Best Practice: Safe Deployments (Feature Flags, Canary, Blue/Green)

# Terraform: CodeDeploy Canary for ECS Service
resource "aws_codedeploy_app" "payment_service" {
  name             = "payment-service"
  compute_platform = "ECS"
}

resource "aws_codedeploy_deployment_group" "production" {
  app_name               = aws_codedeploy_app.payment_service.name
  deployment_group_name  = "payment-service-production"
  service_role_arn       = aws_iam_role.codedeploy_role.arn
  deployment_config_name = "CodeDeployDefault.ECSCanary10Percent5Minutes"
  # Meaning: 10% traffic to new version, observe for 5 minutes, then 100%

  auto_rollback_configuration {
    enabled = true
    events  = [
      "DEPLOYMENT_FAILURE",
      "DEPLOYMENT_STOP_ON_ALARM"
    ]
  }

  alarm_configuration {
    alarms  = [aws_cloudwatch_metric_alarm.canary_error_rate.name]
    enabled = true
  }

  blue_green_deployment_config {
    deployment_ready_option {
      action_on_timeout    = "CONTINUE_DEPLOYMENT"
      wait_time_in_minutes = 0
    }
    terminate_blue_instances_on_deployment_success {
      action                           = "TERMINATE"
      termination_wait_time_in_minutes = 5
    }
  }

  deployment_style {
    deployment_option = "WITH_TRAFFIC_CONTROL"
    deployment_type   = "BLUE_GREEN"
  }

  ecs_service {
    cluster_name = aws_ecs_cluster.production.name
    service_name = aws_ecs_service.payment_service.name
  }

  load_balancer_info {
    target_group_pair_info {
      prod_traffic_route {
        listener_arns = [aws_lb_listener.https.arn]
      }
      target_group {
        name = aws_lb_target_group.blue.name
      }
      target_group {
        name = aws_lb_target_group.green.name
      }
    }
  }
}

# CloudWatch Alarm that triggers automatic rollback
resource "aws_cloudwatch_metric_alarm" "canary_error_rate" {
  alarm_name          = "canary-error-rate-high"
  comparison_operator = "GreaterThanThreshold"
  evaluation_periods  = 1
  threshold           = 20  # Rollback when 5xx > 20 during canary period

  metric_query {
    id          = "error_count"
    expression  = "SELECT SUM(HTTPCode_Target_5XX_Count) FROM SCHEMA(\"AWS/ApplicationELB\", LoadBalancer, TargetGroup)"
    return_data = true
    period      = 60
  }

  alarm_actions = []  # CodeDeploy monitors this alarm directly
}

# Lambda Blue/Green with alias and weighting
resource "aws_lambda_alias" "live" {
  name             = "live"
  function_name    = aws_lambda_function.payment_processor.function_name
  function_version = aws_lambda_function.payment_processor.version

  routing_config {
    additional_version_weights = {
      # 10% traffic to new version (Canary)
      (aws_lambda_function.payment_processor_v2.version) = 0.1
    }
  }
}

resource "aws_lambda_function" "payment_processor" {
  function_name = "payment-processor-stable"
  # ... stable version
}

resource "aws_lambda_function" "payment_processor_v2" {
  function_name = "payment-processor-v2"
  # ... new version
}

resource "aws_appconfig_application" "payment_service" {
  name        = "payment-service"
  description = "Feature flags for payment service"
}

resource "aws_appconfig_environment" "production" {
  name           = "production"
  application_id = aws_appconfig_application.payment_service.id

  # No deployment without monitoring
  monitors {
    alarm_arn      = aws_cloudwatch_metric_alarm.payment_error_rate.arn
    alarm_role_arn = aws_iam_role.appconfig_monitor_role.arn
  }
}

resource "aws_appconfig_configuration_profile" "feature_flags" {
  name           = "payment-features"
  application_id = aws_appconfig_application.payment_service.id
  location_uri   = "hosted"
  type           = "AWS.AppConfig.FeatureFlags"

  validator {
    type    = "JSON_SCHEMA"
    content = jsonencode({
      "$schema" = "http://json-schema.org/draft-07/schema#"
      type      = "object"
      properties = {
        new_checkout_flow = {
          type = "object"
          properties = {
            enabled    = { type = "boolean" }
            percentage = { type = "number", minimum = 0, maximum = 100 }
          }
        }
      }
    })
  }
}

# Feature Flag Values (Hosted Configuration)
resource "aws_appconfig_hosted_configuration_version" "flags_v1" {
  application_id           = aws_appconfig_application.payment_service.id
  configuration_profile_id = aws_appconfig_configuration_profile.feature_flags.configuration_profile_id
  description              = "Initial feature flags"
  content_type             = "application/json"
  content = jsonencode({
    flags = {
      new_checkout_flow = {
        name        = "New Checkout Flow"
        description = "Gradual rollout of redesigned checkout"
        enabled     = false
        attributes = {
          percentage = { constraints = { type = "number" } }
        }
      }
    }
    values = {
      new_checkout_flow = {
        enabled    = false
        percentage = 0
      }
    }
    version = "1"
  })
}

// payment-service/src/checkout.js
const appConfig = require('./appconfig-client');

async function processCheckout(user, cart) {
  // Retrieve feature flag (cached, < 10ms latency)
  const flags = await appConfig.getConfiguration('payment-service', 'production', 'payment-features');

  const useNewFlow = flags.new_checkout_flow.enabled &&
    (Math.random() * 100 < flags.new_checkout_flow.percentage);

  if (useNewFlow) {
    logger.info('Using new checkout flow', {
      trace_id: getCurrentTraceId(),
      user_id: user.id,
      feature_flag: 'new_checkout_flow'
    });
    return processNewCheckout(user, cart);
  }

  return processLegacyCheckout(user, cart);
}