terraform/docs/destroying.md

Terraform Core Resource Destruction Notes

This document intends to describe some of the details and complications involved in the destructions of resources. It covers the ordering defined for related create and destroy operations, as well as changes to the lifecycle ordering imposed by create_before_destroy. It is not intended to enumerate all possible combinations of dependency ordering, only to outline the basics and document some of the more complicated aspects of resource destruction.

The graph diagrams here will continue to use the inverted graph structure used internally by Terraform, where edges represent dependencies rather than order of operations.

Simple Resource Creation

In order to describe resource destruction, we first need to create the resources and define their order. The order of creation is that which fulfills the dependencies for each resource. In this example, A has no dependencies, B depends on A, and C depends on B, and transitively depends on A.

Order of operations:

1. A is created
2. B is created
3. C is created

Resource Updates

An existing resource may be updated with references to a newly created resource. The ordering here is exactly the same as one would expect for creation.

Order of operations:

1. A is created
2. B is created
3. C is created

Simple Resource Destruction

The order for destroying resource is exactly the inverse used to create them. This example shows the graph for the destruction of the same nodes defined above. While destroy nodes will not contain attribute references, we will continue to use the inverted edges showing dependencies for destroy, so the operational ordering is still opposite the flow of the arrows.

Order of operations:

1. C is destroyed
2. B is destroyed
3. A is Destroyed

Resource Replacement

Resource replacement is the logical combination of the above scenarios. Here we will show the replacement steps involved when B depends on A.

In this first example, we simultaneously replace both A and B. Here B is destroyed before A, then A is recreated before B.

Order of operations:

1. B is destroyed
2. A is destroyed
3. A is created
4. B is created

This second example replaces only A, while updating B. Resource B is only updated once A has been destroyed and recreated.

Order of operations:

1. A is destroyed
2. A is created
3. B is updated

While the dependency edge from B update to A destroy isn't necessary in these examples, it is shown here as an implementation detail which will be mentioned later on.

Create Before Destroy

Currently, the only user-controllable method for changing the ordering of create and destroy operations is with the create_before_destroy resource lifecycle attribute. This has the obvious effect of causing a resource to be created before it is destroyed when replacement is required, but has a couple of other effects we will detail here.

Taking the previous replacement examples, we can change the behavior of A to be that of create_before_destroy.

Order of operations:

1. B is destroyed AND A is created
2. B is created
3. A is destroyed

Note that in this first example, the creation of B is inserted in between the creation of A and the destruction of A. This becomes more important in the update example below.

Order of operations:

1. A is created
2. B is updated
3. A is destroyed

Here we can see clearly how B is updated after the creation of A and before the destruction of the deposed resource A. (The prior resource A is sometimes referred to as "deposed" before it is destroyed, to disambiguate it from the newly created A.) This ordering is important for resource that "register" other resources, and require updating before the dependent resource can be destroyed.

The transformation used to create these graphs is also where we use the extra edges mentioned above connecting B to A destroy. The algorithm to change a resource from the default ordering to create_before_destroy simply inverts any incoming edges from other resources, which automatically creates the necessary dependency ordering for dependent updates. This also ensures that reduced versions of this example still adhere to the same ordering rules, such as when the dependency is only being removed:

Order of operations:

1. B is updated
2. A is destroyed

Forced Create Before Destroy

In the previous examples, only resource A was being used as is it were create_before_destroy. The minimal graphs used show that it works in isolation, but that is only when the create_before_destroy resource has no dependencies of it own. When a create_before_resource depends on another resource, that dependency is "infected" by the create_before_destroy lifecycle attribute.

This example demonstrates why forcing create_before_destroy is necessary. B has create_before_destroy while A does not. If we only invert the ordering for B, we can see that results in a cycle.

In order to resolve these cycles, all resources that precede a resource with create_before_destroy must in turn be handled in the same manner. Reversing the incoming edges to A destroy resolves the problem:

Order of operations:

1. A is created
2. B is created
3. B is destroyed
4. A is destroyed

This also demonstrates why create_before_destroy cannot be overridden when it is inherited; changing the behaviour here isn't possible without removing the initial reason for create_before_destroy; otherwise cycles are always introduced into the graph.