
+    if (this.opts.enableEvents === true) {
+      this.emitter = new PoolEmitter()
+    }
+    this.workerChoiceStrategyContext = new WorkerChoiceStrategyContext<
+    Worker,
+    Data,
+    Response
+    >(
+      this,
+      this.opts.workerChoiceStrategy,
+      this.opts.workerChoiceStrategyOptions
+    )
+  }
+
+  private checkFilePath (filePath: string): void {
+    if (
+      filePath == null ||
+      (typeof filePath === 'string' && filePath.trim().length === 0)
+    ) {
+      throw new Error('Please specify a file with a worker implementation')
+    }
+  }
+
+  private checkNumberOfWorkers (numberOfWorkers: number): void {
+    if (numberOfWorkers == null) {
+      throw new Error(
+        'Cannot instantiate a pool without specifying the number of workers'
+      )
+    } else if (!Number.isSafeInteger(numberOfWorkers)) {
+      throw new TypeError(
+        'Cannot instantiate a pool with a non safe integer number of workers'
+      )
+    } else if (numberOfWorkers < 0) {
+      throw new RangeError(
+        'Cannot instantiate a pool with a negative number of workers'
+      )
+    } else if (this.type === PoolTypes.fixed && numberOfWorkers === 0) {
+      throw new Error('Cannot instantiate a fixed pool with no worker')
+    }
+  }
+
+  private checkPoolOptions (opts: PoolOptions<Worker>): void {
+    if (isPlainObject(opts)) {
+      this.opts.workerChoiceStrategy =
+        opts.workerChoiceStrategy ?? WorkerChoiceStrategies.ROUND_ROBIN
+      this.checkValidWorkerChoiceStrategy(this.opts.workerChoiceStrategy)
+      this.opts.workerChoiceStrategyOptions =
+        opts.workerChoiceStrategyOptions ??
+        DEFAULT_WORKER_CHOICE_STRATEGY_OPTIONS
+      this.checkValidWorkerChoiceStrategyOptions(
+        this.opts.workerChoiceStrategyOptions
+      )
+      this.opts.restartWorkerOnError = opts.restartWorkerOnError ?? true
+      this.opts.enableEvents = opts.enableEvents ?? true
+      this.opts.enableTasksQueue = opts.enableTasksQueue ?? false
+      if (this.opts.enableTasksQueue) {
+        this.checkValidTasksQueueOptions(
+          opts.tasksQueueOptions as TasksQueueOptions
+        )
+        this.opts.tasksQueueOptions = this.buildTasksQueueOptions(
+          opts.tasksQueueOptions as TasksQueueOptions
+        )
+      }
+    } else {
+      throw new TypeError('Invalid pool options: must be a plain object')
+    }
+  }
+
+  private checkValidWorkerChoiceStrategy (
+    workerChoiceStrategy: WorkerChoiceStrategy
+  ): void {
+    if (!Object.values(WorkerChoiceStrategies).includes(workerChoiceStrategy)) {
+      throw new Error(
+        `Invalid worker choice strategy '${workerChoiceStrategy}'`
+      )
+    }
+  }
+
+  private checkValidWorkerChoiceStrategyOptions (
+    workerChoiceStrategyOptions: WorkerChoiceStrategyOptions
+  ): void {
+    if (!isPlainObject(workerChoiceStrategyOptions)) {
+      throw new TypeError(
+        'Invalid worker choice strategy options: must be a plain object'
+      )
+    }
+    if (
+      workerChoiceStrategyOptions.weights != null &&
+      Object.keys(workerChoiceStrategyOptions.weights).length !== this.maxSize
+    ) {
+      throw new Error(
+        'Invalid worker choice strategy options: must have a weight for each worker node'
+      )
+    }
+  }
+
+  private checkValidTasksQueueOptions (
+    tasksQueueOptions: TasksQueueOptions
+  ): void {
+    if (tasksQueueOptions != null && !isPlainObject(tasksQueueOptions)) {
+      throw new TypeError('Invalid tasks queue options: must be a plain object')
+    }
+    if ((tasksQueueOptions?.concurrency as number) <= 0) {
+      throw new Error(
+        `Invalid worker tasks concurrency '${
+          tasksQueueOptions.concurrency as number
+        }'`
+      )
+    }
+  }
+
+  /** @inheritDoc */
+  public get info (): PoolInfo {
+    return {
+      type: this.type,
+      worker: this.worker,
+      minSize: this.minSize,
+      maxSize: this.maxSize,
+      workerNodes: this.workerNodes.length,
+      idleWorkerNodes: this.workerNodes.reduce(
+        (accumulator, workerNode) =>
+          workerNode.tasksUsage.running === 0 ? accumulator + 1 : accumulator,
+        0
+      ),
+      busyWorkerNodes: this.workerNodes.reduce(
+        (accumulator, workerNode) =>
+          workerNode.tasksUsage.running > 0 ? accumulator + 1 : accumulator,
+        0
+      ),
+      runningTasks: this.workerNodes.reduce(
+        (accumulator, workerNode) =>
+          accumulator + workerNode.tasksUsage.running,
+        0
+      ),
+      queuedTasks: this.workerNodes.reduce(
+        (accumulator, workerNode) => accumulator + workerNode.tasksQueue.size,
+        0
+      ),
+      maxQueuedTasks: this.workerNodes.reduce(
+        (accumulator, workerNode) =>
+          accumulator + workerNode.tasksQueue.maxSize,
+        0
+      )
+    }
+  }
+
   /**
-   * This is just to avoid not useful warnings message, is used to set `maxListeners` on event emitters (workers are event emitters).
+   * Pool type.
    *
-   * @default 1000
+   * If it is `'dynamic'`, it provides the `max` property.
    */
-  maxTasks?: number
-}
+  protected abstract get type (): PoolType
 
-class PoolEmitter extends EventEmitter {}
+  /**
+   * Gets the worker type.
+   */
+  protected abstract get worker (): WorkerType
 
-export abstract class AbstractPool<
-  Worker extends IWorker,
-  Data = unknown,
-  Response = unknown
-> implements IPool<Data, Response> {
-  public readonly workers: Worker[] = []
-  public nextWorker: number = 0
+  /**
+   * Pool minimum size.
+   */
+  protected abstract get minSize (): number
+
+  /**
+   * Pool maximum size.
+   */
+  protected abstract get maxSize (): number
 
   /**
-   * `workerId` as key and an integer value
+   * Gets the given worker its worker node key.
+   *
+   * @param worker - The worker.
+   * @returns The worker node key if the worker is found in the pool worker nodes, `-1` otherwise.
    */
-  public readonly tasks: Map<Worker, number> = new Map<Worker, number>()
+  private getWorkerNodeKey (worker: Worker): number {
+    return this.workerNodes.findIndex(
+      workerNode => workerNode.worker === worker
+    )
+  }
 
-  public readonly emitter: PoolEmitter
+  /** @inheritDoc */
+  public setWorkerChoiceStrategy (
+    workerChoiceStrategy: WorkerChoiceStrategy,
+    workerChoiceStrategyOptions?: WorkerChoiceStrategyOptions
+  ): void {
+    this.checkValidWorkerChoiceStrategy(workerChoiceStrategy)
+    this.opts.workerChoiceStrategy = workerChoiceStrategy
+    for (const workerNode of this.workerNodes) {
+      this.setWorkerNodeTasksUsage(workerNode, {
+        run: 0,
+        running: 0,
+        runTime: 0,
+        runTimeHistory: new CircularArray(),
+        avgRunTime: 0,
+        medRunTime: 0,
+        waitTime: 0,
+        waitTimeHistory: new CircularArray(),
+        avgWaitTime: 0,
+        medWaitTime: 0,
+        error: 0
+      })
+    }
+    this.workerChoiceStrategyContext.setWorkerChoiceStrategy(
+      this.opts.workerChoiceStrategy
+    )
+    if (workerChoiceStrategyOptions != null) {
+      this.setWorkerChoiceStrategyOptions(workerChoiceStrategyOptions)
+    }
+  }
 
-  protected id: number = 0
+  /** @inheritDoc */
+  public setWorkerChoiceStrategyOptions (
+    workerChoiceStrategyOptions: WorkerChoiceStrategyOptions
+  ): void {
+    this.checkValidWorkerChoiceStrategyOptions(workerChoiceStrategyOptions)
+    this.opts.workerChoiceStrategyOptions = workerChoiceStrategyOptions
+    this.workerChoiceStrategyContext.setOptions(
+      this.opts.workerChoiceStrategyOptions
+    )
+  }
 
-  public constructor (
-    public readonly numWorkers: number,
-    public readonly filePath: string,
-    public readonly opts: PoolOptions<Worker> = { maxTasks: 1000 }
-  ) {
-    if (!this.isMain()) {
-      throw new Error('Cannot start a pool from a worker!')
+  /** @inheritDoc */
+  public enableTasksQueue (
+    enable: boolean,
+    tasksQueueOptions?: TasksQueueOptions
+  ): void {
+    if (this.opts.enableTasksQueue === true && !enable) {
+      this.flushTasksQueues()
     }
-    // TODO christopher 2021-02-07: Improve this check e.g. with a pattern or blank check
-    if (!this.filePath) {
-      throw new Error('Please specify a file with a worker implementation')
+    this.opts.enableTasksQueue = enable
+    this.setTasksQueueOptions(tasksQueueOptions as TasksQueueOptions)
+  }
+
+  /** @inheritDoc */
+  public setTasksQueueOptions (tasksQueueOptions: TasksQueueOptions): void {
+    if (this.opts.enableTasksQueue === true) {
+      this.checkValidTasksQueueOptions(tasksQueueOptions)
+      this.opts.tasksQueueOptions =
+        this.buildTasksQueueOptions(tasksQueueOptions)
+    } else {
+      delete this.opts.tasksQueueOptions
     }
+  }
 
-    this.setupHook()
+  private buildTasksQueueOptions (
+    tasksQueueOptions: TasksQueueOptions
+  ): TasksQueueOptions {
+    return {
+      concurrency: tasksQueueOptions?.concurrency ?? 1
+    }
+  }
+
+  /**
+   * Whether the pool is full or not.
+   *
+   * The pool filling boolean status.
+   */
+  protected abstract get full (): boolean
+
+  /**
+   * Whether the pool is busy or not.
+   *
+   * The pool busyness boolean status.
+   */
+  protected abstract get busy (): boolean
+
+  protected internalBusy (): boolean {
+    return (
+      this.workerNodes.findIndex(workerNode => {
+        return workerNode.tasksUsage.running === 0
+      }) === -1
+    )
+  }
 
-    for (let i = 1; i <= this.numWorkers; i++) {
-      this.internalNewWorker()
+  /** @inheritDoc */
+  public async execute (data?: Data, name?: string): Promise<Response> {
+    const submissionTimestamp = performance.now()
+    const workerNodeKey = this.chooseWorkerNode()
+    const submittedTask: Task<Data> = {
+      name,
+      // eslint-disable-next-line @typescript-eslint/consistent-type-assertions
+      data: data ?? ({} as Data),
+      submissionTimestamp,
+      id: crypto.randomUUID()
     }
+    const res = new Promise<Response>((resolve, reject) => {
+      this.promiseResponseMap.set(submittedTask.id as string, {
+        resolve,
+        reject,
+        worker: this.workerNodes[workerNodeKey].worker
+      })
+    })
+    if (
+      this.opts.enableTasksQueue === true &&
+      (this.busy ||
+        this.workerNodes[workerNodeKey].tasksUsage.running >=
+          ((this.opts.tasksQueueOptions as TasksQueueOptions)
+            .concurrency as number))
+    ) {
+      this.enqueueTask(workerNodeKey, submittedTask)
+    } else {
+      this.executeTask(workerNodeKey, submittedTask)
+    }
+    this.workerChoiceStrategyContext.update(workerNodeKey)
+    this.checkAndEmitEvents()
+    // eslint-disable-next-line @typescript-eslint/return-await
+    return res
+  }
 
-    this.emitter = new PoolEmitter()
+  /** @inheritDoc */
+  public async destroy (): Promise<void> {
+    await Promise.all(
+      this.workerNodes.map(async (workerNode, workerNodeKey) => {
+        this.flushTasksQueue(workerNodeKey)
+        // FIXME: wait for tasks to be finished
+        await this.destroyWorker(workerNode.worker)
+      })
+    )
   }
 
+  /**
+   * Shutdowns the given worker.
+   *
+   * @param worker - A worker within `workerNodes`.
+   */
+  protected abstract destroyWorker (worker: Worker): void | Promise<void>
+
+  /**
+   * Setup hook to execute code before worker node are created in the abstract constructor.
+   * Can be overridden
+   *
+   * @virtual
+   */
   protected setupHook (): void {
-    // Can be overridden
+    // Intentionally empty
   }
 
+  /**
+   * Should return whether the worker is the main worker or not.
+   */
   protected abstract isMain (): boolean
 
-  public async destroy (): Promise<void> {
-    for (const worker of this.workers) {
-      await this.destroyWorker(worker)
+  /**
+   * Hook executed before the worker task execution.
+   * Can be overridden.
+   *
+   * @param workerNodeKey - The worker node key.
+   */
+  protected beforeTaskExecutionHook (workerNodeKey: number): void {
+    ++this.workerNodes[workerNodeKey].tasksUsage.running
+  }
+
+  /**
+   * Hook executed after the worker task execution.
+   * Can be overridden.
+   *
+   * @param worker - The worker.
+   * @param message - The received message.
+   */
+  protected afterTaskExecutionHook (
+    worker: Worker,
+    message: MessageValue<Response>
+  ): void {
+    const workerTasksUsage =
+      this.workerNodes[this.getWorkerNodeKey(worker)].tasksUsage
+    --workerTasksUsage.running
+    ++workerTasksUsage.run
+    if (message.error != null) {
+      ++workerTasksUsage.error
     }
+    this.updateRunTimeTasksUsage(workerTasksUsage, message)
+    this.updateWaitTimeTasksUsage(workerTasksUsage, message)
   }
 
-  protected abstract destroyWorker (worker: Worker): void | Promise<void>
+  private updateRunTimeTasksUsage (
+    workerTasksUsage: TasksUsage,
+    message: MessageValue<Response>
+  ): void {
+    if (this.workerChoiceStrategyContext.getRequiredStatistics().runTime) {
+      workerTasksUsage.runTime += message.runTime ?? 0
+      if (
+        this.workerChoiceStrategyContext.getRequiredStatistics().avgRunTime &&
+        workerTasksUsage.run !== 0
+      ) {
+        workerTasksUsage.avgRunTime =
+          workerTasksUsage.runTime / workerTasksUsage.run
+      }
+      if (
+        this.workerChoiceStrategyContext.getRequiredStatistics().medRunTime &&
+        message.runTime != null
+      ) {
+        workerTasksUsage.runTimeHistory.push(message.runTime)
+        workerTasksUsage.medRunTime = median(workerTasksUsage.runTimeHistory)
+      }
+    }
+  }
+
+  private updateWaitTimeTasksUsage (
+    workerTasksUsage: TasksUsage,
+    message: MessageValue<Response>
+  ): void {
+    if (this.workerChoiceStrategyContext.getRequiredStatistics().waitTime) {
+      workerTasksUsage.waitTime += message.waitTime ?? 0
+      if (
+        this.workerChoiceStrategyContext.getRequiredStatistics().avgWaitTime &&
+        workerTasksUsage.run !== 0
+      ) {
+        workerTasksUsage.avgWaitTime =
+          workerTasksUsage.waitTime / workerTasksUsage.run
+      }
+      if (
+        this.workerChoiceStrategyContext.getRequiredStatistics().medWaitTime &&
+        message.waitTime != null
+      ) {
+        workerTasksUsage.waitTimeHistory.push(message.waitTime)
+        workerTasksUsage.medWaitTime = median(workerTasksUsage.waitTimeHistory)
+      }
+    }
+  }
 
+  /**
+   * Chooses a worker node for the next task.
+   *
+   * The default worker choice strategy uses a round robin algorithm to distribute the load.
+   *
+   * @returns The worker node key
+   */
+  protected chooseWorkerNode (): number {
+    let workerNodeKey: number
+    if (this.type === PoolTypes.dynamic && !this.full && this.internalBusy()) {
+      const workerCreated = this.createAndSetupWorker()
+      this.registerWorkerMessageListener(workerCreated, message => {
+        const currentWorkerNodeKey = this.getWorkerNodeKey(workerCreated)
+        if (
+          isKillBehavior(KillBehaviors.HARD, message.kill) ||
+          (message.kill != null &&
+            this.workerNodes[currentWorkerNodeKey].tasksUsage.running === 0)
+        ) {
+          // Kill message received from the worker: no new tasks are submitted to that worker for a while ( > maxInactiveTime)
+          this.flushTasksQueue(currentWorkerNodeKey)
+          // FIXME: wait for tasks to be finished
+          void (this.destroyWorker(workerCreated) as Promise<void>)
+        }
+      })
+      workerNodeKey = this.getWorkerNodeKey(workerCreated)
+    } else {
+      workerNodeKey = this.workerChoiceStrategyContext.execute()
+    }
+    return workerNodeKey
+  }
+
+  /**
+   * Sends a message to the given worker.
+   *
+   * @param worker - The worker which should receive the message.
+   * @param message - The message.
+   */
   protected abstract sendToWorker (
     worker: Worker,
     message: MessageValue<Data>
   ): void
 
-  protected addWorker (worker: Worker): void {
-    const previousWorkerIndex = this.tasks.get(worker)
-    if (previousWorkerIndex !== undefined) {
-      this.tasks.set(worker, previousWorkerIndex + 1)
-    } else {
-      throw Error('Worker could not be found in tasks map')
+  /**
+   * Registers a listener callback on the given worker.
+   *
+   * @param worker - The worker which should register a listener.
+   * @param listener - The message listener callback.
+   */
+  protected abstract registerWorkerMessageListener<
+    Message extends Data | Response
+  >(worker: Worker, listener: (message: MessageValue<Message>) => void): void
+
+  /**
+   * Returns a newly created worker.
+   */
+  protected abstract createWorker (): Worker
+
+  /**
+   * Function that can be hooked up when a worker has been newly created and moved to the pool worker nodes.
+   *
+   * Can be used to update the `maxListeners` or binding the `main-worker`\<-\>`worker` connection if not bind by default.
+   *
+   * @param worker - The newly created worker.
+   */
+  protected abstract afterWorkerSetup (worker: Worker): void
+
+  /**
+   * Creates a new worker and sets it up completely in the pool worker nodes.
+   *
+   * @returns New, completely set up worker.
+   */
+  protected createAndSetupWorker (): Worker {
+    const worker = this.createWorker()
+
+    worker.on('message', this.opts.messageHandler ?? EMPTY_FUNCTION)
+    worker.on('error', this.opts.errorHandler ?? EMPTY_FUNCTION)
+    worker.on('error', error => {
+      if (this.emitter != null) {
+        this.emitter.emit(PoolEvents.error, error)
+      }
+    })
+    if (this.opts.restartWorkerOnError === true) {
+      worker.on('error', () => {
+        this.createAndSetupWorker()
+      })
     }
-  }
+    worker.on('online', this.opts.onlineHandler ?? EMPTY_FUNCTION)
+    worker.on('exit', this.opts.exitHandler ?? EMPTY_FUNCTION)
+    worker.once('exit', () => {
+      this.removeWorkerNode(worker)
+    })
 
-  protected removeWorker (worker: Worker): void {
-    // Clean worker from data structure
-    const workerIndex = this.workers.indexOf(worker)
-    this.workers.splice(workerIndex, 1)
-    this.tasks.delete(worker)
+    this.pushWorkerNode(worker)
+
+    this.afterWorkerSetup(worker)
+
+    return worker
   }
 
   /**
-   * Execute the task specified into the constructor with the data parameter.
+   * This function is the listener registered for each worker message.
    *
-   * @param data The input for the task specified.
-   * @returns Promise that is resolved when the task is done.
+   * @returns The listener function to execute when a message is received from a worker.
    */
-  public execute (data: Data): Promise<Response> {
-    // configure worker to handle message with the specified task
-    const worker = this.chooseWorker()
-    this.addWorker(worker)
-    const id = ++this.id
-    const res = this.internalExecute(worker, id)
-    this.sendToWorker(worker, { data: data || ({} as Data), id: id })
-    return res
+  protected workerListener (): (message: MessageValue<Response>) => void {
+    return message => {
+      if (message.id != null) {
+        // Task execution response received
+        const promiseResponse = this.promiseResponseMap.get(message.id)
+        if (promiseResponse != null) {
+          if (message.error != null) {
+            promiseResponse.reject(message.error)
+            if (this.emitter != null) {
+              this.emitter.emit(PoolEvents.taskError, {
+                error: message.error,
+                errorData: message.errorData
+              })
+            }
+          } else {
+            promiseResponse.resolve(message.data as Response)
+          }
+          this.afterTaskExecutionHook(promiseResponse.worker, message)
+          this.promiseResponseMap.delete(message.id)
+          const workerNodeKey = this.getWorkerNodeKey(promiseResponse.worker)
+          if (
+            this.opts.enableTasksQueue === true &&
+            this.tasksQueueSize(workerNodeKey) > 0
+          ) {
+            this.executeTask(
+              workerNodeKey,
+              this.dequeueTask(workerNodeKey) as Task<Data>
+            )
+          }
+        }
+      }
+    }
   }
 
-  protected abstract registerWorkerMessageListener (
-    port: Worker,
-    listener: (message: MessageValue<Response>) => void
-  ): void
+  private checkAndEmitEvents (): void {
+    if (this.emitter != null) {
+      if (this.busy) {
+        this.emitter?.emit(PoolEvents.busy, this.info)
+      }
+      if (this.type === PoolTypes.dynamic && this.full) {
+        this.emitter?.emit(PoolEvents.full, this.info)
+      }
+    }
+  }
 
-  protected abstract unregisterWorkerMessageListener (
-    port: Worker,
-    listener: (message: MessageValue<Response>) => void
-  ): void
+  /**
+   * Sets the given worker node its tasks usage in the pool.
+   *
+   * @param workerNode - The worker node.
+   * @param tasksUsage - The worker node tasks usage.
+   */
+  private setWorkerNodeTasksUsage (
+    workerNode: WorkerNode<Worker, Data>,
+    tasksUsage: TasksUsage
+  ): void {
+    workerNode.tasksUsage = tasksUsage
+  }
 
-  protected internalExecute (worker: Worker, id: number): Promise<Response> {
-    return new Promise((resolve, reject) => {
-      const listener: (message: MessageValue<Response>) => void = message => {
-        if (message.id === id) {
-          this.unregisterWorkerMessageListener(worker, listener)
-          this.addWorker(worker)
-          if (message.error) reject(message.error)
-          else resolve(message.data as Response)
-        }
-      }
-      this.registerWorkerMessageListener(worker, listener)
+  /**
+   * Pushes the given worker in the pool worker nodes.
+   *
+   * @param worker - The worker.
+   * @returns The worker nodes length.
+   */
+  private pushWorkerNode (worker: Worker): number {
+    return this.workerNodes.push({
+      worker,
+      tasksUsage: {
+        run: 0,
+        running: 0,
+        runTime: 0,
+        runTimeHistory: new CircularArray(),
+        avgRunTime: 0,
+        medRunTime: 0,
+        waitTime: 0,
+        waitTimeHistory: new CircularArray(),
+        avgWaitTime: 0,
+        medWaitTime: 0,
+        error: 0
+      },
+      tasksQueue: new Queue<Task<Data>>()
     })
   }
 
-  protected chooseWorker (): Worker {
-    this.nextWorker =
-      this.nextWorker === this.workers.length - 1 ? 0 : this.nextWorker + 1
-    return this.workers[this.nextWorker]
+  /**
+   * Sets the given worker in the pool worker nodes.
+   *
+   * @param workerNodeKey - The worker node key.
+   * @param worker - The worker.
+   * @param tasksUsage - The worker tasks usage.
+   * @param tasksQueue - The worker task queue.
+   */
+  private setWorkerNode (
+    workerNodeKey: number,
+    worker: Worker,
+    tasksUsage: TasksUsage,
+    tasksQueue: Queue<Task<Data>>
+  ): void {
+    this.workerNodes[workerNodeKey] = {
+      worker,
+      tasksUsage,
+      tasksQueue
+    }
+  }
+
+  /**
+   * Removes the given worker from the pool worker nodes.
+   *
+   * @param worker - The worker.
+   */
+  private removeWorkerNode (worker: Worker): void {
+    const workerNodeKey = this.getWorkerNodeKey(worker)
+    if (workerNodeKey !== -1) {
+      this.workerNodes.splice(workerNodeKey, 1)
+      this.workerChoiceStrategyContext.remove(workerNodeKey)
+    }
+  }
+
+  private executeTask (workerNodeKey: number, task: Task<Data>): void {
+    this.beforeTaskExecutionHook(workerNodeKey)
+    this.sendToWorker(this.workerNodes[workerNodeKey].worker, task)
   }
 
-  protected abstract newWorker (): Worker
+  private enqueueTask (workerNodeKey: number, task: Task<Data>): number {
+    return this.workerNodes[workerNodeKey].tasksQueue.enqueue(task)
+  }
 
-  protected abstract afterNewWorkerPushed (worker: Worker): void
+  private dequeueTask (workerNodeKey: number): Task<Data> | undefined {
+    return this.workerNodes[workerNodeKey].tasksQueue.dequeue()
+  }
 
-  protected internalNewWorker (): Worker {
-    const worker: Worker = this.newWorker()
-    worker.on('error', this.opts.errorHandler ?? (() => {}))
-    worker.on('online', this.opts.onlineHandler ?? (() => {}))
-    // TODO handle properly when a worker exit
-    worker.on('exit', this.opts.exitHandler ?? (() => {}))
-    this.workers.push(worker)
-    this.afterNewWorkerPushed(worker)
-    // init tasks map
-    this.tasks.set(worker, 0)
-    return worker
+  private tasksQueueSize (workerNodeKey: number): number {
+    return this.workerNodes[workerNodeKey].tasksQueue.size
+  }
+
+  private flushTasksQueue (workerNodeKey: number): void {
+    if (this.tasksQueueSize(workerNodeKey) > 0) {
+      for (let i = 0; i < this.tasksQueueSize(workerNodeKey); i++) {
+        this.executeTask(
+          workerNodeKey,
+          this.dequeueTask(workerNodeKey) as Task<Data>
+        )
+      }
+    }
+  }
+
+  private flushTasksQueues (): void {
+    for (const [workerNodeKey] of this.workerNodes.entries()) {
+      this.flushTasksQueue(workerNodeKey)
+    }
   }
 }