How to provide expression as value inside an ssm document? - aws-ssm

Question

I would like to add a server to an ausostaling-group using SSM document, if the group has n instances running - i want to have (n+1).
Since this stack is managed by cloudformation, i just need to increase the 'DesiredCapacity' variable and update the stack. so i created a document with 2 steps:
get the current value of 'DesiredCapacity'
update stack with value of 'DesiredCapacity' + 1
I didnt find a way to express this simple operation, i guess im doing something wrong ...
SSM Document:
schemaVersion: '0.3'
parameters:
cfnStack:
description: 'The cloudformation stack to be updated'
type: String
mainSteps:
- name: GetDesiredCount
action: 'aws:executeAwsApi'
inputs:
Service: cloudformation
Api: DescribeStacks
StackName: '{{ cfnStack }}'
outputs:
- Selector: '$.Stacks[0].Outputs.DesiredCapacity'
Type: String
Name: DesiredCapacity
- name: UpdateCloudFormationStack
action: 'aws:executeAwsApi'
inputs:
Service: cloudformation
Api: UpdateStack
StackName: '{{ cfnStack }}'
UsePreviousTemplate: true
Parameters:
- ParameterKey: WebServerCapacity
ParameterValue: 'GetDesiredCount.DesiredCapacity' + 1 ### ERROR
# ParameterValue: '{{GetDesiredCount.DesiredCapacity}}' + 1 ### ERROR (trying to concat STR to INT)
# ParameterValue: '{{ GetDesiredCount.DesiredCapacity + 1}}' ### ERROR

Answer 1

There is a way to do calculation inside an SSM document using python runtime.
The additional python step do the following:
Python runtime get variables via the the 'InputPayload' property
The 'current' (str) key added to the event object
The python function script_handler called
The 'current' extracted using event['current']
Converting string to int and adding 1
return a dictionary with the 'desired_capacity' key and value as string
expose the output ($.Payload.desired_capacity referred to the 'desired_capacity' of the returned dictionary)
schemaVersion: '0.3'
parameters:
cfnStack:
description: 'The cloudformation stack to be updated'
type: String
mainSteps:
- name: GetDesiredCount
action: 'aws:executeAwsApi'
inputs:
Service: cloudformation
Api: DescribeStacks
StackName: '{{ cfnStack }}'
outputs:
- Selector: '$.Stacks[0].Outputs.DesiredCapacity'
Type: String
Name: DesiredCapacity
- name: Calculate
action: 'aws:executeScript'
inputs:
Runtime: python3.6
Handler: script_handler
Script: |-
def script_handler(events, context):
desired_capacity = int(events['current']) + 1
return {'desired_capacity': str(desired_capacity)}
InputPayload:
current: '{{ GetDesiredCount.DesiredCapacity }}'
outputs:
- Selector: $.Payload.desired_capacity
Type: String
Name: NewDesiredCapacity
- name: UpdateCloudFormationStack
action: 'aws:executeAwsApi'
inputs:
Service: cloudformation
Api: UpdateStack
StackName: '{{ cfnStack }}'
UsePreviousTemplate: true
Parameters:
- ParameterKey: WebServerCapacity
ParameterValue: '{{ Calculate.NewDesiredCapacity}}'