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