When provisioning with Terraform, how does code obtain a reference to machine IDs (e.g. database machine address) - amazon-ec2

Question

Let's say I'm using Terraform to provision two machines inside AWS:
An EC2 Machine running NodeJS
An RDS instance
How does the NodeJS code obtain the address of the RDS instance?

Answer 1

You've got a couple of options here. The simplest one is to create a CNAME record in Route53 for the database and then always point to that CNAME in your application.
A basic example would look something like this:
resource "aws_db_instance" "mydb" {
allocated_storage = 10
engine = "mysql"
engine_version = "5.6.17"
instance_class = "db.t2.micro"
name = "mydb"
username = "foo"
password = "bar"
db_subnet_group_name = "my_database_subnet_group"
parameter_group_name = "default.mysql5.6"
}
resource "aws_route53_record" "database" {
zone_id = "${aws_route53_zone.primary.zone_id}"
name = "database.example.com"
type = "CNAME"
ttl = "300"
records = ["${aws_db_instance.default.endpoint}"]
}
Alternative options include taking the endpoint output from the aws_db_instance and passing that into a user data script when creating the instance or passing it to Consul and using Consul Template to control the config that your application uses.

Answer 2

You may try Sparrowform - a lightweight provision tool for Terraform based instances, it's capable to make an inventory of Terraform resources and provision related hosts, passing all the necessary data:
$ terrafrom apply # bootstrap infrastructure
$ cat sparrowfile # this scenario
# fetches DB address from terraform cache
# and populate configuration file
# at server with node js code:
#!/usr/bin/env perl6
use Sparrowform;
$ sparrowfrom --ssh_private_key=~/.ssh/aws.pem --ssh_user=ec2 # run provision tool
my $rdb-adress;
for tf-resources() -> $r {
my $r-id = $r[0]; # resource id
if ( $r-id 'aws_db_instance.mydb') {
my $r-data = $r[1];
$rdb-address = $r-data<address>;
last;
}
}
# For instance, we can
# Install configuration file
# Next chunk of code will be applied to
# The server with node-js code:
template-create '/path/to/config/app.conf', %(
source => ( slurp 'app.conf.tmpl' ),
variables => %(
rdb-address => $rdb-address
),
);
# sparrowform --ssh_private_key=~/.ssh/aws.pem --ssh_user=ec2 # run provisioning
PS. disclosure - I am the tool author