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# terraform plan
[plan](https://www.terraform.io/docs/commands/plan.html) 用於執行前進行差異比對，透過 `plan` 來檢查更新是否符合需求，確保 terraform 按計畫進行更新。
terraform 會拿現有的 `state` 與目前定義的 `resource` 進行比對，
### 執行流程
```
$ terraform plan
```
1. plan 會檢查可以 refresh 的資源，像是 data 就會每次都檢查。
```
Refreshing Terraform state in-memory prior to plan...
The refreshed state will be used to calculate this plan, but will not be
persisted to local or remote state storage.
data.aws_ami.ubuntu: Refreshing state...
```
1. 接下來 `plan` 會印出這次執行會有的 `動作`
```
An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
+ create
- destroy
-/+ destroy and the create replacment
-> change to data resource
```
* 新增 resource 顯示 `+`
* 修改 resource 顯示 `~` (僅異動 resource 內參數)
* 刪除 resource 顯示 `-`
* 先刪除後新增 resource 顯示 `-/+`
* 異動 data 顯示 `->`
1. 再來就是詳細資源`異動清單`
```
Terraform will perform the following actions:
# aws_elb.web-elb will be created
+ resource "aws_elb" "web-elb" {
+ arn = (known after apply)
+ availability_zones = [
+ "us-west-2a",
+ "us-west-2b",
...
```
1. 最後是此次異動的總結
```
Plan: 8 to add, 0 to change, 0 to destroy.
------------------------------------------------------------------------
```
### 使用 `-out` 避免 apply 與 plan 的差異
使用自動化執行 terraform 時 `-out` 可以匯出這次 plan 的 `異動清單`，讓 [apply](https://shazi7804.github.io/terraform-manage-guide/command/apply.html) 執行時按照這個 `異動清單` 執行更新，確保 plan 和 apply 執行時是相同的結果。
```
$ terraform plan -out=sample-deployment
$ terraform apply sample-deployment
```