Is it safe to apply Terraform plan when it says the database instance must be replaced?

user_1235074 0
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I'm importing the existing resources (AWS RDS) but the terraform plan command showed a summary:

#aws_db_instance.my_main_db must be replaced 
+/- resource "aws_db_instance" "my_main_db" {
~ address = x
allocated_storage = x
+ apply_immediately = x
~ arn = x
~ username = x
+ password = x
(others arguments with alot of +/- and ~)
}

my_main_db is online with persistent data. My question is as the title; Is it safe for the existing database to run terrafrom apply? I don't want to lose all my customer data.

terraform amazon-rds amazon-web-services
2 Answers

"Replace" in Terraform's terminology means to destroy the existing object and create a new one to replace it. The +/- symbol (as opposed to -/+) indicates that this particular resource will be replaced in the "create before destroy" mode, where there will briefly be two database instances existing during the operation. (This may or may not be possible in practice, depending on whether the instance name is changing as part of this operation.)

For aws_db_instance in particular, destroying an instance is equivalent to deleting the instance in the RDS console: unless you have a backup of the contents of the database, it will be lost. Even if you do have a backup, you'll need to restore it via the RDS console or API rather than with Terraform because Terraform doesn't know about the backup/restore mechanism and so its idea of "create" is to produce an entirely new, empty database.

To sum up: applying a plan like this directly is certainly not generally "safe", because Terraform is planning to destroy your database and all of the contents along with it.

If you need to make changes to your database that cannot be performed without creating an entirely new RDS instance, you'll usually need to make those changes outside of Terraform using RDS-specific tools so that you can implement some process for transferring data between the old and new instances, whether that be backup and then restore (which will require a temporary outage) or temporarily running both instances and setting up replication from old to new until you are ready to shut off the old one. The details of such a migration are outside of Terraform's scope, because they are specific to whatever database engine you are using.

user_2818480

It's most likely not safe, but really only someone familiar with the application can make that decision. Look at the properties and what is going to change or be recreated. Unless you are comfortable with all of those properties changing, then it's not safe.

user_10505360
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