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Resources have a strict lifecycle, and can be thought of as basic state machines. Understanding this lifecycle can help better understand how Terraform generates an execution plan, how it safely executes that plan, and what the resource provider is doing throughout all of this.
Advanced Topic! This page covers technical details of Terraform. You don't need to understand these details to effectively use Terraform. The details are documented here for those who wish to learn about them without having to go spelunking through the source code.
A resource roughly follows the steps below:
ValidateResourceis called to do a high-level structural validation of a resource's configuration. The configuration at this point is raw and the interpolations have not been processed. The value of any key is not guaranteed and is just meant to be a quick structural check.
Diffis called with the current state and the configuration. The resource provider inspects this and returns a diff, outlining all the changes that need to occur to the resource. The diff includes details such as whether or not the resource is being destroyed, what attribute necessitates the destroy, old values and new values, whether a value is computed, etc. It is up to the resource provider to have this knowledge.
Applyis called with the current state and the diff. Apply does not have access to the configuration. This is a safety mechanism that limits the possibility that a provider changes a diff on the fly.
Applymust apply a diff as prescribed and do nothing else to remain true to the Terraform execution plan. Apply returns the new state of the resource (or nil if the resource was destroyed).
If a resource was just created and did not exist before, and the apply succeeded without error, then the provisioners are executed in sequence. If any provisioner errors, the resource is marked as tainted, so that it will be destroyed on the next apply.
If an error happens at any stage in the lifecycle of a resource, Terraform stores a partial state of the resource. This behavior is critical for Terraform to ensure that you don't end up with any zombie resources: resources that were created by Terraform but no longer managed by Terraform due to a loss of state.