Offix Scheduler Introduction

Offix Scheduler is the core component that delivers the offline queueing and storage in packages such as offix-client. The scheduler can be used in an application directly to build offline workflows, or it can be as the foundation to build new packages and clients with offline functionality.

Unlike offix-client which provides many features out of the box, offix-scheduler provides a lower level set of core functionalities which can be used to build offline experiences.

• NetworkStatus interface for defining when an app is and is not offline.
• Executor interface for defining the operations/behaviour to be scheduled. Example: A HTTP request, sending a message, a GraphQL request.
• Offline queue mechanism where all operations are scheduled and fulfilled in order.
• OfflineQueueListener functions that are called at various stages in the queue lifecycle. Can be used to extend the queue with additional behaviour.
• Offline storage mechanism for persisting operations in the queue and restoring them after application restarts.

See Offix-Client for a fully featured implementation that uses the Offix Scheduler.

Installation

Using npm:

npm install offix-scheduler

Or yarn:

yarn add offix-scheduler

Getting Started Example

This example shows how offix-scheduler could be used to schedule HTTP Requests using the fetch API found in most browsers.

Executor

An Executor is a class or an object with an execute method that will be called by the scheduler. This is the core operation/behaviour to be scheduled while offline. In this example we will define a FetchExecutor class that can make requests using fetch.

class FetchExecutor {

// You could pass in some state and initialize it within the constructor

constructor() {}

// The execute function where the http request is made

// options is an object which can contain any properties

// you wish to pass in.

public async execute(options) {

// in this example options will have a url and a body

const { url, body } = options;

// make the request

const res = await fetch(url, {

method: "post",

headers: { "Content-Type": "application/json" },

body: JSON.stringify(body)

});

// return the result

return res.json();

}

}

The execute method defines the business logic to be scheduled. In this example, we're scheduling HTTP POST requests using the fetch API. execute accepts an options object which can contain any properties needed to perform the operation. In this case execute expects the options to have a url which is where we will send the request and a body which will be the body of the request.

This example is simplified as a realistic example would likely need to handle more options and also various error cases.

Initialize the OffixScheduler

The example below shows how to initialize the OffixScheduler with the Executor we defined above.

const offix = new OffixScheduler({

executor: new FetchExecutor()

});

await offix.init();

Schedule an Operation

To schedule an operation, call execute on the scheduler and pass down any options needed by the Executor.

try {

// if online the result is immediately returned

const result = await offix.execute({

url: "http://example.com/tasks",

body: {

title: "A New Task!",

description: "This was created by Offix Scheduler"

}

});

} catch (err) {

// check if an offline error occurred and wait for the result.

if (err.offline) {

const result = await err.watchOfflineChange();

}

}