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website: Initial draft docs for Preconditions and Postconditions

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---
page_title: Preconditions and Postconditions - Configuration Language
---
# Preconditions and Postconditions
Terraform providers can automatically detect and report problems related to
the remote system they are interacting with, but they typically do so using
language that describes implementation details of the target system, which
can sometimes make it hard to find the root cause of the problem in your
Terraform configuration.
Preconditions and postconditions allow you to optionally describe the
assumptions you are making as a module author, so that Terraform can detect
situations where those assumptions don't hold and potentially return an
error earlier or an error with better context about where the problem
originated.
Preconditions and postconditions both follow a similar structure, and differ
only in when Terraform evaluates them: Terraform checks a precondition prior
to evaluating the object it is associated with, and a postcondition _after_
evaluating the object. That means that preconditions are useful for stating
assumptions about data from elsewhere that the resource configuration relies
on, while postconditions are more useful for stating assumptions about the
result of the resource itself.
The following example shows some different possible uses of preconditions and
postconditions.
```hcl
variable "aws_ami_id" {
type = string
# Input variable validation can check that the AMI ID is syntactically valid.
validation {
condition = can(regex("^ami-", var.aws_ami_id))
error_message = "The AMI ID must have the prefix \"ami-\"."
}
}
data "aws_ami" "example" {
id = var.aws_ami_id
lifecycle {
# A data resource with a postcondition can ensure that the selected AMI
# meets this module's expectations, by reacting to the dynamically-loaded
# AMI attributes.
postcondition {
condition = self.tags["Component"] == "nomad-server"
error_message = "The selected AMI must be tagged with the Component value \"nomad-server\"."
}
}
}
resource "aws_instance" "example" {
instance_type = "t2.micro"
ami = "ami-abc123"
lifecycle {
# A resource with a precondition can ensure that the selected AMI
# is set up correctly to work with the instance configuration.
precondition {
condition = data.aws_ami.example.architecture == "x86_64"
error_message = "The selected AMI must be for the x86_64 architecture."
}
# A resource with a postcondition can react to server-decided values
# during the apply step and halt work immediately if the result doesn't
# meet expectations.
postcondition {
condition = self.private_dns != ""
error_message = "EC2 instance must be in a VPC that has private DNS hostnames enabled."
}
}
}
data "aws_ebs_volume" "example" {
# We can use data resources that refer to other resources in order to
# load extra data that isn't directly exported by a resource.
#
# This example reads the details about the root storage volume for
# the EC2 instance declared by aws_instance.example, using the exported ID.
filter {
name = "volume-id"
values = [aws_instance.example.root_block_device.volume_id]
}
}
output "api_base_url" {
value = "https://${aws_instance.example.private_dns}:8433/"
# An output value with a precondition can check the object that the
# output value is describing to make sure it meets expectations before
# any caller of this module can use it.
precondition {
condition = data.aws_ebs_volume.example.encrypted
error_message = "The server's root volume is not encrypted."
}
}
```
The input variable validation rule, preconditions, and postconditions in the
above example declare explicitly some assumptions and guarantees that the
module developer is making in the design of this module:
* The caller of the module must provide a syntactically-valid AMI ID in the
`aws_ami_id` input variable.
This would detect if the caller accidentally assigned an AMI name to the
argument, instead of an AMI ID.
* The AMI ID must refer to an AMI that exists and that has been tagged as
being intended for the component "nomad-server".
This would detect if the caller accidentally provided an AMI intended for
some other system component, which might otherwise be detected only after
booting the EC2 instance and noticing that the expected network service
isn't running. Terraform can therefore detect that problem earlier and
return a more actionable error message for it.
* The AMI ID must refer to an AMI which contains an operating system for the
`x86_64` architecture.
This would detect if the caller accidentally built an AMI for a different
architecture, which might therefore not be able to run the software this
virtual machine is intended to host.
* The EC2 instance must be allocated a private DNS hostname.
In AWS, EC2 instances are assigned private DNS hostnames only if they
belong to a virtual network configured in a certain way. This would
detect if the selected virtual network is not configured correctly,
giving explicit feedback to prompt the user to debug the network settings.
* The EC2 instance will have an encrypted root volume.
This ensures that the root volume is encrypted even though the software
running in this EC2 instance would probably still operate as expected
on an unencrypted volume. Therefore Terraform can draw attention to the
problem immediately, before any other components rely on the
insecurely-configured component.
Writing explicit preconditions and postconditions is always optional, but it
can be helpful to users and future maintainers of a Terraform module by
capturing assumptions that might otherwise be only implied, and by allowing
Terraform to check those assumptions and halt more quickly if they don't
hold in practice for a particular set of input variables.
## Precondition and Postcondition Locations
Terraform supports preconditions and postconditions in a number of different
locations in a module:
* The `lifecycle` block inside a `resource` or `data` block can include both
`precondition` and `postcondition` blocks associated with the containing
resource.
Terraform evaluates resource preconditions before evaluating the resource's
configuration arguments. Resource preconditions can take precedence over
argument evaluation errors.
Terraform evaluates resource postconditions after planning and after
applying changes to a managed resource, or after reading from a data
resource. Resource postcondition failures will therefore prevent applying
changes to other resources that depend on the failing resource.
* An `output` block declaring an output value can include a `precondition`
block.
Terraform evaluates output value preconditions before evaluating the
`value` expression to finalize the result. Output value preconditions
can take precedence over potential errors in the `value` expression.
Output value preconditions can be particularly useful in a root module,
to prevent saving an invalid new output value in the state and to preserve
the value from the previous apply, if any.
Output value preconditions can serve a symmetrical purpose to input
variable `validation` blocks: whereas input variable validation checks
assumptions the module makes about its inputs, output value preconditions
check guarantees that the module makes about its outputs.
## Condition Expressions
`precondition` and `postcondition` blocks both require an argument named
`condition`, whose value is a boolean expression which should return `true`
if the intended assumption holds or `false` if it does not.
Preconditions and postconditions can both refer to any other objects in the
same module, as long as the references don't create any cyclic dependencies.
Resource postconditions can additionally refer to attributes of each instance
of the resource where they are configured, using the special symbol `self`.
For example, `self.private_dns` refers to the `private_dns` attribute of
each instance of the containing resource.
Condition expressions are otherwise just normal Terraform expressions, and
so you can use any of Terraform's built-in functions or language operators
as long as the expression is valid and returns a boolean result.
### Common Condition Expression Features
Because condition expressions must produce boolean results, they can often
use built-in functions and language features that are less common elsewhere
in the Terraform language. The following language features are particularly
useful when writing condition expressions:
* You can use the built-in function `contains` to test whether a given
value is one of a set of predefined valid values:
```hcl
condition = contains(["STAGE", "PROD"], var.environment)
```
* You can use the boolean operators `&&` (AND), `||` (OR), and `!` (NOT) to
combine multiple simpler conditions together:
```hcl
condition = var.name != "" && lower(var.name) == var.name
```
* You can require a non-empty list or map by testing the collection's length:
```hcl
condition = length(var.items) != 0
```
This is a better approach than directly comparing with another collection
using `==` or `!=`, because the comparison operators can only return `true`
if both operands have exactly the same type, which is often ambiguous
for empty collections.
* You can use `for` expressions which produce lists of boolean results
themselves in conjunction with the functions `alltrue` and `anytrue` to
test whether a condition holds for all or for any elements of a collection:
```hcl
condition = alltrue([
for v in var.instances : contains(["t2.micro", "m3.medium"], v.type)
])
```
* You can use the `can` function to concisely use the validity of an expression
as a condition. It returns `true` if its given expression evaluates
successfully and `false` if it returns any error, so you can use various
other functions that typically return errors as a part of your condition
expressions.
For example, you can use `can` with `regex` to test if a string matches
a particular pattern, because `regex` returns an error when given a
non-matching string:
```hcl
condition = can(regex("^[a-z]+$", var.name)
```
You can also use `can` with the type conversion functions to test whether
a value is convertible to a type or type constraint:
```hcl
# This remote output value must have a value that can
# be used as a string, which includes strings themselves
# but also allows numbers and boolean values.
condition = can(tostring(data.terraform_remote_state.example.outputs["name"]))
```
```hcl
# This remote output value must be convertible to a list
# type of with element type.
condition = can(tolist(data.terraform_remote_state.example.outputs["items"]))
```
You can also use `can` with attribute access or index operators to
concisely test whether a collection or structural value has a particular
element or index:
```hcl
# var.example must have an attribute named "foo"
condition = can(var.example.foo)
```
```hcl
# var.example must be a sequence with at least one element
condition = can(var.example[0])
# (although it would typically be clearer to write this as a
# test like length(var.example) > 0 to better represent the
# intent of the condition.)
```
## Early Evaluation
Terraform will evaluate conditions as early as possible.
If the condition expression depends on a resource attribute that won't be known
until the apply phase then Terraform will delay checking the condition until
the apply phase, but Terraform can check all other expressions during the
planning phase, and therefore block applying a plan that would violate the
conditions.
In the earlier example on this page, Terraform would typically be able to
detect invalid AMI tags during the planning phase, as long as `var.aws_ami_id`
is not itself derived from another resource. However, Terraform will not
detect a non-encrypted root volume until the EC2 instance was already created
during the apply step, because that condition depends on the root volume's
assigned ID, which AWS decides only when the EC2 instance is actually started.
For conditions which Terraform must defer to the apply phase, a _precondition_
will prevent taking whatever action was planned for a related resource, whereas
a _postcondition_ will merely halt processing after that action was already
taken, preventing any downstream actions that rely on it but not undoing the
action.
Terraform typically has less information during the initial creation of a
full configuration than when applying subsequent changes to that configuration.
Conditions checked only during apply during initial creation may therefore
be checked during planning on subsequent updates, detecting problems sooner
in that case.
## Error Messages
Each `precondition` or `postcondition` block must include an argument
`error_message`, which provides some custom error sentences that Terraform
will include as part of error messages when it detects an unmet condition.
```
Error: Resource postcondition failed
with data.aws_ami.example,
on ec2.tf line 19, in data "aws_ami" "example":
72: condition = self.tags["Component"] == "nomad-server"
|----------------
| self.tags["Component"] is "consul-server"
The selected AMI must be tagged with the Component value "nomad-server".
```
The `error_message` argument must always be a literal string, and should
typically be written as a full sentence in a style similar to Terraform's own
error messages. Terraform will show the given message alongside the name
of the resource that detected the problem and any outside values used as part
of the condition expression.
## Preconditions or Postconditions?
Because preconditions can refer to the result attributes of other resources
in the same module, it's typically true that a particular check could be
implemented either as a postcondition of the resource producing the data
or as a precondition of a resource or output value using the data.
To decide which is most appropriate for a particular situation, consider
whether the check is representing either an assumption or a guarantee:
* An _assumption_ is a condition that must be true in order for the
configuration of a particular resource to be usable. In the earlier
example on this page, the `aws_instance` configuration had the _assumption_
that the given AMI will always be for the `x86_64` CPU architecture.
Assumptions should typically be written as preconditions, so that future
maintainers can find them close to the other expressions that rely on
that condition, and thus know more about what different variations that
resource is intended to allow.
* A _guarantee_ is a characteristic or behavior of an object that the rest of
the configuration ought to be able to rely on. In the earlier example on
this page, the `aws_instance` configuration had the _guarantee_ that the
EC2 instance will be running in a network that assigns it a private DNS
record.
Guarantees should typically be written as postconditions, so that
future maintainers can find them close to the resource configuration that
is responsible for implementing those guarantees and more easily see
which behaviors are important to preserve when changing the configuration.
In practice though, the distinction between these two is subjective: is the
AMI being tagged as Component `"nomad-server"` a guarantee about the AMI or
an assumption made by the EC2 instance? To decide, it might help to consider
which resource or output value would be most helpful to report in a resulting
error message, because Terraform will always report errors in the location
where the condition was declared.
The decision between the two may also be a matter of convenience. If a
particular resource has many dependencies that _all_ make an assumption about
that resource then it can be pragmatic to declare that just once as a
post-condition of the resource, rather than many times as preconditions on
each of the dependencies.
It may sometimes be helpful to declare the same or similar conditions as both
preconditions _and_ postconditions, particularly if the postcondition is
in a different module than the precondition, so that they can verify one
another as the two modules evolve independently.