Job Manager

A manager for background jobs in Ghost, supporting one-off tasks and recurring jobs.

Table of Contents

- Quick Start
- Job Types
- Background Job Requirements
- Advanced Usage
- Technical Details

Quick Start

``js
const JobManager = require('@tryghost/job-manager');
const jobManager = new JobManager({JobModel: models.Job});

// Simple one-off job
jobManager.addJob({
name: 'hello-world',
job: () => console.log('Hello World'),
offloaded: false
});

// Recurring job every 5 minutes
jobManager.addJob({
name: 'check-emails',
at: 'every 5 minutes',
job: './jobs/check-emails.js'
});
`

Job Types

Ghost supports two types of jobs:

1. Inline Jobs
- Run in the main Ghost process
- Best for quick, simple tasks
- Cannot be scheduled or recurring

2. Offloaded Jobs
- Run in a separate process
- Good for CPU-intensive tasks
- Can be scheduled or recurring

Background Job Requirements

Offloaded jobs must:

- Have a unique name
- Be idempotent (safe to run multiple times)
- Use minimal parameters (prefer using IDs rather than full objects)
- Import their own dependencies
- Primarily use DB or API calls

Advanced Usage

Below is a sample code to wire up job manger and initialize jobs. This is the simplest way to interact with the job manager - these jobs do not persist after reboot:
`js
const JobManager = require('@tryghost/job-manager');

const jobManager = new JobManager({JobModel: models.Job});

// register a job "function" with queued execution in current event loop
jobManager.addJob({
job: (word) => console.log(word),
name: 'hello',
offloaded: false
});

// register a job "module" with queued execution in current even loop
jobManager.addJob({
job:'./path/to/email-module.js',
data: {email: 'send@here.com'},
offloaded: false
});

// register recurring job which needs execution outside of current event loop
jobManager.addJob({
at: 'every 5 minutes',
job: './path/to/jobs/check-emails.js',
name: 'email-checker'
});

// register recurring job with cron syntax running every 5 minutes
// job needs execution outside of current event loop
// for cron builder check https://crontab.guru/ (first value is seconds)
jobManager.addJob({
at: '0 1/5 ',
job: './path/to/jobs/check-emails.js',
name: 'email-checker-cron'
});

// register a job to run immediately running outside of current even loop
jobManager.addJob({
job: './path/to/jobs/check-emails.js',
name: 'email-checker-now'
});

// register a one-off job to be executed immediately within current event loop
jobManager.addOneOffJob({
name: 'members-migrations',
offloaded: false,
job: stripeService.migrations.execute.bind(stripeService.migrations)
});

// register a one-off job to be executed immediately outside of current event loop
jobManager.addOneOffJob({
name: 'generate-backup-2022-09-15',
job: './path/to/jobs/backup.js',
});

// optionally await completion of the one-off job in case
// there are state changes expected to execute the rest of the process
// NOTE: if multiple jobs are submitted using the same name, the first completion will resolve
await jobManager.awaitOneOffCompletion('members-migrations');

// check if previously registered one-off job has been executed
// successfully - it exists and doesn't have a "failed" state.
// NOTE: this is stored in memory and cleared on reboot
const backupSuccessful = await jobManager.hasExecutedSuccessfully('generate-backup-2022-09-15');

if (!backupSuccessful) {
// One-off jobs with "failed" state can be rescheduled
jobManager.addOneOffJob({
name: 'generate-backup-2022-09-15',
job: './path/to/jobs/backup.js',
});
}
`

For more examples of JobManager initialization check test/examples directory.

Technical Details

$3

Background jobs run in separate processes, meaning:
- No shared memory with Ghost
- Must import own dependencies
- Should avoid heavy dependencies
- Should handle graceful shutdown

$3

Offloaded jobs are running on dedicated worker threads which makes their lifecycle a bit different from inline jobs:
1. When starting a job it's only sharing ENV variables with its parent process. The job itself is run on an independent JavaScript execution thread. The script has to re-initialize any modules it will use. For example, it should take care of: model layer initialization, cache initialization, etc.
2. When finishing work in a job prefer to signal successful termination by sending 'done' message to the parent thread: parentPort.postMessage('done') (example use). Finishing work this way terminates the thread through worker.terminate()), which logs termination in parent process and flushes any pipes opened in thread.
3. Jobs that have iterative nature, or need cleanup before interrupting work should allow for graceful shutdown by listening on 'cancel'` message coming from parent thread (example use).
4. When exceptions happen and expected outcome is to terminate current job, leave the exception unhandled allowing it to bubble up to the job manager. Unhandled exceptions terminate current thread and allow for next scheduled job execution to happen.

For more nuances on job structure best practices check bree documentation.