Setup cloudwatch alarms on SST !

FunctionsAlarm.ts

// name: FunctionsAlarm.ts
// can be used with:
//    new FunctionsAlarm(stack, 'ApiAlarmMain', {
//      api,
//      alarmNamePrefix: `API Error`,
//    });
//
//    new FunctionsAlarm(stack, 'ApiAlarmMain', {
//      functions: [cron.jobFunction],
//      alarmNamePrefix: `Cron Job Error`,
//    });

import { Api, ApiAuthorizer, Function } from 'sst/constructs';
import { Duration } from 'aws-cdk-lib';
import { Alarm } from 'aws-cdk-lib/aws-cloudwatch';
import { SnsAction } from 'aws-cdk-lib/aws-cloudwatch-actions';
import { Topic } from 'aws-cdk-lib/aws-sns';
import { EmailSubscription } from 'aws-cdk-lib/aws-sns-subscriptions';
import { Construct } from 'constructs';

export interface FunctionsAlarmProps {
  /**
   * Function to monitor
   */
  readonly functions?: Function[];

  /**
   * The SST API to monitor
   */
  readonly api?: Api<Record<string, ApiAuthorizer>>;

  /**
   * The name of the alarm
   */
  readonly alarmNamePrefix: string;
}

/**
 * This construct is responsible to create an alarm for all API Gateway endpoint.
 */

export default class FunctionsAlarm extends Construct {
  constructor(scope: Construct, id: string, props: FunctionsAlarmProps) {
    super(scope, id);

    if (props.functions === undefined && props.api === undefined) {
      throw new Error('You must provide at least one function or an API');
    }

    if (props.functions && props.api) {
      throw new Error('You must provide either a function or an API, not both');
    }

    if (!process.env.ALERT_EMAIL) {
      throw new Error('You must provide an ALERT_EMAIL environment variable');
    }

    const alarmTopic = new Topic(scope, `${id}-AlarmTopic`);
    alarmTopic.addSubscription(new EmailSubscription(process.env.ALERT_EMAIL));

    // work on Function array
    if (props.functions) {
      for (const func of props.functions) {
        // create a new alarm based on the metric
        createAlarm(scope, func, props.alarmNamePrefix, alarmTopic);
      }
    }

    // work on API construct
    if (props.api) {
      for (const route of props.api.routes) {
        const func = props.api.getFunction(route);

        if (func) {
          createAlarm(scope, func, props.alarmNamePrefix, alarmTopic, route);
        }
      }
    }
  }
}
const createAlarm = (
  scope: Construct,
  func: Function,
  alarmNamePrefix: string,
  alarmTopic: Topic,
  route?: string
) => {
  const alarm = new Alarm(scope, `FunctionAlarm-${func.id}`, {
    metric: func.metricErrors({
      period: Duration.minutes(15),
    }),
    alarmName: route ? `${alarmNamePrefix} ${route}` : `${alarmNamePrefix} ${func.id}`,
    threshold: 5,
    evaluationPeriods: 1,
  });
  alarm.addAlarmAction(new SnsAction(alarmTopic));
};