Scheduler

Unstable API 0.7.0 @project-lakechain/scheduler-event-trigger

---

The Scheduler trigger allows developers to schedule the execution of document processing pipelines on a recurring or one-time schedule.

---

🗓️ One-time Schedules

To use this middleware as part of your pipeline, you have to import it in your CDK stack and specify a Schedule Expression that defines when to schedule the pipeline.

ℹ️ In the below example, we schedule the pipeline to start in 24 hours.

import * as scheduler from '@aws-cdk/aws-scheduler-alpha';
import { SchedulerEventTrigger } from '@project-lakechain/scheduler-event-trigger';
import { CacheStorage } from '@project-lakechain/core';

/**
 * @returns a date object representing the date
 * and time 24 hours from now.
 */
const tomorrow = (): Date => {
  const date = new Date();
  date.setDate(date.getDate() + 1);
  return (date);
};

class Stack extends cdk.Stack {
  constructor(scope: cdk.Construct, id: string) {
    // Create the Scheduler event trigger.
    const trigger = new SchedulerEventTrigger.Builder()
      .withScope(this)
      .withIdentifier('Trigger')
      .withCacheStorage(cache)
      .withScheduleExpression(
        scheduler.ScheduleExpression.at(tomorrow())
      )
      .build();
  }
}

---

🔖 Cron Expressions

You can also use cron expressions to define when to schedule a pipeline.

ℹ️ In the below example, we schedule the pipeline to run at 8:00PM UTC every Monday through Friday.

const trigger = new SchedulerEventTrigger.Builder()
  .withScope(this)
  .withIdentifier('Trigger')
  .withCacheStorage(cache)
  .withScheduleExpression(
    scheduler.ScheduleExpression.expression('cron(0 20 ? * MON-FRI *)')
  )
  .build();

---

⏰ Rate Expressions

Another way to create a recurring schedule is to use a rate expression.

ℹ️ In the below example, we schedule the pipeline to run every 5 minutes.

const trigger = new SchedulerEventTrigger.Builder()
  .withScope(this)
  .withIdentifier('Trigger')
  .withCacheStorage(cache)
  .withScheduleExpression(
    scheduler.ScheduleExpression.rate(cdk.Duration.minutes(5))
  )
  .build();

---

⚠️ Important Note

This middleware does not act as a data source, but rather as a simple trigger because it does not yield any document by default. However, every middleware must provide a valid document that can be interpreted by the next middlewares.

To solve this issue, the Scheduler will send a placeholder document with the application/json+scheduler mime-type when the schedule is reached. This means that subsequent middlewares have to be explicitly configured to accept this mime-type, and know how to react when triggered by the scheduler.

Providing Documents

In some cases, it can be useful to provide input documents on a schedule. A common use-case for this is to create a recurrent pipeline that will fetch information from an external system on a regular basis.

The Scheduler API allows developers to pass an array of URIs identifying documents to use as an input for a scheduled pipeline using the withDocuments method.

ℹ️ The below example will trigger the pipeline every 5 minutes, and create 2 different events for each document.

const trigger = new SchedulerEventTrigger.Builder()
  .withScope(this)
  .withIdentifier('Trigger')
  .withCacheStorage(cache)
  .withScheduleExpression(
    scheduler.ScheduleExpression.rate(cdk.Duration.minutes(5))
  )
  .withDocuments([
    'https://aws.amazon.com/builders-library/dependency-isolation/',
    's3://my-bucket/my-document.json'
  ])
  .build();

When specifying documents, the scheduler will attempt to infer their mime-type automatically. In the previous example, the scheduler would send a document with a mime-type of text/html for the first document, and application/json for the second.

---

🏗️ Architecture

The Scheduler trigger uses the AWS EventBridge Scheduler service to trigger a Lambda function. The Lambda function publishes the appropriate document to its output topic to be consumed by the next middleware in the pipeline.

---

🏷️ Properties

Supported Inputs

This middleware does not accept any inputs from other middlewares.

Supported Outputs

Mime Type	Description
Variant	When specifying documents to the Scheduler, it will attempt to infer the mime-types associated with these documents. If no documents are specified, the Scheduler will send a placeholder document to the next middlewares having a mime-type of application/json+scheduler.

Supported Compute Types

Type	Description
CPU	This middleware is based on a Lambda architecture.

---

📖 Examples

• Article Curation Pipeline - Builds a pipeline converting HTML articles into plain text.