If supported by your backend, Terraform will lock your state for all operations that could write state. This prevents others from acquiring the lock and potentially corrupting your state.
State locking happens automatically on all operations that could write state. You won't see any message that it is happening. If state locking fails, Terraform will not continue. You can disable state locking for most commands with the -lock flag but it is not recommended.
If acquiring the lock is taking longer than expected, Terraform will output a status message. If Terraform doesn't output a message, state locking is still occurring if your backend supports it.
Not all backends support locking. Please view the list of backend types for details on whether a backend supports locking or not.
Terraform has a force-unlock command to manually unlock the state if unlocking failed.
Be very careful with this command. If you unlock the state when someone else is holding the lock it could cause multiple writers. Force unlock should only be used to unlock your own lock in the situation where automatic unlocking failed.
To protect you, the force-unlock command requires a unique lock ID. Terraform will output this lock ID if unlocking fails. This lock ID acts as a nonce, ensuring that locks and unlocks target the correct lock.
© 2018 HashiCorpLicensed under the MPL 2.0 License.
https://www.terraform.io/docs/state/locking.html