fix: natgateway routing

.github/CODEOWNERS

@@ -1,4 +1 @@
-* @mr-robot-in
-* @hritik-verma-sc
-* @rithikb24
-* @gagan1510
+*  @smallcase/core-infra

README.md

@@ -293,3 +293,219 @@ Here’s a breakdown of the configuration options available:
 4. externalSubnets: Specify external subnets if you need to define subnets manually (each with an id, availabilityZone, and routeTableId).
 5. iamPolicyStatements: (Optional) Attach IAM policy statements to control access to the endpoint.
 6. additionalTags: (Optional) Add custom tags to the VPC Endpoint for easier identification and tracking.
+
+## Dynamic Routing Strategy
+
+The module automatically chooses the optimal routing strategy based on the number of NAT Gateways to prevent duplicate `0.0.0.0/0` route entries:
+
+### **Single NAT Gateway (≤1 NAT Gateway)**
+- **Strategy**: One route table per subnet group
+- **Benefits**: 
+  - **Cost Optimized**: Fewer route tables = lower costs
+  - **Simpler Management**: One route table per subnet group
+  - **No Duplicate Routes**: Single NAT Gateway means no duplicate `0.0.0.0/0` entries
+  - **Suitable for**: Development, testing, small workloads
+- **Configuration**: All subnets in a group share the same route table with one NAT route
+
+### **Multiple NAT Gateways (>1 NAT Gateway)**
+- **Strategy**: One route table per subnet
+- **Benefits**:
+  - **Prevents Duplicate Routes**: Each subnet gets its own route table, avoiding duplicate `0.0.0.0/0` entries
+  - **AZ-Aware Routing**: Each subnet uses NAT Gateway in the same AZ when possible
+  - **High Availability**: Better fault tolerance and load distribution
+  - **Cross-AZ Cost Reduction**: Reduced data transfer costs
+  - **Suitable for**: Production workloads, high availability requirements
+- **Configuration**: Each subnet gets its own route table with one NAT route
+
+### **Why This Strategy?**
+
+AWS route tables don't support duplicate entries for the same destination CIDR (like `0.0.0.0/0`). When you have multiple NAT Gateways:
+
+- **❌ Wrong Approach**: Multiple NAT routes in same route table → `AlreadyExists` error
+- **✅ Correct Approach**: One route table per subnet → Each gets one NAT route
+
+### **Automatic Strategy Selection**
+
+```typescript
+// Single NAT Gateway - Cost Optimized
+new Network(this, 'NETWORK', {
+  vpc: {
+    cidr: '10.10.0.0/16',
+    subnetConfiguration: [],
+  },
+  subnets: [
+    {
+      subnetGroupName: 'NATGateway',
+      subnetType: ec2.SubnetType.PUBLIC,
+      cidrBlock: ['10.10.0.0/28'], // Only one subnet
+      availabilityZones: ['ap-south-1a'], // Only one AZ
+      useSubnetForNAT: true,
+    },
+    {
+      subnetGroupName: 'Private',
+      subnetType: ec2.SubnetType.PRIVATE_WITH_NAT,
+      cidrBlock: ['10.10.5.0/24', '10.10.6.0/24', '10.10.7.0/24'],
+      availabilityZones: ['ap-south-1a', 'ap-south-1b', 'ap-south-1c'],
+    },
+  ],
+});
+
+// Multiple NAT Gateways - Performance Optimized
+new Network(this, 'NETWORK', {
+  vpc: {
+    cidr: '10.10.0.0/16',
+    subnetConfiguration: [],
+  },
+  natEipAllocationIds: [
+    'eipalloc-1234567890abcdef',
+    'eipalloc-0987654321fedcba',
+    'eipalloc-abcdef1234567890',
+  ],
+  subnets: [
+    {
+      subnetGroupName: 'NATGateway',
+      subnetType: ec2.SubnetType.PUBLIC,
+      cidrBlock: ['10.10.0.0/28', '10.10.0.16/28', '10.10.0.32/28'],
+      availabilityZones: ['ap-south-1a', 'ap-south-1b', 'ap-south-1c'],
+      useSubnetForNAT: true,
+    },
+    {
+      subnetGroupName: 'Private',
+      subnetType: ec2.SubnetType.PRIVATE_WITH_NAT,
+      cidrBlock: ['10.10.5.0/24', '10.10.6.0/24', '10.10.7.0/24'],
+      availabilityZones: ['ap-south-1a', 'ap-south-1b', 'ap-south-1c'],
+    },
+  ],
+});
+```
+
+### **CloudFormation Outputs**
+
+The module provides outputs to show which strategy is being used:
+
+- **RoutingStrategy**: Shows "Route Table per Subnet Group" or "Route Table per Subnet"
+- **NATGatewayCount**: Shows the number of NAT Gateways configured
+
+### **Route Table Distribution**
+
+| Scenario | Route Tables | NAT Routes per Table | Strategy |
+|----------|-------------|---------------------|----------|
+| Single NAT | 1 per subnet group | 1 | Cost optimized |
+| Multiple NAT | 1 per subnet | 1 | Performance optimized |
+
+### **Migration Strategy**
+
+You can easily migrate between strategies by changing your configuration:
+
+1. **Development → Production**: Add more NAT Gateway subnets
+2. **Production → Development**: Reduce to single NAT Gateway subnet
+3. **Cost Optimization**: Monitor usage and adjust NAT Gateway count
+
+The module handles the migration automatically without manual route table changes.
+
+## NAT Gateway EIP Allocation
+
+You can specify existing Elastic IP (EIP) allocation IDs to use with your NAT Gateways. This is useful when you want to:
+
+- Use pre-existing EIPs
+- Maintain consistent IP addresses across deployments
+- Control EIP costs and management
+
+### **Using EIP Allocation IDs**
+
+```typescript
+new Network(this, 'NETWORK', {
+  vpc: {
+    cidr: '10.10.0.0/16',
+    subnetConfiguration: [],
+  },
+  // Specify existing EIP allocation IDs
+  natEipAllocationIds: [
+    'eipalloc-1234567890abcdef', // EIP for NAT Gateway 1
+    'eipalloc-0987654321fedcba', // EIP for NAT Gateway 2
+    'eipalloc-abcdef1234567890', // EIP for NAT Gateway 3
+  ],
+  subnets: [
+    {
+      subnetGroupName: 'NATGateway',
+      subnetType: ec2.SubnetType.PUBLIC,
+      cidrBlock: ['10.10.0.0/28', '10.10.0.16/28', '10.10.0.32/28'],
+      availabilityZones: ['ap-south-1a', 'ap-south-1b', 'ap-south-1c'],
+      useSubnetForNAT: true,
+    },
+    {
+      subnetGroupName: 'Private',
+      subnetType: ec2.SubnetType.PRIVATE_WITH_NAT,
+      cidrBlock: ['10.10.5.0/24', '10.10.6.0/24', '10.10.7.0/24'],
+      availabilityZones: ['ap-south-1a', 'ap-south-1b', 'ap-south-1c'],
+    },
+  ],
+});
+```
+
+### **EIP Allocation ID Requirements**
+
+- **Count**: Should match the number of NAT Gateway subnets (optional)
+- **Format**: Must be valid EIP allocation IDs (e.g., `eipalloc-xxxxxxxxx`)
+- **Region**: Must be in the same region as your VPC
+- **Account**: Must be owned by the same AWS account
+
+### **Benefits of Using EIP Allocation IDs**
+
+1. **Cost Control**: Reuse existing EIPs instead of creating new ones
+2. **IP Consistency**: Maintain the same public IP addresses across deployments
+3. **Compliance**: Meet requirements for static IP addresses
+4. **DNS**: Use existing DNS records that point to specific EIPs
+
+### **CloudFormation Outputs**
+
+When EIP allocation IDs are provided, the module outputs:
+
+- **NATEipAllocationIds**: Comma-separated list of EIP allocation IDs used
+- **RoutingStrategy**: Shows the routing strategy being used
+- **NATGatewayCount**: Number of NAT Gateways configured
+
+### **Example Output**
+
+```json
+{
+  "NATEipAllocationIds": "eipalloc-1234567890abcdef,eipalloc-0987654321fedcba,eipalloc-abcdef1234567890",
+  "RoutingStrategy": "Route Table per Subnet",
+  "NATGatewayCount": "3"
+}
+```
+
+### **Creating EIPs for NAT Gateways**
+
+If you need to create EIPs first, you can do so using CDK:
+
+```typescript
+import * as ec2 from 'aws-cdk-lib/aws-ec2';
+
+// Create EIPs
+const eip1 = new ec2.CfnEIP(this, 'NATEip1', {
+  domain: 'vpc',
+});
+
+const eip2 = new ec2.CfnEIP(this, 'NATEip2', {
+  domain: 'vpc',
+});
+
+const eip3 = new ec2.CfnEIP(this, 'NATEip3', {
+  domain: 'vpc',
+});
+
+// Use them in your Network construct
+new Network(this, 'NETWORK', {
+  vpc: {
+    cidr: '10.10.0.0/16',
+    subnetConfiguration: [],
+  },
+  natEipAllocationIds: [
+    eip1.attrAllocationId,
+    eip2.attrAllocationId,
+    eip3.attrAllocationId,
+  ],
+  // ... rest of configuration
+});
+```

src/constructs/network.ts

@@ -186,8 +186,8 @@ export class Network extends Construct {
       vpcId: this.vpc.vpcId,
     });
 
-    // Initialize NAT provider after collecting all subnets
-    const natProvider = props.natEipAllocationIds?.length === this.natSubnets?.length && props.natEipAllocationIds?.length > 0
+    // Initialize NAT provider with EIP allocation IDs if provided
+    const natProvider = props.natEipAllocationIds && props.natEipAllocationIds.length > 0
       ? ec2.NatProvider.gateway({
         eipAllocationIds: props.natEipAllocationIds,
       }) : ec2.NatProvider.gateway();
@@ -226,22 +226,37 @@ export class Network extends Construct {
       });
     }
 
-    // Second pass: configure routes after NAT is configured
-    props.subnets.forEach((subnetProps) => {
-      const routeTableManager = new RouteTableManager(this, `${subnetProps.subnetGroupName}RouteTableManager`, {
-        vpc: this.vpc,
-        subnetGroupName: subnetProps.subnetGroupName,
-        routes: subnetProps.routes,
-        peeringConnectionId: this.peeringConnectionIds,
-        subnetType: subnetProps.subnetType,
-        natProvider: natProvider,
-        internetGateway: internetGateway,
-      });
-      this.subnets[subnetProps.subnetGroupName].forEach((subnet, index) => {
-        routeTableManager.associateSubnet(subnet, index);
-      });
+    // Determine routing strategy based on number of NAT Gateways
+    const natGatewayCount = this.natSubnets.length;
+    const useSingleRouteTable = natGatewayCount <= 1;
+
+    // Add output to show which strategy is being used
+    new CfnOutput(this, 'RoutingStrategy', {
+      value: useSingleRouteTable ? 'Route Table per Subnet Group' : 'Route Table per Subnet',
+      description: 'Routing strategy based on NAT Gateway count',
+    });
+
+    new CfnOutput(this, 'NATGatewayCount', {
+      value: natGatewayCount.toString(),
+      description: 'Number of NAT Gateways configured',
     });
 
+    // Add output for EIP allocation IDs if provided
+    if (props.natEipAllocationIds && props.natEipAllocationIds.length > 0) {
+      new CfnOutput(this, 'NATEipAllocationIds', {
+        value: props.natEipAllocationIds.join(','),
+        description: 'EIP allocation IDs used for NAT Gateways',
+      });
+    }
+
+    if (useSingleRouteTable) {
+      // Single NAT Gateway: One route table per subnet group
+      this.configureSubnetGroupRouteTables(props, natProvider, internetGateway);
+    } else {
+      // Multiple NAT Gateways: One route table per subnet to avoid duplicate 0.0.0.0/0 entries
+      this.configureSubnetRouteTables(props, natProvider, internetGateway);
+    }
+
     // this.pbSubnets.forEach((pb) => {
     //   pb.addDefaultInternetRoute(internetGateway.ref, att);
     // });
@@ -260,6 +275,76 @@ export class Network extends Construct {
     }
   }
 
+  /**
+   * Configure route tables per subnet group (for single NAT Gateway)
+   */
+  private configureSubnetGroupRouteTables(
+    props: VPCProps,
+    natProvider: ec2.NatProvider,
+    internetGateway: ec2.CfnInternetGateway,
+  ) {
+    // One route table per subnet group
+    props.subnets.forEach((subnetProps) => {
+      const routeTableManager = new RouteTableManager(this, `${subnetProps.subnetGroupName}RouteTableManager`, {
+        vpc: this.vpc,
+        subnetGroupName: subnetProps.subnetGroupName,
+        routes: subnetProps.routes,
+        peeringConnectionId: this.peeringConnectionIds,
+        subnetType: subnetProps.subnetType,
+        natProvider: natProvider,
+        internetGateway: internetGateway,
+      });
+      this.subnets[subnetProps.subnetGroupName].forEach((subnet, index) => {
+        routeTableManager.associateSubnet(subnet, index);
+      });
+    });
+  }
+
+  /**
+   * Configure route tables per subnet (for multiple NAT Gateways)
+   * This prevents duplicate 0.0.0/0 entries in the same route table
+   */
+  private configureSubnetRouteTables(
+    props: VPCProps,
+    natProvider: ec2.NatProvider,
+    internetGateway: ec2.CfnInternetGateway,
+  ) {
+    // One route table per subnet to avoid duplicate 0.0.0.0/0 entries
+    props.subnets.forEach((subnetProps) => {
+      this.subnets[subnetProps.subnetGroupName].forEach((subnet, index) => {
+        // Find the NAT Gateway in the same AZ as the subnet
+        //let specificNatGateway: ec2.CfnNatGateway | undefined;
+        // if (subnetProps.subnetType === ec2.SubnetType.PRIVATE_WITH_NAT && natProvider.configuredGateways) {
+        //   // Try to find NAT Gateway in the same AZ
+        //   const subnetAZ = subnet.availabilityZone;
+        //   const natGatewayInSameAZ = natProvider.configuredGateways.find(natGateway => {
+        //     return natGateway.az === subnetAZ;
+        //   });
+
+        //   if (natGatewayInSameAZ) {
+        //     // Use the NAT Gateway from the same AZ
+        //     specificNatGateway = natGatewayInSameAZ as any; // Type assertion for CfnNatGateway
+        //   } else {
+        //     // Fallback to first NAT Gateway if no match found
+        //     specificNatGateway = natProvider.configuredGateways[0] as any;
+        //   }
+        // }
+
+        const routeTableManager = new RouteTableManager(this, `${subnetProps.subnetGroupName}Subnet${index}RouteTableManager`, {
+          vpc: this.vpc,
+          subnetGroupName: `${subnetProps.subnetGroupName}Subnet${index}`,
+          routes: subnetProps.routes,
+          peeringConnectionId: this.peeringConnectionIds,
+          subnetType: subnetProps.subnetType,
+          natProvider: natProvider,
+          internetGateway: internetGateway,
+          subnetAvailabilityZone: subnet.availabilityZone,
+        });
+        routeTableManager.associateSubnet(subnet, 0); // Only one subnet per route table
+      });
+    });
+  }
+
   createSubnet(option: ISubnetsProps, vpc: ec2.Vpc) {
     const subnets: ec2.Subnet[] = [];
     const SUBNETTYPE_TAG = 'aws-cdk:subnet-type';