NCrontab.Scheduler 1.0.2-pre

NCrontab.Scheduler

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NCrontab.Scheduler is a simple, open source task scheduling system that can be used in any .NET application. The main component of this project is a thread-safe scheduler which facilitates very basic scheduling operations like adding, remove or changing task schedules. NCrontab.Scheduler is built on top of NCrontab.

Download and Install NCrontab.Scheduler

This library is available on NuGet: https://www.nuget.org/packages/NCrontab.Scheduler Use the following command to install NCrontab.Scheduler using NuGet package manager console:

PM> Install-Package NCrontab.Scheduler

You can use this library in any .NET Standard or .NET Core project.

API Usage

Creating a Scheduler

Scheduler implements the main scheduler operations. You can either create a new instance of Scheduler manually:

IScheduler scheduler = new Scheduler();

Alternatively, you can access the provided singleton instance Scheduler.Current or register/resolve Scheduler and IScheduler in your dependency injection framework.

serviceCollection.AddSingleton<IScheduler>(x => new Scheduler(x.GetRequiredService<ILogger<Scheduler>>()));

Add scheduled tasks

Use method AddTask with all the provided convenience overloads to add tasks to the scheduler. A task is composed of a cron pattern which specifies the recurrance interval and an action (for synchronous callbacks) or a task (for asynchronous callbacks).

Tasks can be added to the scheduler either before or after the scheduler has been started.

scheduler.AddTask(
    cronExpression: CrontabSchedule.Parse("* * * * *"), 
    action: ct => { Console.WriteLine($"{DateTime.Now:O} -> Task runs every minutes"); });

scheduler.AddTask(
    cronExpression: CrontabSchedule.Parse("*/2 * * * *"), 
    action: ct => { Console.WriteLine($"{DateTime.Now:O} -> Task runs every second minutes"); });

scheduler.AddTask(
    cronExpression: CrontabSchedule.Parse("*/3 * * * *"), 
    action: ct => { Console.WriteLine($"{DateTime.Now:O} -> Task runs every third minutes"); });
           

Starting and stopping

Use method StartAsync to start the scheduler operations. This method can be awaited which blocks all further calls until all scheduled tasks have been canceled or removed. Use Start if you prefer to start the scheduler without blocking the current execution path of your program. If the scheduler is started without having added any tasks, it just waits (and blocks) until tasks are added.

Stop attempts to cancel all scheduled tasks immediately.

Design Considerations

Scheduler algorithm

The scheduler calculates its priorities based on the given cron expression for each task. Each scheduling iteration attempts to find the task with the earliest possible execution date/time. It calculates the delay from the current date/time to selected earliest possible execution date/time. If tasks are added or removed during this wait period, the evaluation of the earliest possible execution date/time has to be repeated.

Thread-safety

All scheduler operations are kept thread-safe. Adding and removing tasks as well as starting and stopping the scheduler can be done concurrently.

Task isolation

Each task run is isolated from all other scheduled tasks. The success or failure of execution does not have any negative side effect on other scheduled tasks.

License

This project is Copyright © 2022 Thomas Galliker. Free for non-commercial use. For commercial use please contact the author.