Why

The Cron module is amazing! No doubt!

But sometimes you want to use it and at the same time prevent running jobs until the last run has finished. This is a tricky thing for heavy computational jobs where the completion time cannot be precisely determined prior to running the job.

This module is a thin wrapper on top of Cron that manages a map of running processes on redis and only allows the cron function to run once the last job is done.

Additionally, it adds a feature called Wait Cycles that allows job keys to be expired after a job has taken longer that cron-time x 10 (or whatever value you set)

Finally I threw in the Human to Cron package because I prefer to work with human-readable crons.

To use

yarn add long-cron

Then...

    
    //Require
    const Cron = require('long-cron');
    
    //Initialize with options
    const options = {
        //How long to wait to clear keys for hung jobs
        //One cycle is equal to your cron duration/interval
        waitCycles: 10,

        // You can optionally pass your own redis (client) interval
        // This module uses ioredis by default
        // redis: YOUR REDIS INSTANCE,

        //You can set your time zone here. Refer to the cron module
        timeZone: 'America/Los_Angeles',
        
        //Optional prefix for your redis keys. The default is as shown below
        redisKeyPrefix: "long-cron:cache:"
    }
    
  
    const cron = new Cron(options);
    
    // now run your cron!
    // pass the duration in human (see human-to-cron module) or as a cron pattern

    cron.start('each 1 seconds', async function() {
    
        console.time("Long Cron");
        console.log('Running Cron Job');
    
        await new Promise((resolve, reject) => {
            setTimeout(resolve, 5000);
        });
    
        console.timeEnd("Long Cron");
    
    
    })

This will output the following...

    Running Cron Job
    Long Cron: 5.003s
    Running Cron Job
    Long Cron: 5.000s
    Running Cron Job
    Long Cron: 5.001s
    Running Cron Job
    Long Cron: 5.001s

As shown in the output, even though the cron job is set to run every one second we actually only run the function once the last job has returned (after 5 seconds).

If this job was to hung, the keys preventing it to run would automatically expire after 1x10 seconds.

It therefore means that when callig long running processes, set your cron interval and waitCycles to values that are reasonable based on the task at hand.

Enjoy!