Terraform Fundamentals: CloudWatch Synthetics

Proactive Infrastructure Monitoring with Terraform and CloudWatch Synthetics

Modern infrastructure demands proactive monitoring, not just reactive alerting. Traditional monitoring often relies on observing system metrics after an issue impacts users. This is insufficient for complex, distributed systems. CloudWatch Synthetics provides a way to actively probe your applications and APIs, simulating user behavior and identifying problems before they escalate. Integrating this into a Terraform-based infrastructure as code (IaC) pipeline allows for consistent, version-controlled, and automated synthetic monitoring setup alongside your core infrastructure. This isn’t simply about adding another monitoring tool; it’s about shifting left on observability and embedding proactive checks directly into your infrastructure definition. It fits naturally within a platform engineering stack, providing self-service monitoring capabilities for application teams.

What is CloudWatch Synthetics in a Terraform Context?

CloudWatch Synthetics allows you to create canaries – configurable scripts that run on a schedule, mimicking user interactions with your applications. These canaries execute steps like making HTTP requests, performing browser-based interactions, and validating responses. Terraform manages the lifecycle of these canaries, ensuring they are defined as code and deployed consistently.

The primary Terraform resource is aws_cloudwatch_synthetic_monitor. This resource defines the canary configuration, including the runtime, script, schedule, and failure settings. There isn’t a dedicated provider beyond the standard aws provider.

Terraform-Specific Behavior & Caveats:

• State Management: Changes to canary scripts require a Terraform apply, meaning version control of the script itself is crucial.
• Idempotency: Terraform ensures that the canary exists in the desired state. Repeated applies with the same configuration will not create duplicate monitors.
• Dependencies: Canary scripts often depend on other infrastructure components (e.g., API Gateway endpoints, S3 buckets). Terraform’s dependency graph ensures these components are created before the canary is deployed.
• Update Limitations: Significant changes to the canary configuration (e.g., changing the runtime from Node.js to Python) may require resource destruction and recreation.

Use Cases and When to Use

1. API Endpoint Health Checks: Verify the availability and response times of critical API endpoints. Essential for SREs responsible for service level objectives (SLOs).
2. E-commerce Checkout Flow Validation: Simulate a complete user journey (add to cart, checkout) to detect issues in the payment processing pipeline. Critical for revenue-generating applications.
3. Third-Party Service Dependency Monitoring: Monitor the responsiveness of external APIs your application relies on. Proactively identify issues with external dependencies.
4. Single Page Application (SPA) Performance Monitoring: Measure the load times and functionality of SPAs, identifying performance bottlenecks in the client-side code. Important for front-end engineering teams.
5. Database Connection Verification: Regularly test database connectivity and query performance. Essential for database administrators and application developers.

Key Terraform Resources

1. aws_cloudwatch_synthetic_monitor: Defines the canary itself.
   

   resource "aws_cloudwatch_synthetic_monitor" "example" {
     name        = "my-api-health-check"
     runtime     = "nodejs"
     script      = file("canary.js")
     schedule    = "rate(5 minutes)"
     timeout     = 60
     failureThreshold = 1
     start_date  = "2024-01-01T00:00:00Z"
   }
   

2. aws_iam_role: Creates an IAM role for the canary to assume.
   

   resource "aws_iam_role" "canary_role" {
     name               = "cloudwatch-synthetics-role"
     assume_role_policy = jsonencode({
       Version = "2012-10-17",
       Statement = [
         {
           Action = "sts:AssumeRole",
           Principal = {
             Service = "synthetics.amazonaws.com"
           },
         },
       ],
     })
   }
   

3. aws_iam_policy: Defines the permissions granted to the canary role.
   

   resource "aws_iam_policy" "canary_policy" {
     name        = "cloudwatch-synthetics-policy"
     description = "Policy for CloudWatch Synthetics canaries"
     policy      = jsonencode({
       Version = "2012-10-17",
       Statement = [
         {
           Action = [
             "logs:CreateLogGroup",
             "logs:CreateLogStream",
             "logs:PutLogEvents",
           ],
           Resource = "*",
         },
         {
           Action = [
             "ec2:DescribeVpcs",
             "ec2:DescribeSubnets",
             "ec2:DescribeSecurityGroups",
           ],
           Resource = "*",
         },
       ],
     })
   }
   

4. aws_iam_role_policy_attachment: Attaches the policy to the role.
   

   resource "aws_iam_role_policy_attachment" "canary_attachment" {
     role       = aws_iam_role.canary_role.name
     policy_arn = aws_iam_policy.canary_policy.arn
   }
   

5. aws_cloudwatch_log_group: Creates a CloudWatch Logs group for canary logs.
   

   resource "aws_cloudwatch_log_group" "canary_logs" {
     name              = "cloudwatch-synthetics-logs"
     retention_in_days = 7
   }
   

6. data.aws_region: Dynamically retrieves the current AWS region.
   

   data "aws_region" "current" {}
   

7. local.canary_script: Stores the canary script as a local variable. Useful for complex scripts.
   

   locals {
     canary_script = file("canary.js")
   }
   

8. aws_cloudwatch_event_rule: Triggers actions based on canary failures (e.g., sending SNS notifications).
   

   resource "aws_cloudwatch_event_rule" "canary_failure_rule" {
     name        = "canary-failure-notification"
     description = "Rule to trigger notification on canary failure"
     event_pattern = jsonencode({
       source = ["aws.cloudwatch"],
       detail-type = ["CloudWatch Synthetic Monitor Alarm"],
       detail = {
         alarmName = [aws_cloudwatch_synthetic_monitor.example.name]
       }
     })
   }
   

Common Patterns & Modules

• Remote Backend: Store Terraform state remotely (e.g., S3 with DynamoDB locking) for collaboration and versioning.
• Dynamic Blocks: Use for_each or dynamic blocks to create multiple canaries based on a list of endpoints or services.
• Environment-Based Configuration: Use Terraform workspaces or separate directories to manage canary configurations for different environments (dev, staging, prod).
• Monorepo Structure: Organize canary configurations alongside other infrastructure code in a monorepo for better maintainability.

While no single canonical public module exists, several community-driven modules provide basic canary configurations. Searching the Terraform Registry for "cloudwatch synthetics" will yield relevant results. However, tailoring a module to your specific application requirements is often necessary.

Hands-On Tutorial

This example creates a simple canary that checks the HTTP status code of a public endpoint.

Provider Setup:

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 5.0"
    }
  }
}

provider "aws" {
  region = "us-east-1" # Replace with your desired region

}

Resource Configuration (canary.js):

// canary.js
exports.handler = async (event) => {
  const url = 'https://www.example.com'; // Replace with your endpoint
  const https = require('https');

  return new Promise((resolve, reject) => {
    https.get(url, (res) => {
      if (res.statusCode >= 200 && res.statusCode < 300) {
        resolve({
          statusCode: res.statusCode,
          body: 'Successfully checked ' + url
        });
      } else {
        reject(new Error('Request failed with status code: ' + res.statusCode));
      }
    }).on('error', (err) => {
      reject(err);
    });
  });
};

Terraform Configuration (main.tf):

resource "aws_cloudwatch_synthetic_monitor" "example" {
  name        = "example-http-check"
  runtime     = "nodejs"
  script      = file("canary.js")
  schedule    = "rate(1 minute)"
  timeout     = 30
  failureThreshold = 1
}

Apply & Destroy:

terraform init
terraform plan
terraform apply
terraform destroy

The terraform plan output will show the creation of the aws_cloudwatch_synthetic_monitor resource. terraform apply will deploy the canary. terraform destroy will remove it.

Enterprise Considerations

Large organizations leverage Terraform Cloud/Enterprise for state management, remote runs, and collaboration. Sentinel or Open Policy Agent (OPA) can enforce policy-as-code constraints on canary configurations (e.g., requiring specific tags, limiting runtime options). IAM design should follow the principle of least privilege, granting canaries only the necessary permissions. State locking is critical to prevent concurrent modifications. Costs are primarily driven by canary execution frequency and data transfer. Multi-region deployments require careful consideration of canary placement and data replication.

Security and Compliance

• Least Privilege: Use IAM roles with narrowly scoped permissions.
• RBAC: Control access to Terraform workspaces and canary configurations using IAM policies.
• Policy Constraints: Enforce tagging policies and runtime restrictions using Sentinel or OPA.
• Drift Detection: Regularly compare the Terraform state with the actual canary configuration in AWS.
• Auditability: Log all Terraform operations and canary execution events.

Example IAM Policy:

resource "aws_iam_policy" "canary_restrict_access" {
  name        = "canary-restrict-access-policy"
  description = "Restrict access to CloudWatch Synthetics canaries"
  policy      = jsonencode({
    Version = "2012-10-17",
    Statement = [
      {
        Effect   = "Allow",
        Action   = ["cloudwatch:GetSyntheticMonitor", "cloudwatch:ListSyntheticMonitors"],
        Resource = "*"
      },
    ],
  })
}

Integration with Other Services

graph LR
    A[Terraform] --> B(CloudWatch Synthetics);
    B --> C{CloudWatch Alarms};
    C --> D[SNS Notifications];
    B --> E[CloudWatch Logs];
    A --> F(API Gateway);
    B --> F;

1. CloudWatch Alarms: Trigger alarms based on canary failures.
2. SNS Notifications: Send notifications to Slack, email, or other channels when alarms are triggered.
3. CloudWatch Logs: Store canary execution logs for debugging and analysis.
4. API Gateway: Monitor the health and performance of API endpoints.
5. Lambda: Canaries can invoke Lambda functions to perform more complex checks.

Module Design Best Practices

• Abstraction: Encapsulate canary configurations into reusable modules.
• Input/Output Variables: Define clear input variables for customization (e.g., endpoint URL, schedule, runtime).
• Locals: Use locals to simplify complex expressions and improve readability.
• Backends: Utilize remote backends for state management and collaboration.
• Documentation: Provide comprehensive documentation for the module, including usage examples and parameter descriptions.

CI/CD Automation

# .github/workflows/deploy-canaries.yml

name: Deploy CloudWatch Synthetics Canaries

on:
  push:
    branches:
      - main

jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - uses: hashicorp/setup-terraform@v2
      - run: terraform fmt
      - run: terraform validate
      - run: terraform plan -out=tfplan
      - run: terraform apply tfplan

Pitfalls & Troubleshooting

1. IAM Permissions: Canaries failing due to insufficient IAM permissions. Solution: Verify the canary role has the necessary permissions.
2. Script Errors: Errors in the canary script causing failures. Solution: Review the CloudWatch Logs for detailed error messages.
3. Timeout Issues: Canaries timing out due to slow response times. Solution: Increase the timeout value or optimize the endpoint being monitored.
4. State Corruption: Terraform state corruption leading to inconsistencies. Solution: Restore from a backup or manually correct the state.
5. Incorrect Schedule: Canaries not running as expected due to an incorrect schedule. Solution: Verify the schedule expression is valid and aligned with the desired frequency.
6. Network Connectivity: Canaries failing due to network connectivity issues. Solution: Ensure the canary can reach the target endpoint.

Pros and Cons

Pros:

• Proactive Monitoring: Detects issues before they impact users.
• IaC Integration: Consistent and version-controlled monitoring setup.
• Automation: Automated canary deployment and management.
• Early Warning System: Provides an early warning system for application failures.

Cons:

• Scripting Overhead: Requires writing and maintaining canary scripts.
• Cost: Canary execution costs can accumulate over time.
• Complexity: Setting up and configuring canaries can be complex.
• Maintenance: Requires ongoing maintenance and updates to canary scripts.

Conclusion

CloudWatch Synthetics, when integrated with Terraform, provides a powerful mechanism for proactive infrastructure monitoring. It shifts the focus from reactive alerting to preventative checks, improving application reliability and user experience. Engineers should prioritize evaluating this service, building reusable modules, and integrating it into their CI/CD pipelines to unlock its full potential. Start with a proof-of-concept for a critical API endpoint, then expand to cover more complex user journeys and dependencies.