# Persistent ALB Pattern (bg-common)

The persistent ALB pattern ensures that ALB DNS names remain constant even when service stacks are deleted and recreated. This matches the legacy `bg-common` pattern.

## How It Works

The bg-common ALB stack is **always created separately**. Services reference it via `albLoadBalancerArn`:

1. **bg-common ALB stack is deployed separately** (`{serviceStackName}-alb` or via `spicyALB`)
2. **The ALB stack persists** across service deployments
3. **Service stacks reference the existing ALB** via `albLoadBalancerArn` (maps to `existingALB` in construct)
4. **ALB DNS name stays constant** even when service stacks are deleted/recreated

### Stack Naming

- **ALB Stack**: `{serviceStackName}-alb` (shared between blue/green)
- **Service Stack (Rolling)**: `{serviceStackName}`
- **Service Stack (Blue/Green)**: `{serviceStackName}-blue` and `{serviceStackName}-green`

For blue/green deployments, both blue and green services share the same ALB stack.

## Usage

### Rolling Deployment (Non-Blue/Green)

```groovy
@Library(["spicy-automation@main"]) _

spicyECSService(
    jenkinsAwsCredentialsId: "aws-credentials",
    region: "ca-central-1",
    stackName: "my-api-dev",
    serviceName: "my-api",

    // ... cluster/VPC config ...

    // Container
    image: "nexus.kodeniks.com/docker-hosted/my-api:latest",
    containerPort: 3000,

    // Reference bg-common ALB (always created separately) - auto-imports from ALB stack
    albStackName: "my-api-dev-alb",  // Auto-imports ALB ARN and listener ARNs

    // Routing
    hostHeader: "api-dev.example.com",
    priority: 100,

    // Tags
    ownerTag: "MyTeam",
    productTag: "my-product",
    componentTag: "api",
    environment: "dev",
)
```

**What happens:**

1. Pipeline deploys `my-api-dev-alb` stack (ALB)
2. Pipeline gets ALB outputs (DNS, listener ARNs)
3. Pipeline deploys `my-api-dev` stack (service) referencing the ALB
4. ALB DNS name stays constant: `my-api-dev-alb-123456789.ca-central-1.elb.amazonaws.com`

### Blue/Green Deployment

```groovy
@Library(["spicy-automation@main"]) _

spicyECSService(
    jenkinsAwsCredentialsId: "aws-credentials",
    region: "ca-central-1",
    stackName: "my-api-prod",  // Base name (blue/green suffixes added automatically)
    serviceName: "my-api",

    // ... cluster/VPC config ...

    // Container
    image: "nexus.kodeniks.com/docker-hosted/my-api:latest",
    containerPort: 3000,

    // Blue/Green
    blueGreen: true,
    activeHostname: "api.example.com",
    inactiveHostname: "inactive-api.example.com",
    bgHostedZoneId: "Z1234567890",  // Route53 hosted zone

    // Reference bg-common ALB (always created separately) - auto-imports from ALB stack
    albStackName: "my-api-prod-alb",  // Auto-imports ALB ARN and listener ARNs
    healthCheckPath: "/health",

    // Tags
    ownerTag: "MyTeam",
    productTag: "my-product",
    componentTag: "api",
    environment: "prod",
)
```

**What happens:**

1. Pipeline deploys `my-api-prod-alb` stack (ALB) - **only once, shared by blue/green**
2. Pipeline gets ALB outputs (or uses `albStackName` for auto-import)
3. Pipeline deploys `my-api-prod-green` stack (inactive service) referencing the ALB
4. Service creates listener rules on **both** HTTP and HTTPS listeners (if both available)
5. Pipeline swaps hostname routing via ALB listener rule priorities (no DNS changes)
6. ALB DNS name stays constant across all deployments

**How Blue/Green Routing Works:**

- **DNS Records**: Both `api.example.com` and `inactive-api.example.com` DNS records are created and always point to the same ALB
- **Traffic Routing**: Controlled by ALB listener rule priorities, not DNS:
  - Active service: Listener rule with priority 100 for `api.example.com`
  - Inactive service: Listener rule with priority 200 for `inactive-api.example.com`
- **Swapping**: When swapping, only the listener rule priorities are updated (via CDK deployment). DNS records never change.
- **Result**: Zero-downtime swaps in ~30 seconds (just ALB rule updates, no DNS propagation delays)

## Benefits

1. **Static DNS**: ALB DNS name never changes (unless ALB stack is deleted)
2. **No DNS Updates**: Cloudflare/Route53 records set once and never need updating
3. **Faster Deployments**: ALB stack only updates when ALB config changes
4. **Cost Efficient**: ALB persists, avoiding recreation costs

## ALB Stack Lifecycle

- **Created**: Deployed separately via `spicyALB` pipeline function or manually
- **Updated**: When ALB configuration changes (certificate, scheme, etc.)
- **Deleted**: Only when explicitly destroyed (not deleted with service stacks)

## Stack Outputs

The ALB stack exports these outputs (used by service stacks via CloudFormation imports):

| Output Key         | Description        | Export Name                                  |
| ------------------ | ------------------ | -------------------------------------------- |
| `LoadBalancerDNS`  | ALB DNS name       | `{stackName}-internet-facing-url`            |
| `LoadBalancerArn`  | ALB ARN            | `{stackName}-internet-facing-arn`            |
| `HTTPListenerArn`  | HTTP listener ARN  | `{stackName}-internet-facing-http-listener`  |
| `HTTPSListenerArn` | HTTPS listener ARN | `{stackName}-internet-facing-https-listener` |

**Minimal Props:** When using `albStackName` in service stack, these are auto-imported via CloudFormation `Fn::ImportValue`. The service creates listener rules on **both** HTTP and HTTPS listeners (if both are available), matching the old template behavior.
| `SecurityGroupId` | ALB security group ID | `{stackName}-internet-facing-security-group` |

## Deploying the bg-common ALB Stack

The bg-common ALB stack must be deployed separately before deploying services:

```groovy
@Library(["spicy-automation@main"]) _

// Deploy bg-common ALB stack
spicyALB(
    jenkinsAwsCredentialsId: "aws-credentials",
    region: "ca-central-1",
    stackName: "my-api-dev-alb",
    vpcId: "vpc-12345678",
    availabilityZones: "ca-central-1a,ca-central-1b",
    subnetIds: "subnet-xxx,subnet-yyy",
    scheme: "internet-facing",
    certificateArn: "arn:aws:acm:...",
    ownerTag: "MyTeam",
    productTag: "my-product",
    componentTag: "alb",
    environment: "dev",
)

// Then deploy service referencing the ALB
spicyECSService(
    // ... config ...
    albLoadBalancerArn: "arn:aws:elasticloadbalancing:...",  // From ALB stack outputs
    albHttpsListenerArn: "arn:aws:elasticloadbalancing:...",  // From ALB stack outputs
)
```

## Comparison with Legacy bg-common

| Feature             | Legacy bg-common       | New Persistent ALB                         |
| ------------------- | ---------------------- | ------------------------------------------ |
| **Stack Name**      | `{envName}-bg-common`  | `{serviceStackName}-alb`                   |
| **Deployment**      | Separate pipeline step | Separate pipeline step (via `spicyALB`)    |
| **Blue/Green**      | Shared ALB             | Shared ALB                                 |
| **DNS Management**  | Manual                 | Automatic (Route53) or Manual (Cloudflare) |
| **Stack Lifecycle** | Manual                 | Manual (deployed separately)               |

## Troubleshooting

### ALB Stack Not Found

If the ALB stack doesn't exist, the pipeline will create it automatically. If you see errors:

1. Check the ALB stack name: `{serviceStackName}-alb`
2. Verify the stack exists: `aws cloudformation describe-stacks --stack-name {stackName}-alb`
3. Check stack outputs: `aws cloudformation describe-stacks --stack-name {stackName}-alb --query 'Stacks[0].Outputs'`

### ALB DNS Name Changed

This should never happen with the persistent ALB pattern. If it does:

1. Check if the ALB stack was deleted/recreated
2. Verify the ALB stack name is consistent
3. Check CloudFormation stack events for ALB replacement

### Service Can't Find ALB

If the service stack can't reference the ALB:

1. Verify ALB stack outputs are available
2. Check that `albLoadBalancerArn` is set in the service context
3. Verify the ALB ARN is correct