Скачать или смотреть How to Prevent Overriding AWS desired_count with Terraform

Learn how to manage the `desired_count` of your AWS services using Terraform without unwanted overrides. This guide provides step-by-step solutions tailored for microservices.
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How to Prevent Overriding AWS desired_count with Terraform

Managing cloud infrastructure efficiently is crucial for any modern organization. When using AWS and Terraform, one common issue users encounter is the unintended overriding of the desired_count for their autoscaling group or ECS service. This can be particularly problematic when you manage multiple microservices with a shared module governing the number of running instances. In this guide, we will delve into the problem and present a solution that allows you to maintain control over your desired_count on a service-by-service basis.

The Problem

When working with AWS through Terraform, the default behavior is for Terraform to take ownership of all specified configurations in your infrastructure. This includes the desired_count, which dictates how many instances of a service should be running. Every time you apply your Terraform configurations, it will try to set the desired_count to the value defined in your module. For many users, this is not desirable, especially for microservices where the count may vary based on different operational needs.

Specific Challenges:

You want to control the desired_count without altering the shared module for all microservices.

Using lifecycle { ignore_changes = ... } isn't feasible due to Terraform's limitations on parameterizing lifecycle attributes.

The goal is to try and read the current state of desired_count without changing the underlying module.

The Solution

While there is no straightforward method to handle this with a single resource definition, the use of conditional resource creation in Terraform can effectively manage this issue. The approach involves creating two separate aws_ecs_service resources and toggling between them based on your requirements.

Step 1: Define Your ECS Services

You will create two ECS service resources:

The first service will manage the standard desired_count.

The second will utilize the lifecycle block to ignore changes to desired_count.

Here is a simplified example of how this can be set up in your Terraform configuration:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Reference the Active Service

To manage which service you are currently deploying, utilize a local variable. This code will help you reference the created services dynamically based on whether you wish to use the lifecycle ignore feature:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Code:

The count attribute dynamically adjusts how many instances of each resource are created based on the var.use_lifecycle boolean variable. When use_lifecycle is true, it only initializes the lifecycle-aware service, effectively ignoring changes to desired_count.

The local variable service allows you to easily reference the correct service throughout your Terraform module without hardcoding any resource references.

Conclusion

By using this two-resource strategy in combination with local variables, you can efficiently manage the desired_count of your AWS microservices with Terraform without affecting the shared module for all services. This approach provides flexibility and control, allowing your infrastructure to scale effectively while meeting individual service needs.

Now that you are equipped with this knowledge, you can manage your AWS infrastructure more confidently, ensuring that you maintain the behavior you expect without unwanted changes to your desired_count. Happy terraforming!