Wow. What a lot of pain.

So let me first confirm what won’t work. Whether Apache 1, Apache 2, leader_only or EB_IS_COMMAND_LEADER testing, you cannot consistenty get exactly one (not zero, or 2+) instances to run a cron job over a long period of time.

The issue is the the leader information isn’t safe. It’s only available during deployments, when you usually first set up the cron job. But when an instance replacement happens—for whatever reason—there is no related deployment, the test to check if the new instance is the leader fails, and you end up with no crons.

So for anything you do at the moment of deployment—database migrations, logging, whatever— that you only want to run once, even on multi-instance environments in an Elastic Beanstalk Application, you can use and trust the leader tests available to you.

But for anything you want to run regularly, on a schedule across weeks and months, you cannot trust those options.

After a lot of research, and trial and error there are two options available to you.

Worker environments

The ‘official’ way is probably via a worker environment. Essentially an instance set up specifically to schedule and run short or long lives processes.

You can learn more about them via the links below, but for the rest of this article we’ll be talking about the alternative option of ‘runtime leader testing’.

https://medium.com/@joelennon/running-cron-jobs-on-amazon-web-services-aws-elastic-beanstalk-a41d91d1c571

Runtime leader testing

This option means running the cron on every instance of an environment, but the initial section of the code being run checks the if the instance id that it’s running on is the first to be returned in the list of all instances. That is, only the first instance id returned by the aws elastic beanstalk api will continue to run the task.

Below the examples are in PHP, but it’s a valid method for other languages too.

We’re assuming you’ve already set up a cron and know how to alter it to make it run on all instances of your environment.

The steps in this guide:

• Create an IAM user just for this process and give it limited read permissions
• Add the credentials to EB configuration
• Write code to check the instance id from the api against the current instance id

Adding the user

• Open IAM in AWS console
• Click Users
• Click Add users
• Type your User name – we used “[applicationCode]-eb-read-user”
• Click the ‘Access Key’ access type
• Click Next: Permissions
• Click the ‘Attach existing policies directly’ tab
• Type ‘AWSElasticBeanstalkReadOnly’ and select it
  • NB: This permission actually gives more access than is required and you could trim it further
• Click Next: Tags
• Click Next: Review
• Click Create user
• Take a copy of the KEY and SECRET

Storing the key and secret in the environment

• Open your environment
• Click configuration
• Click ‘edit’ on the ‘Software’ section
• Add two new items, one for the key (examples use AWS_KEY) and one for the secret (examples use AWS_SECRET)

The code

So this is php, but the basic idea is:

• Install the aws library – here we use composer
• Add some code to call the library
• Compare the first returned instance id against the id of the instance running the code

Install the aws library

$composer require aws/aws-sdk-php

Add the code to do the check

$client = ElasticBeanstalkClient::factory([
    'credentials' => [
        'key' => $_ENV['AWS_KEY'],
        'secret' => $_ENV['AWS_SECRET'],
    ],
    'region'  => '[[your region]]',
    'version' => 'latest'
]);

Replace [[your_region]] above with your own

$result = $client->describeEnvironmentResources([
    'EnvironmentName' => '[[your_environment_name]]'
]);

Then load in the current instance’s id

$currentId = file_get_contents("http://instance-data/latest/meta-data/instance-id");

Then compare the API’s first result to the current one. If the same, so something, otherwise don’t.

if ($currentId == $result['EnvironmentResources']['Instances'][0]['Id'])
{
    // do something. Only one instance will get here
}
else
{
    // don't do anything. All other instances will be here
}

Other useful resources:

https://rotaready.com/blog/scheduled-tasks-elastic-beanstalk-cron

In October 2024 I rewrote this guide. The updated version contains less information, but it simplified to 7 steps with limited downtime (unless your install is huge) and limited chance of failure.

Feel free to read the rest of this article if you like a broad knowledge of things before you start.

Updated 27th September 2020 to consider restart of all services where restarting just apache isn’t enough, and also to share choice when it comes to import first vs SSL first as you may lose permalinks. Thanks to Peter and Simon in the comments.

Updated 25th January 2021 to add an initial paragraph clarifying that you can’t ‘choose’ a PHP version on Lightsail and to further clarify that you must first check which version of PHP a new AWS Lightsail instance will be built with to confirm it will resolve the message being displayed in WordPress.

Updated 11th May 2021 to confirm that snapshots cannot be used to update the PHP version of a Lightsail Bitnami instance.

Updated 22nd September 2022 to add a link to a guide to check which PHP version you’ll get if you spin up a new Lightsail WordPress instance. Also added a note about WordPress supported PHP versions.

Updated 7th September 2023 to add a mention of the option to use Bitnami’s bncert tool to create an SSL certificate

NB: If you’re hoping for a way to click ‘update php’ or ‘use X version of PHP’ in Lightsail, it doesn’t (at time of writing) exist. This guidance will walk you though creating a new Lightsail instance with a newer version of PHP installed and moving everything over to it.

The AWS Lightsail service is great. Click a few buttons and you have a powerful VPS with WordPress up and running for only a few dollars a month. The Bitnami integration allows you to choose from a whole set of Stacks to include upon setup. The LAMP option allows a competent PHP developer to run almost anything on Lightsail.

But after a while running the Lightsail VPS the setup is going to get stale, and because Bitnami bundles everything up, it’s not recommended to go poking around with only a few parts of it.

That’s the challenge I was faced with. Logging into the WordPress admin console a message is displayed which is telling me WordPress would prefer a newer version of PHP please. My version was old and not receiving security updates.

I read a fair bit about my options, but after looking at the choices I decided the best thing was to do an export and import onto a new Lightsail instance.