Budget alarms for AWS accounts and services using Terraform

Who never had a surprise when checking the billing panel from Amazon, google, or another cloud provider? It happened with me, a couple of times actually, and I am sure it happens with you as well as in your organization 😅

Common problems and motivations 💸

It’s not rare that we sometimes think service would cost X and it ends up costing 2X in the final of the month, sometimes we also create resources manually for experimentations and forget to clean them up, on the other hand, it is not healthy to access the budgets panel every day to check whether the costs are okay or not, so the best way to avoid surprises at the end of the month would be automating this process and creating some sort of alarms when something is wrong.

Is this post we are going to create a process to keep track of our budgets in AWS by defining a threshold in the account level as well as by services. When the threshold is reached we are going to receive an alert via email (Slack users can use the Email Slack App so that they can receive the email in a specific channel).

Prerequisites

This guide assumes some basic familiarity with the usual Terraform workflow (init, plan, and apply). It also assumes that you have your terraform and AWS provider properly configured.

Implementation Scenario

Using AWS Billings it is possible to keep track of costs using a lot of approaches, for instance, resource tags where we can filter costs by a specific business or teams, resource names using amazon services names, etc. It is also possible to set alarms for forecast values or current values.

Let's say that we have a development account in AWS and we usually use two services: EC2 and S3 which costs $ 15/month.

EC2: $ 10,00
S3: $ 5,00

We also want to reserve $ 5,00 in the account for eventual costs, in the end, what we need to achieve is:

Receive an alert when:

Forecast amount for the Development account is greater than 20,00
Forecast amount for EC2 is greater than 10,00
Forecast amount for S3 is greater than 5,00

After implementing the module in terraform, we are going to use it like this:

module "billing_alarm" {
  source = "./../../modules/budgets"

  account_name         = "Development"
  account_budget_limit = "20.0"

  services = {
    EC2 = {
      budget_limit = "10.0"
    },
    S3 = {
      budget_limit = "5.0"
    }
  }
}

Before Starting

If you in a hurry, you can find this module ready to use here

Terraform Module Structure

Let’s get started by creating a terraform module so that we can reuse in cases where we have more than one account, our folder's structure will be something like this:

.
├── accounts
│   └── development
│       ├── budgets.tf
│       └── provider.tf # not configured in this post    
│       
└── modules
    └── budgets
        ├── main.tf
        ├── services.tf
        └── variables.tf

Input Variables

First, create the directory modules/budgets and the file variables.tf, it will have input variables for our module:

Account name: Name of the AWS account to be tracked.
Account budget limit: Threshold cost for the account level.
Services: List of AWS services to be tracked, we need to inform the name and values for each service.

# modules/budgets/variables.tf

variable "account_name" {}

variable "account_budget_limit" {}

variable "services" {
  description = "List of AWS services to be monitored in terms of costs"

  type = map(object({
    budget_limit = string
  }))
}

Services Helper

The name of the service should match the name AWS expects otherwise the filter will not work properly, thus, we can create a support map to define some common services (if the service you want is not listed here you can add it later), in the same folder create a new a file called services.tf

# modules/budgets/services.tf

locals {
  aws_services = {
    Athena         = "Amazon Athena"
    EC2            = "Amazon Elastic Compute Cloud - Compute"
    ECR            = "Amazon EC2 Container Registry (ECR)"
    ECS            = "Amazon EC2 Container Service"
    Kubernetes     = "Amazon Elastic Container Service for Kubernetes"
    EBS            = "Amazon Elastic Block Store"
    CloudFront     = "Amazon CloudFront"
    CloudTrail     = "AWS CloudTrail"
    CloudWatch     = "AmazonCloudWatch"
    Cognito        = "Amazon Cognito"
    Config         = "AWS Config"
    DynamoDB       = "Amazon DynamoDB"
    DMS            = "AWS Database Migration Service"
    ElastiCache    = "Amazon ElastiCache"
    Elasticsearch  = "Amazon Elasticsearch Service"
    ELB            = "Amazon Elastic Load Balancing"
    Gateway        = "Amazon API Gateway"
    Glue           = "AWS Glue"
    Kafka          = "Managed Streaming for Apache Kafka"
    KMS            = "AWS Key Management Service"
    Kinesis        = "Amazon Kinesis"
    Lambda         = "AWS Lambda"
    Lex            = "Amazon Lex"
    Matillion      = "Matillion ETL for Amazon Redshift"
    Pinpoint       = "AWS Pinpoint"
    Polly          = "Amazon Polly"
    Rekognition    = "Amazon Rekognition"
    RDS            = "Amazon Relational Database Service"
    Redshift       = "Amazon Redshift"
    S3             = "Amazon Simple Storage Service"
    SFTP           = "AWS Transfer for SFTP"
    Route53        = "Amazon Route 53"
    SageMaker      = "Amazon SageMaker"
    SecretsManager = "AWS Secrets Manager"
    SES            = "Amazon Simple Email Service"
    SNS            = "Amazon Simple Notification Service"
    SQS            = "Amazon Simple Queue Service"
    Tax            = "Tax"
    VPC            = "Amazon Virtual Private Cloud"
    WAF            = "AWS WAF"
    XRay           = "AWS X-Ray"
  }
}

Now, we can implement the budgets module, let's start by creating an SNS topic so that when an alarm is triggered, it will send a message in this topic and everyone subscribed in this topic will receive the message, in our case, it will be an email, we will get into that later.

Budget Module

Here we are creating a new topic and defining a policy that allows AWS budgets to Publish Message.

# modules/budgets/main.tf

resource "aws_sns_topic" "account_billing_alarm_topic" {
  name = "account-billing-alarm-topic"
}

resource "aws_sns_topic_policy" "account_billing_alarm_policy" {
  arn    = aws_sns_topic.account_billing_alarm_topic.arn
  policy = data.aws_iam_policy_document.sns_topic_policy.json
}

data "aws_iam_policy_document" "sns_topic_policy" {

  statement {
    sid = "AWSBudgetsSNSPublishingPermissions"
    effect = "Allow"

    actions = [
      "SNS:Receive",
      "SNS:Publish"
    ]

    principals {
      type        = "Service"
      identifiers = ["budgets.amazonaws.com"]
    }

    resources = [
      aws_sns_topic.account_billing_alarm_topic.arn
    ]
  }
}

Once we have a topic, we can create a budget resource for the account level and subscribe to the topic.

# modules/budgets/main.tf

resource "aws_budgets_budget" "budget_account" {
  name              = "${var.account_name} Account Monthly Budget"
  budget_type       = "COST"
  limit_amount      = var.account_budget_limit
  limit_unit        = "USD"
  time_unit         = "MONTHLY"
  time_period_start = "2020-01-01_00:00"

  notification {
    comparison_operator       = "GREATER_THAN"
    threshold                 = 100
    threshold_type            = "PERCENTAGE"
    notification_type         = "FORECASTED"
    subscriber_sns_topic_arns = [
      aws_sns_topic.account_billing_alarm_topic.arn
    ]
  }

  depends_on = [
    aws_sns_topic.account_billing_alarm_topic
  ]
}

The last resource will be the budgets for services, it will receive a list of services and create a budget for each one.

# modules/budgets/main.tf

resource "aws_budgets_budget" "budget_resources" {
  for_each = var.services

  name              = "${var.account_name} ${each.key} Monthly Budget"
  budget_type       = "COST"
  limit_amount      = each.value.budget_limit
  limit_unit        = "USD"
  time_unit         = "MONTHLY"
  time_period_start = "2020-01-01_00:00"

  cost_filters = {
    Service = lookup(local.aws_services, each.key)
  }

  notification {
    comparison_operator       = "GREATER_THAN"
    threshold                 = 100
    threshold_type            = "PERCENTAGE"
    notification_type         = "FORECASTED"
    subscriber_sns_topic_arns = [
      aws_sns_topic.account_billing_alarm_topic.arn
    ]
  }

  depends_on = [
    aws_sns_topic.account_billing_alarm_topic
  ]
}

The module is complete, now we can easily reuse in our amazon accounts, to do so, create a budgets.tf in the account folder and import the module, like this:

# accounts/development/budgets.tf

module "billing_alarm" {
  source = "../../modules/budgets"

  account_name         = "Development"
  account_budget_limit = "20.0"

  services = {
    EC2 = {
      budget_limit = "10.0"
    },
    S3 = {
      budget_limit = "5.0"
    }
  }
}

To deploy this infrastructure, go to the development folder and type:

$ terraform init
$ terraform plan

# After check the plan command we can apply 
$ terraform apply

After applying you should have the budgets as shown below:

And the SNS topic:

The last step is to subscribe to this topic via Email app in Slack or any email.

Slack or Email Integration

If you have a slack paid plan, you can use the Email Slack App integration, it provides a custom email to be used in the slack channel, thus, every email that is sent to this account will appear in the channel.

I don't have a pro slack plan, so for simplicity, I will use my email for it, however, the process would be the same using the email integration app.

Note: This part of the process is not supported by Terraform, so we are going to do it manually.

Subscribing the email to the topic

Go to SNS panel in AWS and select Subscriptions and create a new subscription by choosing the topic that was created in the previous step and in Protocol chose email, after creating the subscription you should receive an email from AWS SNS.

From now on, every time you hit a threshold you will receive an email notification like this:

AWS Budget Notification May 04, 2020
AWS Account XXXXXXXXX

Dear AWS Customer,

You requested that we alert you when the FORECASTED Cost associated with your Development EC2 Monthly Budget is greater than $10.00 for the current month. 

The FORECASTED Cost associated with this budget is $15.00. 

You can find additional details below and by accessing the AWS Budgets dashboard.

Budget Name: Development EC2 Monthly Budget
Budget Type: Cost
Budgeted Amount: $10.00
Alert Type: FORECASTED
Alert Threshold: > $10.00
FORECASTED Amount: $15.00

That's it!

Note: This implementation uses the version 0.12 of Terraform, you can find the same implementation using the version 0.11 here

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