Configurable instance replacement via lifecycle replace_triggered_by
#30900
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Evaluate the expressions stored in replace_triggered_by into the *addrs.Reference needed to lookup changes in the plan.
The EvalContext is the only place with all the information to be able to complete the evaluation of the replace_triggered_by expressions. These need to be evaluated into a reference, which is then looked up in the pending changes which the context has access too. On top of needing the plan changes, we also need access to all providers and schemas to decode the changes if we need to traverse the resource values for individual attributes.
The replace_triggered_by expressions create edges in the graph, so must be returned in the References method.
Check for triggered resource replacement in the plan. While the functionality of the feature works here, we ill want to follow up with a way to indicate in the plan _why_ the resource was replaced.
Only immediate changes to the resource are considered.
replace_triggered_by references are scoped to the current module, so we need to filter changes for the current module instance. Rather than creating a ConfigResource and filtering the result, make a Changes.InstancesForAbsResource method to get only the AbsResource changes.
Set ResourceInstanceReplaceByTriggers in the change.
alisdair
approved these changes
Apr 20, 2022
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Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch. |
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This is fantastic, thank you. |
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I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions. |
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Add a
replace_triggered_byoption in thelifecyclemeta block for managed resources, which can be used to couple the lifecycle of resources which may need to be replaced based on conditions in the configuration. References to other managed resources can be used to force replacement of the containing resource, acting similarly totriggersin the ubiquitousnull_resource, a manualtaint, or aplan -replaceoperation.The
replace_triggered_byargument is a list of pseudo-expressions which refer to other managed resources. The syntax is limited to resources, resource instances, and attributes of a resource instance. The only variable values allowed in the expression arecount.indexandeach.key, which can be used to tie individual instances of expanded resources together.The references in the argument list are not evaluated as we typically do within Terraform, rather they are used to lookup changes in the plan. If any of the references do correspond to a change, the containing resource will be planned to be replaced.
The steps for determining if a
replace_triggered_byis going to force replacement are as follows:Update,DeleteThenCreate, orCreateThenDeleteaction on any instance will force replacement.Update,DeleteThenCreate, orCreateThenDeletewill force replacement.Update,DeleteThenCreate, orCreateThenDelete; then the before and after cty values of the change will be compared for equality. Unknown After values are considered a change in value for this purpose as well.TODO: This initial implementation uses the existing mechanism in place for forced replacement (
-replace=addr) to trigger the correct plan. The diff output will show thatreplace_triggered_byis the reason for replacement, but we may want to make a new mechanism for handing these replacements and a more specific message about the change in the diff output.TODO: Cut a release of hcl before the RC.
Docs will be added in a separate PR.
Closes #8099
Closes #11418
Closes #22572
Closes #30210