Terraform从S3加载JSON文件并注入变量的实现方案问询

场景说明 #

本地模板正常工作 #

我们有一个包含变量占位符的JSON文件：

// fruits.json
{ 
   "fruits" : 
      {
          "apples": 3,
          "bananas": 4,
          "pears": "${pears}"
      }
} 

本地环境下，使用templatefile函数可以直接注入变量：

# main.tf
variable "pears" {
    type    = number
    default = 2
}

locals { 
      fruits = templatefile("fruits.json", { pears = var.pears })
}

S3存储后的问题 #

将该JSON文件上传到S3后，通过aws_s3_bucket_object数据源获取内容时，无法直接用templatefile解析占位符，得到的pears字段是字面量${pears}，直接替换也无效：

data "aws_s3_bucket_object" "fruits" {
    bucket = "fruits"
    key    = "subdir/fruits.json"
}

locals {    
    data = jsondecode(data.aws_s3_bucket_object.fruits.body)  
}

解决方案 #

方法1：单占位符用replace直接替换 #

如果只有少量占位符，直接用replace函数替换后再解析JSON：

data "aws_s3_bucket_object" "fruits" {
    bucket = "fruits"
    key    = "subdir/fruits.json"
}

variable "pears" {
    type    = number
    default = 2
}

locals {
    templated_body = replace(
        data.aws_s3_bucket_object.fruits.body,
        "${pears}",
        tostring(var.pears)
    )
    fruits = jsondecode(local.templated_body)
}

方法2：多占位符用foldl批量替换 #

如果有多个变量占位符，用foldl遍历变量映射表批量替换：

data "aws_s3_bucket_object" "fruits" {
    bucket = "fruits"
    key    = "subdir/fruits.json"
}

variable "pears" {
    type    = number
    default = 2
}

variable "oranges" {
    type    = number
    default = 5
}

locals {
    var_mapping = {
        pears = tostring(var.pears)
        oranges = tostring(var.oranges)
    }

    templated_body = foldl(
        data.aws_s3_bucket_object.fruits.body,
        [for key, val in local.var_mapping : { k = key, v = val }],
        (current_body, item) => replace(current_body, "${${item.k}}", item.v)
    )

    fruits = jsondecode(local.templated_body)
}

方法3：临时文件配合templatefile（不推荐） #

这种方法依赖本地执行环境，兼容性差，仅作备选：

data "aws_s3_bucket_object" "fruits" {
    bucket = "fruits"
    key    = "subdir/fruits.json"
}

variable "pears" {
    type    = number
    default = 2
}

# 生成临时模板文件
resource "null_resource" "temp_template" {
    triggers = {
        s3_content = data.aws_s3_bucket_object.fruits.body
        pears_val  = var.pears
    }

    provisioner "local-exec" {
        command = "echo '${data.aws_s3_bucket_object.fruits.body}' > ./temp_fruits.json"
    }
}

locals {
    fruits = templatefile("./temp_fruits.json", { pears = var.pears })
}

# 清理临时文件
resource "null_resource" "clean_temp" {
    depends_on = [null_resource.temp_template]
    provisioner "local-exec" {
        command = "rm ./temp_fruits.json"
    }
}

内容的提问来源于stack exchange，提问作者yuvi