From 92cc26657d1fdc48968bc77da789d5f8c99e41ab Mon Sep 17 00:00:00 2001
From: Mike Neverov <mike.neverov@mapbox.com>
Date: Tue, 5 May 2026 12:46:34 +0100
Subject: [PATCH] init

---
 changelog.md             |  4 ++++
 functions/default-vpc.js |  5 +++++
 package.json             |  2 +-
 readme.md                |  2 ++
 test/default-vpc.test.js | 12 +++++++-----
 5 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/changelog.md b/changelog.md
index fc1b58f..1a1d2f3 100644
--- a/changelog.md
+++ b/changelog.md
@@ -1,3 +1,7 @@
+### 4.1.0
+
+Adds `FargateArm64PrivateSubnets` to the DefaultVpc resource.
+
 ### 4.0.0
 
 Updates to Node 22.x
diff --git a/functions/default-vpc.js b/functions/default-vpc.js
index 1e839fb..3a639fe 100644
--- a/functions/default-vpc.js
+++ b/functions/default-vpc.js
@@ -2,6 +2,8 @@ var AWS = require('aws-sdk');
 var utils = require('../lib/utils');
 var Response = require('../lib/response');
 
+var FARGATE_ARM64_UNSUPPORTED_AZ_IDS = ['use1-az3'];
+
 module.exports = function(event, context) {
   if (!utils.validCloudFormationEvent(event))
     return context.done(null, 'ERROR: Invalid CloudFormation event');
@@ -44,6 +46,9 @@ module.exports = function(event, context) {
 
     info.PublicSubnets = publicSubnets.map((subnet) => subnet.SubnetId);
     info.PrivateSubnets = privateSubnets.map((subnet) => subnet.SubnetId);
+    info.FargateArm64PrivateSubnets = privateSubnets
+      .filter((subnet) => !FARGATE_ARM64_UNSUPPORTED_AZ_IDS.includes(subnet.AvailabilityZoneId))
+      .map((subnet) => subnet.SubnetId);
     info.RouteTable = results[1].RouteTables[0].RouteTableId;
     info.RouteTables = results[1].RouteTables.map((rt) => rt.RouteTableId);
     response.setId(info.VpcId);
diff --git a/package.json b/package.json
index 1083192..9ec8ff0 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@mapbox/magic-cfn-resources",
-  "version": "4.0.0",
+  "version": "4.1.0",
   "description": "Build Lambda-backed custom CloudFormation resources",
   "main": "index.js",
   "directories": {
diff --git a/readme.md b/readme.md
index 5143ef5..c2d924a 100644
--- a/readme.md
+++ b/readme.md
@@ -50,6 +50,8 @@ You can use `Fn::GetAtt` to obtain the following data:
 - AvailabilityZones: an array of strings representing the VPC's availability zones
 - AvailabilityZoneCount: the number of availability zones
 - PublicSubnets: an array of strings representing the VPC's public subnets
+- PrivateSubnets: an array of strings representing the VPC's private subnets
+- FargateArm64PrivateSubnets: an array of private subnet IDs excluding availability zones where Fargate ARM64 is unsupported
 - RouteTable: the ID for the first route table in the VPC
 - RouteTables: IDs of all of the route tables in the VPC
 
diff --git a/test/default-vpc.test.js b/test/default-vpc.test.js
index 017c56c..3f993fb 100644
--- a/test/default-vpc.test.js
+++ b/test/default-vpc.test.js
@@ -28,7 +28,8 @@ test('[default VPC] success', (assert) => {
       Subnets: [
         { SubnetId: 'a', AvailabilityZone: '1a', MapPublicIpOnLaunch: true },
         { SubnetId: 'b', AvailabilityZone: '1b', MapPublicIpOnLaunch: true },
-        { SubnetId: 'c', AvailabilityZone: '1c', MapPublicIpOnLaunch: false }
+        { SubnetId: 'c', AvailabilityZone: '1c', AvailabilityZoneId: 'use1-az3', MapPublicIpOnLaunch: false },
+        { SubnetId: 'd', AvailabilityZone: '1d', AvailabilityZoneId: 'use1-az6', MapPublicIpOnLaunch: false }
       ]
     }));
   });
@@ -51,12 +52,13 @@ test('[default VPC] success', (assert) => {
       VpcId: 'vpcid',
       AvailabilityZones: ['1a','1b'],
       AvailabilityZoneCount: 2,
-      PrivateSubnetAvailabilityZones: ['1c'],
-      PrivateSubnetAvailabilityZoneCount: 1,
+      PrivateSubnetAvailabilityZones: ['1c', '1d'],
+      PrivateSubnetAvailabilityZoneCount: 2,
       AzIndexedPublicSubnets: ['a', 'b', { Ref: 'AWS::NoValue' }, { Ref: 'AWS::NoValue' }, { Ref: 'AWS::NoValue' }, { Ref: 'AWS::NoValue' }],
-      AzIndexedPrivateSubnets: [{ Ref: 'AWS::NoValue' }, { Ref: 'AWS::NoValue' }, 'c', { Ref: 'AWS::NoValue' }, { Ref: 'AWS::NoValue' }, { Ref: 'AWS::NoValue' }],
+      AzIndexedPrivateSubnets: [{ Ref: 'AWS::NoValue' }, { Ref: 'AWS::NoValue' }, 'c', 'd', { Ref: 'AWS::NoValue' }, { Ref: 'AWS::NoValue' }],
       PublicSubnets: ['a', 'b'],
-      PrivateSubnets: ['c'],
+      PrivateSubnets: ['c', 'd'],
+      FargateArm64PrivateSubnets: ['d'],
       RouteTable: 'x',
       RouteTables: ['x', 'z']
     }, 'returns expected info');