59 lines
2.6 KiB
Markdown
59 lines
2.6 KiB
Markdown
|
---
|
||
|
|
layout: "docs"
|
||
|
|
page_title: "Resource Lifecycle"
|
||
|
|
sidebar_current: "docs-internals-lifecycle"
|
||
|
|
---
|
||
|
|
|
||
|
|
# Resource Lifecycle
|
||
|
|
|
||
|
|
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.
|
||
|
|
|
||
|
|
<div class="alert alert-block alert-warning">
|
||
|
|
<strong>Advanced Topic!</strong> 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.
|
||
|
|
</div>
|
||
|
|
|
||
|
|
## Lifecycle
|
||
|
|
|
||
|
|
A resource roughly follows the steps below:
|
||
|
|
|
||
|
|
1. `ValidateResource` is 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.
|
||
|
|
|
||
|
|
1. `Diff` is 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.
|
||
|
|
|
||
|
|
1. `Apply` is 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. `Apply` must 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).
|
||
|
|
|
||
|
|
1. 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.
|
||
|
|
|
||
|
|
## Partial State and Error Handling
|
||
|
|
|
||
|
|
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.
|