Dependency between pubsub topic and subscription using terraform script - google-cloud-platform

Question

I am using one terraform script to create a pub sub topic and subscription. If the subscription needs to subscribes from the topic created by the same script, is there a way to create a dependency such that terraform attempts to create the pub/sub subscription only after the topic is created?
My main file looks like this :
version = ""
project = var.project_id
region = var.region
zone = var.zone
}
# module "Dataflow" {
#source = "../modules/cloud-dataflow"
#}
module "PubSubTopic" {
source = "../modules/pubsub_topic"
}
#module "PubSubSubscription" {
# source = "../modules/pubsub_subscription"
#}
#module "CloudFunction" {
# source = "../modules/cloud-function"
#}

Answer 1

Terraform will attempt to create the resources following the proper order but to answer your question and what your looking for is modules dependency "depends_on".
For example, subscription module will be created only once topic resource has been already created. That way you should add the depends_on on the subscription module.
Example:
resource "aws_iam_policy_attachment" "example" {
name = "example"
roles = [aws_iam_role.example.name]
policy_arn = aws_iam_policy.example.arn
}
module "uses-role" {
# ...
depends_on = [aws_iam_policy_attachment.example]
}
Official documentation: https://www.terraform.io/docs/language/meta-arguments/depends_on.html

Answer 2

You can create a simple pubsub topic and a subscription with this snippet (just add the .json for a service account with enough privileges) on your filesystem:
provider "google" {
credentials = "${file("account.json")}" # Or use GOOGLE_APPLICATION_CREDENTIALS
project = "__your_project_id__"
region = "europe-west4" # Amsterdam
}
resource "google_pubsub_topic" "incoming_data" {
name = "incoming-data"
}
resource "google_pubsub_subscription" "incoming_subs" {
name = "Subscription_for_incoming_data"
topic = google_pubsub_topic.incoming_data.name
# Time since Pubsub receives a message to deletion.
expiration_policy {
ttl = "300000s"
}
# Time from client reception to ACK
message_retention_duration = "1200s"
retain_acked_messages = false
enable_message_ordering = false
}
To link a subscription with a topic in terraform, you just need to link it with:
topic = google_pubsub_topic.TERRAFORM_TOPIC.name
Be carefull with Google requirements for topic and subscription identifiers. If they're not valid, terraform plan will pass, but you'll get an Error 400 : You have passed an invalid argument to the service