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@ChristophShyper
Last active February 17, 2021 12:24
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Pass metadata between Terraform modules using DynamoDB. Only scalar values are possible. Maps or sets will cause issues. Much faster than remote state.

Save metadata in DynamoDB

resource "aws_dynamodb_table_item" "organization" {
  table_name = "my-dyn-db-table"
  hash_key = "KeyName"
  item = jsonencode({
    KeyName = {
      S = "organization"
    }
    Outputs = {
      M = {
        organization_arn = {
          S = aws_organizations_organization.default.arn
        }
        organization_id = {
          S = aws_organizations_organization.default.id
        }
      }
    }
  })
}

Use Python

Create the script for reading data

import boto3
import json
import sys


def handler(arg):
    table_name = arg['table_name']
    key_name = arg['key_name']
    item_name = arg['item_name']
    attr_name = arg['attr_name']
    dynamodb = boto3.resource('dynamodb')
    table = dynamodb.Table(table_name)
    ret = table.get_item(
        Key={
            key_name: item_name
        }
    )['Item'][attr_name]
    print("{}".format(json.dumps(ret)))


if __name__ == '__main__':
    data = json.load(sys.stdin)
    handler(data)
    exit(0)

Read the data via data source

data "external" "example" {
  program = ["python3", "${path.module}/read-dynamodb.py"]

  query = {
    table_name = "my-dyn-db-table"
    key_name = "ModulePath"
    item_name = "organization"
    attr_name = "Outputs"
  }
}

Use the data

output "organization_id" {
  value = data.external.example.result.organization_id
}

Use Bash

Create the script for reading data

#!/usr/bin/env bash
set -e
eval "$(jq -r '@sh "TABLE_NAME=\(.table_name) KEY_NAME=\(.key_name) ITEM_NAME=\(.item_name)"')"
RESP=$(aws dynamodb get-item --table-name "${TABLE_NAME}" --key "{ \"${KEY_NAME}\": { \"S\": \"${ITEM_NAME}\" }}" --region eu-west-1 | jq '.Item.Outputs.M')
jq -n --arg resp "${RESP}" '{"Outputs":$resp}'

Read the data via data source

data "external" "example" {
  program = ["sh", "${path.module}/read-dynamodb.sh"]

  query = {
    table_name = "my-dyn-db-table"
    key_name = "KeyName"
    item_name = "organization"
  }
}

Use the data

output "organization" {
  value = data.external.example.result["Outputs"]
}
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