» Purpose of Terraform State
State is a necessary requirement for Terraform to function. It is often asked if it is possible for Terraform to work without state, or for Terraform to not use state and just inspect cloud resources on every run. This page will help explain why Terraform state is required.
As you'll see from the reasons below, state is required. And in the scenarios where Terraform may be able to get away without state, doing so would require shifting massive amounts of complexity from one place (state) to another place (the replacement concept).
» Mapping to the Real World
Terraform requires some sort of database to map Terraform config to the real
world. When you have a resource
resource "aws_instance" "foo" in your
configuration, Terraform uses this map to know that instance
is that resource.
For some providers like AWS, Terraform could theoretically use something like AWS tags. Early prototypes of Terraform actually had no state files and used this method. However, we quickly ran into problems. The first major issue was a simple one: not all resources support tags, and not all cloud providers support tags.
Therefore, for mapping configuration to resources in the real world, Terraform requires states.
Terraform needs to store more than just resource mappings. Terraform must keep track of metadata such as dependencies.
Terraform typically uses the configuration to determine dependency order. However, when you delete a resource from a Terraform configuration, Terraform must know how to delete that resource. Terraform can see that a mapping exists for a resource not in your configuration and plan to destroy. However, since the configuration no longer exists, it no longer knows the proper destruction order.
To work around this, Terraform stores the creation-time dependencies within the state. Now, when you delete one or more items from the configuration, Terraform can still build the correct destruction ordering based only on the state.
One idea to avoid this is for Terraform to understand the proper ordering of resources. For example, Terraform could know that servers must be deleted before the subnets they are a part of. The complexity for this approach quickly explodes, however: in addition to Terraform having to understand the ordering semantics of every resource for every cloud, Terraform must also understand the ordering across providers.
In addition to dependencies, Terraform will store more metadata in the future such as last run time, creation time, update time, lifecycle options such as prevent destroy, etc.
In addition to basic mapping, Terraform stores a cache of the attribute values for all resources in the state. This is the most optional feature of Terraform state and is done only as a performance improvement.
When running a
terraform plan, Terraform must know the current state of
resources in order to effectively determine the changes that it needs to make
to reach your desired configuration.
For small infrastructures, Terraform can query your providers and sync the latest attributes from all your resources. This is the default behavior of Terraform: for every plan and apply, Terraform will sync all resources in your state.
For larger infrastructures, querying every resource is too slow. Many cloud
providers do not provide APIs to query multiple resources at once, and the
round trip time for each resource is hundreds of milliseconds. On top of this,
cloud providers almost always have API rate limiting so Terraform can only
request a certain number of resources in a period of time. Larger users
of Terraform make heavy use of the
-refresh=false flag as well as the
-target flag in order to work around this. In these scenarios, the cached
state is treated as the record of truth.
The primary motivation people have for using remote state files is in an attempt to improve using Terraform with teams. State files can easily result in conflicts when two people modify infrastructure at the same time.
Remote state is the recommended solution to this problem. At the time of writing, remote state works well but there are still scenarios that can result in state conflicts. A priority for future versions of Terraform is to improve this.