feat(sns): add support for attribute key matching in message filtering (

#3709)

* feat(sns): add support for attribute key matching in message filtering

SNS message filtering now supports attribute key matching. This feature lets you create an Amazon
SNS subscription filter policy that matches incoming messages which contain an attribute key,
regardless of the attribute value associated with this key. This feature lets you offload additional
message filtering logic to Amazon SNS.

Update `SubscriptionFilter` to support this new feature.

See https://aws.amazon.com/about-aws/whats-new/2019/08/amazon-sns-message-filtering-adds-support-for-attribute-key-matching/

* update README

* add new test for existsFilter

packages/@aws-cdk/aws-sns/README.md

@@ -57,6 +57,7 @@ const fn = new lambda.Function(this, 'Function', ...);
 // color: 'red' or 'orange' or begins with 'bl'
 // size: anything but 'small' or 'medium'
 // price: between 100 and 200 or greater than 300
+// store: attribute must be present
 topic.subscribeLambda(new subs.LambdaSubscription(fn, {
     filterPolicy: {
         color: sns.SubscriptionFilter.stringFilter({
@@ -69,7 +70,8 @@ topic.subscribeLambda(new subs.LambdaSubscription(fn, {
         price: sns.SubscriptionFilter.numericFilter({
             between: { start: 100, stop: 200 },
             greaterThan: 300
-        })
+        }),
+        store: sns.SubscriptionFilter.existsFilter(),
     }
 }));
 ```

packages/@aws-cdk/aws-sns/lib/subscription-filter.ts

@@ -135,5 +135,12 @@ export class SubscriptionFilter {
     return new SubscriptionFilter(conditions);
   }
 
+  /**
+   * Returns a subscription filter for attribute key matching.
+   */
+  public static existsFilter() {
+    return new SubscriptionFilter([{ exists: true }]);
+  }
+
   constructor(public readonly conditions: any[] = []) {}
 }

packages/@aws-cdk/aws-sns/test/test.subscription.ts

@@ -80,6 +80,30 @@ export = {
     test.done();
   },
 
+  'with existsFilter'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const topic = new sns.Topic(stack, 'Topic');
+
+    // WHEN
+    new sns.Subscription(stack, 'Subscription', {
+      endpoint: 'endpoint',
+      filterPolicy: {
+        size: sns.SubscriptionFilter.existsFilter(),
+      },
+      protocol: sns.SubscriptionProtocol.LAMBDA,
+      topic
+    });
+
+    // THEN
+    expect(stack).to(haveResource('AWS::SNS::Subscription', {
+      FilterPolicy: {
+        size: [{ exists: true }]
+      },
+    }));
+    test.done();
+  },
+
   'throws with raw delivery for protocol other than http, https or sqs'(test: Test) {
     // GIVEN
     const stack = new cdk.Stack();