CODEWITHSUNDEEP
pythonamazon-web-servicesbotoaws-cloudformation
Hugo Lopes Tavares
Hugo Lopes Tavares

Reputation: 30374

Boto: What is the best way to check if a CloudFormation stack is exists?

What is the best way to check if a CloudFormation stack exists and is not in a broken state using Boto? By broken I mean failed and rollback states.

I don't want to use a try/except solution, because boto logs it as an error, and in my scenario it's going to send the exception log to an alarm system.

At the moment I have the following solutions:

1) Use boto.cloudformation.connection.CloudFormationConnection.describe_stacks()

valid_states = '''\
CREATE_IN_PROGRESS
CREATE_COMPLETE
UPDATE_IN_PROGRESS
UPDATE_COMPLETE_CLEANUP_IN_PROGRESS
UPDATE_COMPLETE'''.splitlines()

def describe_stacks():
    result = []
    resp = cf_conn.describe_stacks()
    result.extend(resp)
    while resp.next_token:
        resp = cf_conn.describe_stacks(next_token=resp.next_token)
        result.extend(resp)
    return result


stacks = [stack for stack in describe_stacks() if stack.stack_name == STACK_NAME and stack.stack_status in valid_states]
exists = len(stacks) >= 1

This is slow because I have lots of stacks.

2) Use boto.cloudformation.connection.CloudFormationConnection.list_stacks()

def list_stacks(filters):
    result = []
    resp = cf_conn.list_stacks(filters)
    result.extend(resp)
    while resp.next_token:
        resp = cf_conn.list_stacks(filters, next_token=resp.next_token)
        result.extend(resp)
    return result

stacks = [stack for stack in list_stacks(valid_states) if stack.stack_name == STACK_NAME]
exists = len(stacks) >= 1

This takes forever, because summaries are kept for 90 days and I have lots of stacks.

Question: What is the ideal solution to check if a given stack exists and is not in a failure or rollback state?

Upvotes: 13

Views: 20214

Answers (6)

samtoddler
samtoddler

Reputation: 9605

I would check via the paginator combined with ListStacks API call because it might be possible my lambda or I doesn't have permissions for cloudformation if that happens, I should return relevant information instead of saying stack doesn't exist.

import boto3
from botocore.exceptions import ClientError

cfn = boto3.client('cloudformation')
def stack_exists(stack_name: str, stack_status: str) -> bool:
    try:
        paginator = cfn.get_paginator('list_stacks')
        response_iterator = paginator.paginate()
        for page in response_iterator:
            for stack in page['StackSummaries']:
                if stack_name == stack.get('StackName') and stack.get('StackStatus') != stack_status:
                    return True
    except ClientError as e:
        logger.exception(f'Client error while checking stack : {e}')
        raise
    except Exception:
        logger.exception('Error while checking stack')
        raise
    return False

Or if we don't want to iterate over all the stacks:

import boto3
from botocore.exceptions import ClientError

cfn = boto3.client('cloudformation')
def stack_exists(stack_name: str, required_status='DELETE_COMPLETE'):
    try:
        stacks_summary = cfn.describe_stacks(StackName=stack_name)
        stack_info = stacks_summary.get('Stacks')[0]
        return stack_name == stack_info.get('StackName') and stack_info.get('StackStatus') != required_status
    except ClientError as e:
        stack_not_found_error = f'Stack with id {stack_name} does not exist'
        error_received = e.response('Error')
        error_code_received = error_received.get('Code')
        error_message_received = error_received.get('Message')
        if error_code_received == 'ValidationError' and error_message_received == stack_not_found_error:
            return True
        logger.exception(f'Client error while describing stacks: {e}')
        raise
    except Exception:
        logger.exception('Error while checking stack')
        raise

Upvotes: 1

Justin Barton
Justin Barton

Reputation: 131

I implemented the following which works:

import boto3
from botocore.exceptions import ClientError

client = boto3.client('cloudformation')

def stack_exists(name, required_status = 'CREATE_COMPLETE'):
    try:
        data = client.describe_stacks(StackName = name)
    except ClientError:
        return False
    return data['Stacks'][0]['StackStatus'] == required_status

I didn't find any of the previous solutions complete, nor is there any quick way of doing it with boto3 so I created the above.

Upvotes: 13

Nathan Thompson
Nathan Thompson

Reputation: 335

You can silence the Boto logger by setting its level above ERROR. This will let you capture the exception without the logging alarm:

import boto3
import botocore.exceptions
import logging

cf = boto3.resource('cloudformation')
logging.Logger.manager.loggerDict['boto3'].setLevel(logging.CRITICAL)
logging.Logger.manager.loggerDict['botocore'].setLevel(logging.CRITICAL)
try:
    stack.Stack('foo').stack_status
except botocore.exceptions.ClientError:
    logging.info('stack doesnt exist')

logging.Logger.manager.loggerDict['boto3'].setLevel(logging.WARNING)
logging.Logger.manager.loggerDict['botocore'].setLevel(logging.WARNING)

Upvotes: 0

Sathed
Sathed

Reputation: 846

I know this is old, but someone asked if there was a solution to it just a couple of weeks ago, so here goes...

If you read the boto3 docs, it mentions deleted stacks quite frequently. In order to do this, you have to use the full stack ID. You can't use the stack name. This is because the only thing truly unique about the stack is the ID.

Example:

resource = boto3.resource('cloudformation')
status = resource.Stack('id:of:stack').stack_status

The only time this will return an exception is if that stack ID doesn't exist.

Cheers!

Upvotes: 1

Liyan Chang
Liyan Chang

Reputation: 8051

The best way to do this will be to split this into two separate problems:

  1. Find out which stacks don't exist.
  2. Find out which stacks are in an error state.

Might look something like this:

failure_states = ['CREATE_FAILED', ... ]
stack_names = ['prod', 'dev', 'test']

c = boto.cloudformation.connect_to_region(region)
existing_stacks = [s.stack_name for s in c.describe_stacks()]
nonexistent_stacks = set(stack_names) - set(existing_stacks)
error_stacks = c.list_stacks(stack_status_filters=failure_states)

You may have many more stacks then I do so maybe you need to use next_token to paginate, which would be a bit of a hassle. However, you can see that both operations can be done with a single request each and will not throw an exception.

Upvotes: 0

Ben Whaley
Ben Whaley

Reputation: 34406

From the boto docs:

describe_stacks(stack_name_or_id=None, next_token=None)

Returns the description for the specified stack; if no stack name was specified, then it returns the description for all the stacks created.

Parameters: stack_name_or_id (string) – The name or the unique identifier associated with the stack.

Since you know the stack name, you can use describe_stacks(stack_name_or_id=STACK_NAME). That should speed things up for you.

Upvotes: 4

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