Updated readme with instructions about jobs

refs https://github.com/TryGhost/Ghost/issues/12402

- Describes different types of jobs that could be executed depending on the nature of the jobs.
- Lays down ground rules on how to design scheduled jobs

ghost/job-manager/README.md

@@ -13,6 +13,7 @@ or
 
 ## Usage
 
+Below is a sample code to wire up job manger and initialize jobs:
 ```js
 const JobManager = require('@tryghost/job-manager');
 
@@ -39,6 +40,29 @@ jobManager.scheduleJob('every 5 minutes', './path/to/jobs/check-emails.js', {},
 jobManager.scheduleJob('0 1/5 * * * *', './path/to/jobs/check-emails.js', {}, 'email-checker-cron');
 ```
 
+### Job types and definitions
+
+Job manager's instance accepts a "job" as a parameter in it's `addJob` and `scheduleJob` methods. Both methods should be used based on the nature of jobs they are going to run.
+
+`addJob` method should be used to queue a "function" for execution which is not computationally intensive and contains small amount of asynchronous operations. When registering such job it should always be accounted that code will be executed on the caller's event loop/thread/process.
+
+`scheduleJob` method should be used to register execution of a "worker" (script defined in a separate file) in the future or in recurring manner. Comparing to "function" jobs, **scheduled jobs are safer to execute as they are run on a dedicated thread**. Scheduled jobs *can* contain heavy computations or less safe to execute code, for example: statistical information processing, memory intensive computations, processing that requires blocking I/O operations etc. 
+
+### Jobs
+
+Jobs can be defined in multiple ways depending on the method they will be registered with.
+
+Short, non-blocking, asap executed jobs - should come through `addJob` method. Those can be standard [JavaScript function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function) or a path to a module that exports a function as default.
+
+Scheduled job can be registered through `scheduleJob` method. Jobs created this way are managed by [bree](https://github.com/breejs/bree) job scheduling library. For examples of job scripts check out [this section](https://github.com/breejs/bree#nodejs-email-queue-job-scheduling-example) of bree's documentation.
+
+### Job rules of thumb
+To prevent complications around failed job retries and and handling of specific job states here are some rules that should be followed for all scheduled jobs:
+1. Jobs are **self contained** - meaning job manager should be able to run the job with the state information included within the job's parameters. Job script should look up for the rest of needed information from somewhere else, like a database, API, or file.
+2. Jobs should be [idempotent](https://en.wikipedia.org/wiki/Idempotence) - consequent job executions should be safe.
+3. Job **parameters** should be **kept to the minimum**. When passing large amounts of data around performance can suffer from slow JSON serialization. Also, storage size restrictions that can arise if there is a need to store parameters in the future.Job parameters should be kept to only information that is needed to retrieve the rest of information from somewhere else. For example, it's recommended to pass in only an *id* of the resource that could be fetched from the data storage during job execution or pass in a file path which could be read during execution.
+4. Scheduled **job execution time should not overlap**. It's up to the registering service to assure job execution time does not ecceed time between subsequent scheduled jobs. For example, if job is scheduled to run every 5 minutes it should always run under 5 minutes, otherwise next scheduled job would fail to start.
+
 ## Develop
 
 This is a mono repository, managed with [lerna](https://lernajs.io/).