Delayed Async Task

The DelayedAsyncTask class provides a modern substitute for JavaScript's built-in setTimeout function, specifically tailored for asynchronous tasks (callbacks returning a Promise). This one-time scheduler is designed to handle the delayed execution of a single asynchronous task, offering advanced capabilities beyond basic delay.

Key features include:

• Status Communication: Easily check the current status of the scheduled task.
• Graceful Await: Await the completion of an ongoing execution, ensuring deterministic termination when needed.

This class is ideal for scenarios where precise control over the execution and termination of asynchronous tasks is required.

Key Features

• Modern Substitute for Javascript's 'setTimeout': Specifically designed for scheduling asynchronous tasks.
• Execution Status Getters: Allows users to check the task's execution status, helping to prevent potential race conditions.
• Graceful and Deterministic Termination: The awaitCompletionIfCurrentlyExecuting method resolves once the currently executing task finishes or resolves immediately if the task is not executing.
• Robust Error Handling: If the task throws an uncaught error, the error is captured and accessible via the uncaughtRejection getter.
• Comprehensive Documentation: The class is thoroughly documented, enabling IDEs to provide helpful tooltips that enhance the coding experience.
• Fully Tested: Covered extensively by unit tests.
• No External Runtime Dependencies: Lightweight component, only development dependencies are used.
• Non-Durable Scheduling: Scheduling stops if the application crashes or goes down.
• ES6 Compatibility.
• TypeScript support.

Execution Status Getters

The DelayedAsyncTask class provides five getter methods to communicate the task's status to users:

• isPending: Indicates that the execution has not started yet.
• isAborted: Indicates that the task was aborted by the tryAbort method.
• isExecuting: Indicates that the task is currently executing.
• isCompleted: Indicates that the task has completed.
• isUncaughtRejectionOccurred: Indicates that the task threw an uncaught error. The error can be accessed using the uncaughtRejection getter.

Graceful and Deterministic Termination

In the context of asynchronous tasks and schedulers, graceful and deterministic termination is often overlooked. DelayedAsyncTask provides an out-of-the-box mechanism to await the completion of an asynchronous task that has already started but not yet finished, via the awaitCompletionIfCurrentlyExecuting method.

Without deterministic termination, leftover references from incomplete executions can cause issues such as unexpected behavior during unit tests. A clean state is essential for each test, as ongoing tasks from a previous test can interfere with subsequent ones.

This feature is crucial whenever your component has a stop or terminate method. Consider the following example:

class Component {
  private readonly _timeout: NodeJS.Timeout?;

  public start(): void {
    this._timeout = setTimeout(this._prolongedTask.bind(this), 8000);
  }

  public async stop(): Promise<void> {
    if (this._timeout) {
      clearTimeout(this._timeout);
      this._timeout = null;
      // The dangling promise of _prolongedTask might still be running in the
      // background, leading to non-deterministic termination and potential
      // race conditions or unexpected behavior.
    }
  }

  private async _prolongedTask(): Promise<void> {
    // Perform your task here.
  }
}

While it is possible to manually address this issue by avoiding dangling promises and introducing more state properties, doing so can compromise the Single Responsibility Principle of your component. It can also decrease readability and likely introduce code duplication, as this need is frequent.
The above example can be fixed using the DelayedAsyncTask class as follows:

import { DelayedAsyncTask } from 'delayed-async-task';

class Component {
  private readonly _delayedTask: AsyncDelayedTask;

  public start(): void {
    this._delayedTask = new AsyncDelayedTask(
      this._prolongedTask.bind(this),
      8000
    );
  }

  public async stop(): Promise<void> {
    if (!this._delayedTask.tryAbort()) {
      await this._delayedTask.awaitCompletionIfCurrentlyExecuting();
    }
  }

  private async _prolongedTask(): Promise<void> {
    // Perform your task here.
  }
}

Another scenario where this feature is highly recommended is when a schedule might be aborted, such as in an abort-and-reschedule situation. If the task is currently executing, you may not be able to abort it. In such cases, you can ignore the reschedule request, await the current execution to complete, or implement any other business logic that suits your requirements.

Zero Over-Engineering, No External Dependencies

This component provides a lightweight, dependency-free solution. It is designed to be simple and efficient, ensuring minimal overhead. Additionally, it can serve as a building block for more advanced implementations, such as a Delayed Async Tasks Manager, if needed.

Non-Persistent Scheduling

This component features non-durable scheduling, which means that if the app crashes or goes down, scheduling stops.

If you need to guarantee durability over a multi-node deployment, consider using this scheduler as a building block or use other custom-made solutions for that purpose.

Error Handling

Unlike setTimeout in Node.js, where errors from rejected promises propagate to the event loop and trigger an uncaughtRejection event, this package offers robust error handling:

• Errors thrown during task's execution are captured, and accessible via the uncaughtRejection getter.
• Use the isUncaughtRejectionOccurred getter to determine if an uncaught error occurred during execution.

Ideally, a delayed task should handle its own errors and avoid throwing uncaught exceptions.

Use-case Example

Consider a Background Updates Manager. For simplicity, we assume that updates occur only following an on-demand request by the admin, triggered by the execution of the scheduleNextJob method. Additionally, assume that only one future background update task can be scheduled at any given time. This means that scheduling a new task will abort any previously scheduled task that has not yet started.

Please note that this example is overly simplified. Real-world usage examples can be more complex, often involving durability and synchronization with external resources, as most modern applications are stateless.

import { DelayedAsyncTask } from 'delayed-async-task';

class BackgroundUpdatesManager {
  private _delayedBackgroundTask: DelayedAsyncTask?;

  public async scheduleNextJob(nextExecutionDate: Date): Promise<void> {
    // This method may override a previous schduled date, thus we need to abort
    // such a previous-schedule if possible.
    await this._abortIfPossibleOrAwaitCompletion();

    const msTillNextExecution = nextExecutionDate.getTime() - Date.now();
    this._delayedBackgroundTask = new DelayedAsyncTask(
      this._installNewestUpdates.bind(this),
      msTillNextExecution
    );
  }

  private async _abortIfPossibleOrAwaitCompletion(): Promise<void> {
    if (!this._delayedBackgroundTask) {
      return; // No previous execution exists.
    }

    if (this._delayedBackgroundTask.tryAbort()) {
      logger.info(`Aborted a previously scheduled background updates task`);
      return;
    }
        
    if (this._delayedBackgroundTask.isExecuting) {
      logger.info('Waiting for a previously scheduled background updates task to finish execution...');
      await this._delayedBackgroundTask.awaitCompletionIfCurrentlyExecuting();
    }

    const uncaughtError = this._delayedBackgroundTask.uncaughtRejection;
    if (uncaughtError) {
      logger.error(
        `Previous background updates execution failed with uncaught error: ${uncaughtError.message}`
      );
      return;
    }

    // At this stage, necessarily this._delayedBackgroundTask.isCompleted === true
    // as all other options were eliminated.
    logger.info('Previously scheduled background updates task has finished successfully');
  }
    
  private async _installNewestUpdates(): Promise<void> {
    // Potentially a prolonged operation:
    // * Checks for the newest-uninstalled updates.
    // * Downloads them.
    // * Installs them.
  }
}

License

MIT