feat(core): Fn.findInMap supports default value

packages/aws-cdk-lib/core/README.md

@@ -1058,6 +1058,28 @@ declare const regionTable: CfnMapping;
 regionTable.findInMap(Aws.REGION, 'regionName');
 ```
 
+An optional default value can also be passed to `findInMap`. If either key is not found in the map and the mapping is lazy, `findInMap` will return the default value. If the mapping is not lazy or either key is an unresolved token, the call to `findInMap` will return a token that resolves to `{ "Fn::FindInMap": [ "MapName", "TopLevelKey", "SecondLevelKey", { "DefaultValue": "DefaultValue" } ] }`.
+
+For example, the following code will again not produce anything in the "Mappings" section. The
+call to `findInMap` will be able to resolve the value during synthesis and simply return
+`'Region not found'`.
+
+```ts
+const regionTable = new CfnMapping(this, 'RegionTable', {
+  mapping: {
+    'us-east-1': {
+      regionName: 'US East (N. Virginia)',
+    },
+    'us-east-2': {
+      regionName: 'US East (Ohio)',
+    },
+  },
+  lazy: true,
+});
+
+regionTable.findInMap('us-west-1', 'regionName', 'Region not found');
+```
+
 [cfn-mappings]: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/mappings-section-structure.html
 
 ### Dynamic References

packages/aws-cdk-lib/core/lib/cfn-fn.ts

@@ -235,8 +235,8 @@ export class Fn {
    * keys in a two-level map that is declared in the Mappings section.
    * @returns a token represented as a string
    */
-  public static findInMap(mapName: string, topLevelKey: string, secondLevelKey: string): string {
-    return Fn._findInMap(mapName, topLevelKey, secondLevelKey).toString();
+  public static findInMap(mapName: string, topLevelKey: string, secondLevelKey: string, defaultValue?: string): string {
+    return Fn._findInMap(mapName, topLevelKey, secondLevelKey, defaultValue).toString();
   }
 
   /**
@@ -245,8 +245,8 @@ export class Fn {
    *
    * @internal
    */
-  public static _findInMap(mapName: string, topLevelKey: string, secondLevelKey: string): IResolvable {
-    return new FnFindInMap(mapName, topLevelKey, secondLevelKey);
+  public static _findInMap(mapName: string, topLevelKey: string, secondLevelKey: string, defaultValue?: string): IResolvable {
+    return new FnFindInMap(mapName, topLevelKey, secondLevelKey, defaultValue);
   }
 
   /**
@@ -500,9 +500,10 @@ class FnFindInMap extends FnBase {
    * @param mapName The logical name of a mapping declared in the Mappings section that contains the keys and values.
    * @param topLevelKey The top-level key name. Its value is a list of key-value pairs.
    * @param secondLevelKey The second-level key name, which is set to one of the keys from the list assigned to TopLevelKey.
+   * @param defaultValue The value of the default value returned if either the key is not found in the map
    */
-  constructor(mapName: string, topLevelKey: any, secondLevelKey: any) {
-    super('Fn::FindInMap', [mapName, topLevelKey, secondLevelKey]);
+  constructor(mapName: string, topLevelKey: any, secondLevelKey: any, defaultValue?: string) {
+    super('Fn::FindInMap', [mapName, topLevelKey, secondLevelKey, defaultValue !== undefined ? { DefaultValue: defaultValue } : undefined]);
   }
 }
 

packages/aws-cdk-lib/core/lib/cfn-mapping.ts

@@ -40,8 +40,8 @@ export interface CfnMappingProps {
 export class CfnMapping extends CfnRefElement {
   private mapping: Mapping;
   private readonly lazy?: boolean;
-  private lazyRender = false;
-  private lazyInformed = false;
+  private lazyRender = false; // prescribes `_toCloudFormation()` to pass nothing if value from map is returned lazily.
+  private lazyInformed = false; // keeps track if user has been sent a message informing them of the possibility to use lazy synthesis.
 
   constructor(scope: Construct, id: string, props: CfnMappingProps = {}) {
     super(scope, id);
@@ -63,30 +63,41 @@ export class CfnMapping extends CfnRefElement {
   }
 
   /**
-   * @returns A reference to a value in the map based on the two keys.
+   * @returns A reference to a value in the map based on the two keys. If mapping is lazy, the value from the map or default value is returned instead of the reference.
    */
-  public findInMap(key1: string, key2: string): string {
+  public findInMap(key1: string, key2: string, defaultValue?: string): string {
     let fullyResolved = false;
+    let notInMap = false;
     if (!Token.isUnresolved(key1)) {
       if (!(key1 in this.mapping)) {
-        throw new Error(`Mapping doesn't contain top-level key '${key1}'`);
-      }
-      if (!Token.isUnresolved(key2)) {
+        if (defaultValue === undefined) {
+          throw new Error(`Mapping doesn't contain top-level key '${key1}'`);
+        } else {
+          notInMap = true;
+        }
+      } else if (!Token.isUnresolved(key2)) {
         if (!(key2 in this.mapping[key1])) {
-          throw new Error(`Mapping doesn't contain second-level key '${key2}'`);
+          if (defaultValue === undefined) {
+            throw new Error(`Mapping doesn't contain second-level key '${key2}'`);
+          } else {
+            notInMap = true;
+          }
         }
         fullyResolved = true;
       }
     }
-    if (fullyResolved) {
-      if (this.lazy) {
+
+    if (this.lazy) {
+      if (notInMap && defaultValue !== undefined) {
+        return defaultValue;
+      } else if (fullyResolved) {
         return this.mapping[key1][key2];
       }
-    } else {
-      this.lazyRender = true;
     }
 
-    return new CfnMappingEmbedder(this, this.mapping, key1, key2).toString();
+    this.lazyRender = !fullyResolved;
+
+    return new CfnMappingEmbedder(this, this.mapping, key1, key2, defaultValue).toString();
   }
 
   /**
@@ -130,12 +141,16 @@ export class CfnMapping extends CfnRefElement {
 class CfnMappingEmbedder implements IResolvable {
   readonly creationStack: string[] = [];
 
-  constructor(private readonly cfnMapping: CfnMapping, readonly mapping: Mapping, private readonly key1: string, private readonly key2: string) { }
+  constructor(private readonly cfnMapping: CfnMapping,
+    readonly mapping: Mapping,
+    private readonly key1: string,
+    private readonly key2: string,
+    private readonly defaultValue?: string) { }
 
   public resolve(context: IResolveContext): string {
     const consumingStack = Stack.of(context.scope);
     if (consumingStack === Stack.of(this.cfnMapping)) {
-      return Fn.findInMap(this.cfnMapping.logicalId, this.key1, this.key2);
+      return Fn.findInMap(this.cfnMapping.logicalId, this.key1, this.key2, this.defaultValue);
     }
 
     const constructScope = consumingStack;
@@ -148,7 +163,7 @@ class CfnMappingEmbedder implements IResolvable {
       });
     }
 
-    return Fn.findInMap(mappingCopy.logicalId, this.key1, this.key2);
+    return Fn.findInMap(mappingCopy.logicalId, this.key1, this.key2, this.defaultValue);
   }
 
   public toString() {

packages/aws-cdk-lib/core/test/mappings.test.ts

@@ -259,7 +259,7 @@ describe('lazy mapping', () => {
     });
   });
 
-  it('throws if keys can be resolved but are not found in backing', () => {
+  it('throws if keys can be resolved but are not found in mapping', () => {
     expect(() => mapping.findInMap('NonExistentKey', 'SecondLevelKey1'))
       .toThrowError(/Mapping doesn't contain top-level key .*/);
     expect(() => mapping.findInMap('TopLevelKey1', 'NonExistentKey'))
@@ -316,6 +316,121 @@ describe('eager by default', () => {
   });
 });
 
+describe('defaultValue included', () => {
+  const backing = {
+    TopLevelKey1: {
+      SecondLevelKey1: [1, 2, 3],
+      SecondLevelKey2: { Hello: 'World' },
+    },
+  };
+
+  const defValue = 'foo';
+
+  let app: App;
+  let stack: Stack;
+  let mapping: CfnMapping;
+
+  describe('lazy mapping', () => {
+    beforeEach(() => {
+      stack = new Stack();
+      mapping = new CfnMapping(stack, 'Lazy Mapping', {
+        mapping: backing,
+        lazy: true,
+      });
+    });
+
+    it('does not create CfnMapping if findInMap keys can be resolved', () => {
+      const retrievedValue = mapping.findInMap('TopLevelKey1', 'SecondLevelKey1', defValue);
+
+      expect(stack.resolve(retrievedValue)).toStrictEqual([1, 2, 3]);
+      expect(toCloudFormation(stack)).toStrictEqual({});
+    });
+
+    it('creates CfnMapping if top level key cannot be resolved', () => {
+      const retrievedValue = mapping.findInMap(Aws.REGION, 'SecondLevelKey1', defValue);
+
+      expect(stack.resolve(retrievedValue)).toStrictEqual({ 'Fn::FindInMap': ['LazyMapping', { Ref: 'AWS::Region' }, 'SecondLevelKey1', { DefaultValue: defValue }] }); // should I use string or variable here? variable works
+      expect(toCloudFormation(stack)).toStrictEqual({
+        Mappings: {
+          LazyMapping: backing,
+        },
+      });
+    });
+
+    it('creates CfnMapping if second level key cannot be resolved', () => {
+      const retrievedValue = mapping.findInMap('TopLevelKey1', Aws.REGION, defValue);
+
+      expect(stack.resolve(retrievedValue)).toStrictEqual({ 'Fn::FindInMap': ['LazyMapping', 'TopLevelKey1', { Ref: 'AWS::Region' }, { DefaultValue: defValue }] });
+      expect(toCloudFormation(stack)).toStrictEqual({
+        Mappings: {
+          LazyMapping: backing,
+        },
+      });
+    });
+
+    it('returns default value if keys can be resolved but are not found in mapping', () => {
+      const retrievedValue1 = mapping.findInMap('NonExistentKey', 'SecondLevelKey1', defValue);
+      const retrievedValue2 = mapping.findInMap('TopLevelKey1', 'NonExistentKey', defValue);
+
+      expect(stack.resolve(retrievedValue1)).toStrictEqual('foo');
+      expect(stack.resolve(retrievedValue2)).toStrictEqual('foo');
+
+      expect(toCloudFormation(stack)).toStrictEqual({});
+    });
+  });
+
+  describe('eager by default', () => {
+    beforeEach(() => {
+      app = new App();
+      stack = new Stack(app, 'Stack');
+      mapping = new CfnMapping(stack, 'Lazy Mapping', {
+        mapping: backing,
+      });
+    });
+
+    it('emits warning if every findInMap resolves immediately', () => {
+      mapping.findInMap('TopLevelKey1', 'SecondLevelKey1', defValue);
+
+      const assembly = app.synth();
+
+      expect(getInfoAnnotations(assembly)).toStrictEqual([{
+        path: '/Stack/Lazy Mapping',
+        message: 'Consider making this CfnMapping a lazy mapping by providing `lazy: true`: either no findInMap was called or every findInMap could be immediately resolved without using Fn::FindInMap',
+      }]);
+    });
+
+    it('does not emit warning if a findInMap could not resolve immediately', () => {
+      mapping.findInMap('TopLevelKey1', Aws.REGION, defValue);
+
+      const assembly = app.synth();
+
+      expect(getInfoAnnotations(assembly)).toStrictEqual([]);
+    });
+
+    it('creates CfnMapping if top level key cannot be resolved', () => {
+      const retrievedValue = mapping.findInMap(Aws.REGION, 'SecondLevelKey1', defValue);
+
+      expect(stack.resolve(retrievedValue)).toStrictEqual({ 'Fn::FindInMap': ['LazyMapping', { Ref: 'AWS::Region' }, 'SecondLevelKey1', { DefaultValue: defValue }] }); // should I use string or variable here? variable works
+      expect(toCloudFormation(stack)).toStrictEqual({
+        Mappings: {
+          LazyMapping: backing,
+        },
+      });
+    });
+
+    it('creates CfnMapping if second level key cannot be resolved', () => {
+      const retrievedValue = mapping.findInMap('TopLevelKey1', Aws.REGION, defValue);
+
+      expect(stack.resolve(retrievedValue)).toStrictEqual({ 'Fn::FindInMap': ['LazyMapping', 'TopLevelKey1', { Ref: 'AWS::Region' }, { DefaultValue: defValue }] });
+      expect(toCloudFormation(stack)).toStrictEqual({
+        Mappings: {
+          LazyMapping: backing,
+        },
+      });
+    });
+  });
+});
+
 function getInfoAnnotations(casm: CloudAssembly) {
   const result = new Array<{ path: string, message: string }>();
   for (const stack of Object.values(casm.manifest.artifacts ?? {})) {