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feat(aws-lambda): high level API for event sources (#1063)
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Adds a uniform high-level API for AWS Lambda event sources. This API
allows developers to use a single API `lambda.addEventSource(source)`
to connect a function with any type of event source.

The `IEventSource` class is defined in the aws-lambda module
and the aws-lambda-event-sources module includes implementations
for various event sources. Each implementation utilizes the
relevant mechanism.

Currently supported:

 - SQS
 - SNS
 - S3

We eventually want to support all event sources as described in the
Lambda developer guide.
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Elad Ben-Israel committed Nov 1, 2018
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## AWS Lambda Event Sources

This module includes classes that allow using various AWS services as event
sources for AWS Lambda via the high-level `lambda.addEventSource(source)` API.

NOTE: In most cases, it is also possible to use the resource APIs to invoke an
AWS Lambda function. This library provides a uniform API for all Lambda event
sources regardless of the underlying mechanism they use.

### SQS

Amazon Simple Queue Service (Amazon SQS) allows you to build asynchronous
workflows. For more information about Amazon SQS, see Amazon Simple Queue
Service. You can configure AWS Lambda to poll for these messages as they arrive
and then pass the event to a Lambda function invocation. To view a sample event,
see [Amazon SQS Event](https://docs.aws.amazon.com/lambda/latest/dg/eventsources.html#eventsources-sqs).

To set up Amazon Simple Queue Service as an event source for AWS Lambda, you
first create or update an Amazon SQS queue and select custom values for the
queue parameters. The following parameters will impact Amazon SQS's polling
behavior:

* __visibilityTimeoutSec__: May impact the period between retries.
* __receiveMessageWaitTimeSec__: Will determine [long
poll](https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-long-polling.html)
duration. The default value is 20 seconds.

```ts
import { SqsEventSource } from '@aws-cdk/aws-lambda-event-sources';

const queue = new sqs.Queue(this, 'MyQueue', {
visibilityTimeoutSec: 30 // default,
receiveMessageWaitTimeSec: 20 // default
});

lambda.addEventSource(new SqsEventSource(queue, {
batchSize: 10 // default
});
```
### S3
You can write Lambda functions to process S3 bucket events, such as the
object-created or object-deleted events. For example, when a user uploads a
photo to a bucket, you might want Amazon S3 to invoke your Lambda function so
that it reads the image and creates a thumbnail for the photo.
You can use the bucket notification configuration feature in Amazon S3 to
configure the event source mapping, identifying the bucket events that you want
Amazon S3 to publish and which Lambda function to invoke.
```ts
import { S3EventSource } from '@aws-cdk/aws-lambda-event-sources';

const bucket = new s3.Bucket(...);

lambda.addEventSource(new S3EventSource(bucket, {
events: [ s3.EventType.ObjectCreated, s3.EventType.ObjectDeleted ],
filters: [ { prefix: 'subdir/' } ] // optional
}));
```
### SNS
You can write Lambda functions to process Amazon Simple Notification Service
notifications. When a message is published to an Amazon SNS topic, the service
can invoke your Lambda function by passing the message payload as a parameter.
Your Lambda function code can then process the event, for example publish the
message to other Amazon SNS topics, or send the message to other AWS services.
This also enables you to trigger a Lambda function in response to Amazon
CloudWatch alarms and other AWS services that use Amazon SNS.
For an example event, see [Appendix: Message and JSON
Formats](https://docs.aws.amazon.com/sns/latest/dg/json-formats.html) and
[Amazon SNS Sample
Event](https://docs.aws.amazon.com/lambda/latest/dg/eventsources.html#eventsources-sns).
For an example use case, see [Using AWS Lambda with Amazon SNS from Different
Accounts](https://docs.aws.amazon.com/lambda/latest/dg/with-sns.html).
```ts
import { SnsEventSource } from '@aws-cdk/aws-lambda-event-sources';

const topic = new sns.Topic(...);

lambda.addEventSource(new SnsEventSource(topic));
```
When a user calls the SNS Publish API on a topic that your Lambda function is
subscribed to, Amazon SNS will call Lambda to invoke your function
asynchronously. Lambda will then return a delivery status. If there was an error
calling Lambda, Amazon SNS will retry invoking the Lambda function up to three
times. After three tries, if Amazon SNS still could not successfully invoke the
Lambda function, then Amazon SNS will send a delivery status failure message to
CloudWatch.
## Roadmap
Eventually, this module will support all the event sources described under
[Supported Event
Sources](https://docs.aws.amazon.com/lambda/latest/dg/invoking-lambda-function.html)
in the AWS Lambda Developer Guide.
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export * from './sqs';
export * from './s3';
export * from './sns';
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import lambda = require('@aws-cdk/aws-lambda');
import s3 = require('@aws-cdk/aws-s3');

export interface S3EventSourceProps {
/**
* The s3 event types that will trigger the notification.
*/
events: s3.EventType[];

/**
* S3 object key filter rules to determine which objects trigger this event.
* Each filter must include a `prefix` and/or `suffix` that will be matched
* against the s3 object key. Refer to the S3 Developer Guide for details
* about allowed filter rules.
*/
filters?: s3.NotificationKeyFilter[];
}

/**
* Use S3 bucket notifications as an event source for AWS Lambda.
*/
export class S3EventSource implements lambda.IEventSource {
constructor(readonly bucket: s3.Bucket, private readonly props: S3EventSourceProps) {

}

public bind(target: lambda.FunctionRef) {
const filters = this.props.filters || [];
for (const event of this.props.events) {
this.bucket.onEvent(event, target, ...filters);
}
}
}
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import lambda = require('@aws-cdk/aws-lambda');
import sns = require('@aws-cdk/aws-sns');

/**
* Use an Amazon SNS topic as an event source for AWS Lambda.
*/
export class SnsEventSource implements lambda.IEventSource {
constructor(readonly topic: sns.TopicRef) {
}

public bind(target: lambda.FunctionRef) {
this.topic.subscribeLambda(target);
}
}

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