feat(synthetics): add static cron method to schedule class

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

@@ -97,13 +97,15 @@ object to the `schedule` property.
 Configure a run rate of up to 60 minutes with `Schedule.rate`:
 
 ```ts
-Schedule.rate(Duration.minutes(5)), // Runs every 5 minutes.
+Schedule.rate(Duration.minutes(5)) // Runs every 5 minutes.
 ```
 
-You can also specify a [cron expression](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch_Synthetics_Canaries_cron.html) via `Schedule.expression`:
+You can also specify a [cron expression](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch_Synthetics_Canaries_cron.html) with `Schedule.cron`:
 
 ```ts
-Schedule.expression('cron(0 0,8,16 * * ? *)'), // Run at 12am, 8am, 4pm UTC every day
+Schedule.cron({
+  hour: '0,8,16' // Run at 12am, 8am, 4pm UTC every day
+})
 ```
 
 If you want the canary to run just once upon deployment, you can use `Schedule.once()`.

packages/@aws-cdk/aws-synthetics/lib/schedule.ts

@@ -42,9 +42,81 @@ export class Schedule {
     return new Schedule(`rate(${minutes} minutes)`);
   }
 
+  /**
+   * Create a schedule from a set of cron fields
+   */
+  public static cron(options: CronOptions): Schedule {
+    if (options.weekDay !== undefined && options.day !== undefined) {
+      throw new Error('Cannot supply both \'day\' and \'weekDay\', use at most one');
+    }
+
+    const minute = fallback(options.minute, '*');
+    const hour = fallback(options.hour, '*');
+    const month = fallback(options.month, '*');
+
+    // Weekday defaults to '?' if not supplied. If it is supplied, day must become '?'
+    const day = fallback(options.day, options.weekDay !== undefined ? '?' : '*');
+    const weekDay = fallback(options.weekDay, '?');
+
+    // '*' is only allowed in the year field
+    const year = '*';
+
+    return new Schedule(`cron(${minute} ${hour} ${day} ${month} ${weekDay} ${year})`);
+  }
+
   private constructor(
     /**
      * The Schedule expression
      */
     public readonly expressionString: string) {}
 }
+
+
+/**
+ * Options to configure a cron expression
+ *
+ * All fields are strings so you can use complex expressions. Absence of
+ * a field implies '*' or '?', whichever one is appropriate.
+ *
+ * @see https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch_Synthetics_Canaries_cron.html
+ */
+export interface CronOptions {
+  /**
+   * The minute to run this rule at
+   *
+   * @default - Every minute
+   */
+  readonly minute?: string;
+
+  /**
+   * The hour to run this rule at
+   *
+   * @default - Every hour
+   */
+  readonly hour?: string;
+
+  /**
+   * The day of the month to run this rule at
+   *
+   * @default - Every day of the month
+   */
+  readonly day?: string;
+
+  /**
+   * The month to run this rule at
+   *
+   * @default - Every month
+   */
+  readonly month?: string;
+
+  /**
+   * The day of the week to run this rule at
+   *
+   * @default - Any day of the week
+   */
+  readonly weekDay?: string;
+}
+
+function fallback(x: string | undefined, def: string): string {
+  return x ?? def;
+}

packages/@aws-cdk/aws-synthetics/test/canary.test.ts

@@ -292,6 +292,27 @@ test('Schedule can be set to 1 minute', () => {
   });
 });
 
+test('Schedule can be set with Cron', () => {
+  // GIVEN
+  const stack = new Stack();
+
+  // WHEN
+  new synthetics.Canary(stack, 'Canary', {
+    schedule: synthetics.Schedule.cron({ minute: '30' }),
+    test: synthetics.Test.custom({
+      handler: 'index.handler',
+      code: synthetics.Code.fromInline('/* Synthetics handler code */'),
+    }),
+    runtime: synthetics.Runtime.SYNTHETICS_NODEJS_PUPPETEER_3_3,
+  });
+
+  // THEN
+  Template.fromStack(stack).hasResourceProperties('AWS::Synthetics::Canary', {
+    Schedule: Match.objectLike({ Expression: 'cron(30 * * * ? *)' }),
+  });
+});
+
+
 test('Schedule can be set with Expression', () => {
   // GIVEN
   const stack = new Stack();

packages/@aws-cdk/aws-synthetics/test/schedule.test.ts

@@ -0,0 +1,26 @@
+import * as synthetics from '../lib';
+
+describe('cron', () => {
+  test('day and weekDay are mutex: given week day', () => {
+    expect(synthetics.Schedule.cron({
+      weekDay: 'MON-FRI',
+    }).expressionString).toEqual('cron(* * ? * MON-FRI *)');
+  });
+
+  test('day and weekDay are mutex: given month day', () => {
+    expect(synthetics.Schedule.cron({
+      day: '1',
+    }).expressionString).toEqual('cron(* * 1 * ? *)');
+  });
+
+  test('day and weekDay are mutex: given neither', () => {
+    expect(synthetics.Schedule.cron({}).expressionString).toEqual('cron(* * * * ? *)');
+  });
+
+  test('day and weekDay are mutex: throw if given both', () => {
+    expect(() => synthetics.Schedule.cron({
+      day: '1',
+      weekDay: 'MON',
+    })).toThrow('Cannot supply both \'day\' and \'weekDay\', use at most one');
+  });
+});