AWS CloudFormation identify IP of another instance - circular dependency - amazon-web-services

Question

I'm provisioning two instances with CloudFormation template. "MASTER" and "SLAVE".
At the userdata script I need to pass slave's private IP to master and master's IP to slave.
Here is my template:
{
"AWSTemplateFormatVersion" : "2010-09-09",
"Description" : "",
"Parameters" : {
},
"Resources" : {
"InstanceSecurityGroup" : {
"Type" : "AWS::EC2::SecurityGroup",
"Properties" : {
"GroupDescription" : "Default Ports",
"SecurityGroupIngress" : [
{ "IpProtocol" : "tcp", "FromPort" : "22", "ToPort" : "22", "CidrIp" : "0.0.0.0/0"}
]
}
},
"MASTER" : {
"Type" : "AWS::EC2::Instance",
"Properties" : {
"Tags":[{"Key":"Name", "Value":"MASTER"}],
"SecurityGroups" : [ { "Ref" : "InstanceSecurityGroup" } ],
"KeyName" : "mykey",
"ImageId" : "ami-a25415cb",
"InstanceType": "m1.large",
"UserData" : { "Fn::Base64" : { "Fn::Join" : ["", [
"#!/bin/bash -ex", "\n",
"wget https://s3.amazonaws.com/mybucket/bootstrap.sh","\n",
"ROLE=MASTER SLAVEIP=",?????," sh bootstrap.sh","\n"
] ] } }
}
},
"SLAVE" : {
"Type" : "AWS::EC2::Instance",
"Properties" : {
"Tags":[{"Key":"Name", "Value":"SLAVE"}],
"SecurityGroups" : [ { "Ref" : "InstanceSecurityGroup" } ],
"KeyName" : "mykey",
"ImageId" : "ami-a25415cb",
"InstanceType": "m1.large",
"UserData" : { "Fn::Base64" : { "Fn::Join" : ["", [
"#!/bin/bash -ex", "\n",
"wget https://s3.amazonaws.com/mybucket/bootstrap.sh","\n",
"ROLE=SLAVE MASTERIP=",?????," sh bootstrap.sh","\n"
] ] } }
}
},
},
"Outputs" : {
}
}
What would be right replacement for ????? if it's possible at all and if not - what alternative can I use?
UPD: found this: {"Fn::GetAtt": ["MASTER","PrivateIp"]}, it works fine on it's own, but fails with "Template validation error: Circular dependency between resources: [SLAVE, MASTER]" if I'm trying to do both master and slave IPs.

Answer 1

If you're using VPCs and Subnets, you can do this by creating an AWS::EC2::NetworkInterface for each instance. Then use { "Fn::GetAtt": [ "MyNetworkInterface", "PrimaryPrivateIpAddress" ] } in user data to reference the internal IP address of the network interface
You associate a network interface with an EC2 instance using the NetworkInterfaces property
...
"MasterNetInt" : {
"Type" : "AWS::EC2::NetworkInterface",
"Properties" : {
"SubnetId": { "Ref" : "MySubnet" }
}
},
"SlaveNetInt" : {
"Type" : "AWS::EC2::NetworkInterface",
"Properties" : {
"SubnetId": { "Ref" : "MySubnet" }
}
},
"Master" : {
"Type" : "AWS::EC2::Instance",
"Properties" : {
"SecurityGroups" : [ { "Ref" : "InstanceSecurityGroup" } ],
"KeyName" : "mykey",
"ImageId" : "ami-a25415cb",
"InstanceType": "m1.large",
"SubnetId": { "Ref" : "MySubnet" },
"NetworkInterfaces": [ { "Ref" : "MasterNetInt" } ],
"UserData": { "Fn::Base64" : { "Fn::Join" : ["", [
"#!/bin/bash -ex", "\n",
"wget https://s3.amazonaws.com/mybucket/bootstrap.sh","\n",
"ROLE=MASTER SLAVEIP=", { "Fn::GetAtt": [ "SlaveNetInt", "PrimaryPrivateIpAddress" ] }," sh bootstrap.sh","\n"
] ] } }
}
},
"Slave" : {
"Type" : "AWS::EC2::Instance",
"Properties" : {
"SecurityGroups" : [ { "Ref" : "InstanceSecurityGroup" } ],
"KeyName" : "mykey",
"ImageId" : "ami-a25415cb",
"InstanceType": "m1.large",
"SubnetId": { "Ref" : "MySubnet" },
"NetworkInterfaces": [ { "Ref" : "SlaveNetInt" } ],
"UserData" : { "Fn::Base64" : { "Fn::Join" : ["", [
"#!/bin/bash -ex", "\n",
"wget https://s3.amazonaws.com/mybucket/bootstrap.sh","\n",
"ROLE=SLAVE MASTERIP=", { "Fn::GetAtt": [ "MasterNetInt", "PrimaryPrivateIpAddress" ] }," sh bootstrap.sh","\n"
] ] } }
}
}
...
If you're unfamiliar with setting up a VPC and Subnets, read the following documentation: http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Scenario1.html
and reference these template examples: http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/sample-templates-services-us-west-2.html#d0e113371
The basic requirements are:
AWS::EC2::VPC
AWS::EC2::InternetGateway
AWS::EC2::VPCGatewayAttachment
AWS::EC2::RouteTable
AWS::EC2::Route
AWS::EC2::Subnet
AWS::EC2::SubnetRouteTableAssociation
AWS::EC2::NetworkAcl
AWS::EC2::SubnetNetworkAclAssociation
AWS::EC2::NetworkAclEntry
AWS::EC2::NetworkInterface
AWS::EC2::Instance