neat_periodic_task 2.0.1

Auxiliary classes for reliably running a periodic task in a long-running process such as web-server.

Neat Periodic Task Scheduling

When writing long-running processes such as a web-server in Dart it is often useful to run maintenance operations in a periodic background task. This package provides an auxiliary framework for reliably running such tasks.

Disclaimer: This is not an officially supported Google product.

Backup tasks, database maintenance operations, garbage collection of cloud storage, pre-computed data aggregations are often tasks that can't be done on-line every time a request arrives at a web-server. While doing the task more than once isn't a problem, it is wasteful to run it too often and could lead to performance degradation or an expensive cloud bill.

One solution is off-load such tasks to a clouding scheduling system similar to cron, but such services typically require out-of-band configuration. Further more, a scheduling service needs a way to trigger your task, requiring that the web server exposes a public web API for triggering the task.

However, if the web-server is a set of long-processes serving traffic, the web-server could simply run the task on a loop with a long delay. That is effectively what this package facilitates. Obviously, this package has few more tricks such as.

• Run a periodic task on a loop.
• Limit maximum run-time to prevent a hanging loop.
• Retry failed task executions after a timeout.
• Catch and log all exceptions/errors from the task.
• (Optionally) prevent concurrent tasks using status in a storage service that supports optimistic concurrency (eg. Cloud Datastore, Google Cloud Storage).
• Print log messages liveness monitoring of the periodic tasks.

Example

import 'dart:io';
import 'package:neat_periodic_task/neat_periodic_task.dart';

void main() async {
  // Create a periodic task that prints 'Hello World' every 30s
  final scheduler = NeatPeriodicTaskScheduler(
    interval: Duration(seconds: 30),
    name: 'hello-world',
    timeout: Duration(seconds: 5),
    task: () async => print('Hello World'),
    minCycle: Duration(seconds: 5),
  );

  scheduler.start();
  await ProcessSignal.sigterm.watch().first;
  await scheduler.stop();
}

Monitoring

When a NeatPeriodicTaskScheduler is created it is given a task name. When the scheduler is running the following log messages is written to package logging.

• '### [ALIVE] neat-periodic-task: "<name>"', when task status is verified to be idle,
• '### [START] neat-periodic-task: "<name>"', when task is started,
• '### [FAILED] neat-periodic-task: "<name>"', when task has failed, and,
• '### [FINISHED] neat-periodic-task: "<name>"', when task is finished.

To be alerted when a task consistently fails to run, it is sufficient to alert if the log message '### [ALIVE] neat-periodic-task: "<name>"' is absent from logs for more than 24 hours. This message is printed when the status is checked and the scheduler decided not to run the task because it was completely successfully less than interval time ago.

A simple robust monitoring strategy is to alert if the message '### [ALIVE] neat-periodic-task: "<name>"' have not occurred in 24 hours. This works regardless of the interval length, and will fire alerts even if the server fails to boot.

If the task is intended to run frequently it is also possible to monitor for '### [FINISHED] neat-periodic-task: "<name>"'. Simply alert if this message does not appear at-least once every 3 * interval.