Terraform提供程序应如何处理服务器端应用的默认值？

Question

Context: I am implementing (my first) Terraform plugin/provider as a wrapper towards existing public API.

One of the create operations in the API specifies an integer field which takes positive values or -1 serving as a default. If you specify -1 in the create API call, the value gets replaced by some default on the server side (say field = 1000) and is stored as 1000 from now on.

If I present this to my Terraform plugin (terraform apply):

resource "something" "mysomething" {
  name  = "someName"
  field = -1
}

the call is not idempotent. Terraform continues to see a drift and subsequently offers:

  # something.mysomething will be updated in-place
  ~ resource "something" "mysomething" {
        id               = "165-1567498530352"
        name             = "someName"
      ~ field            = 1000 -> -1
    }

Plan: 0 to add, 1 to change, 0 to destroy.

How to deal with such API?

Answer 1

You can use the DiffSuppressFunc flag on schema attributes to conditionally suppress a diff so that Terraform doesn't choose to do anything with the diff.

Something like this should work for you:

package something

import (
    "github.com/hashicorp/terraform/helper/schema"
)

func somethingSomething() *schema.Resource {
    return &schema.Resource{
        // ...
        Schema: map[string]*schema.Schema{
            // ...
            "field": {
                Type:     schema.TypeInt,
                Optional: true,
                Default:  -1,
                DiffSuppressFunc: func(k, old, new string, d *schema.ResourceData) bool {
                    if new == "-1" {
                        return true
                    }
                    return false
                },
            },
        },
    }
}

Martin's answer provides probably a better alternative by using the Computed flag and making the attribute Optional. For this to work nicely you'd want to prevent people from specifying -1 as a value for this which you can do with a ValidateFunc, using the IntAtLeast validator from the list of predefined validations in the core SDK:

package something

import (
    "github.com/hashicorp/terraform/helper/schema"
    "github.com/hashicorp/terraform/helper/validation"
)

func somethingSomething() *schema.Resource {
    return &schema.Resource{
        // ...
        Schema: map[string]*schema.Schema{
            // ...
            "field": {
                Type:         schema.TypeInt,
                Optional:     true,
                Computed:     true,
                ValidateFunc: validation.IntAtLeast(1),
            },
        },
    }
}