Remove all default healthchecks

This is a change that just kept on giving!

So I started out wanting to remove the health check that verifies
whether next-metrics is configured properly. We don't need this any
more, I couldn't care less if an app stops sending metrics 🙈 we will
rethink this in future, the right way is probably a heartbeat metric in
OpenTelemetry.

Anyway, once I removed the metrics health check there were no more
default checks. This means there are no more ticking checks at all as
far as I can tell. This allowed me to remove a bunch of redundant code
from the internals.

I tested this by symlinking it into several different n-express apps and
everything starts up fine, including for apps that define their own
healthchecks.

main.js

@@ -3,7 +3,6 @@
  * @typedef {import("./typings/n-express").Callback} Callback
  * @typedef {import("./typings/n-express").AppOptions} AppOptions
  * @typedef {import("./typings/n-express").AppContainer} AppContainer
- * @typedef {import("./typings/metrics").TickingMetric} TickingMetric
  */
 
 require('isomorphic-fetch');
@@ -26,7 +25,7 @@ const metrics = require('next-metrics');
 const logger = require('@dotcom-reliability-kit/logger');
 
 // utils
-const healthChecks = require('./src/lib/health-checks');
+const setupHealthEndpoint = require('./src/lib/health-checks');
 const InstrumentListen = require('./src/lib/instrument-listen');
 const guessAppDetails = require('./src/lib/guess-app-details');
 
@@ -105,7 +104,7 @@ const getAppContainer = (options) => {
 	app.use(vary);
 
 	if (!options.demo) {
-		instrumentListen.addMetrics(healthChecks(app, options, meta));
+		setupHealthEndpoint(app, options, meta);
 	}
 
 	// Debug related headers.

src/lib/health-checks.js

@@ -1,47 +1,26 @@
 /**
  * @typedef {import("express").Application} ExpressApp
- * @typedef {import("../../typings/metrics").Healthcheck} Healthcheck
- * @typedef {import("../../typings/metrics").TickingMetric} TickingMetric
  * @typedef {import("../../typings/n-express").AppOptions} AppOptions
  */
 
 const logger = require('@dotcom-reliability-kit/logger');
 
-const metricsHealthCheck = require('./metrics-healthcheck');
-
 /**
  * @param {ExpressApp} app
  * @param {AppOptions} options
  * @param {{name: string, description: string}} meta
- * @returns {(Healthcheck & TickingMetric)[]}
+ * @returns {void}
  */
 module.exports = (app, options, meta) => {
 	const defaultAppName = `Next FT.com ${meta.name} in ${
 		process.env.REGION || 'unknown region'
 	}`;
-	/**
-	 * Add checks to this array if they use an interval or similar
-	 * to poll for data. This allows them to be properly stopped
-	 * alongside the n-express app.
-	 *
-	 * @type {(Healthcheck & TickingMetric)[]}
-	 */
-	const tickingMetricChecks = [];
-
-	/** @type {Healthcheck[]} */
-	const defaultChecks = [
-		...tickingMetricChecks,
-		metricsHealthCheck(meta.name)
-	];
-
-	/** @type {Healthcheck[]} */
-	const healthChecks = options.healthChecks.concat(defaultChecks);
 
 	app.get(
 		/\/__health(?:\.([123]))?$/,
 		/** @type {ExpressApp} */ (req, res) => {
 			res.set({ 'Cache-Control': 'private, no-cache, max-age=0' });
-			const checks = healthChecks.map((check) => check.getStatus());
+			const checks = options.healthChecks.map((check) => check.getStatus());
 
 			checks.forEach(check => {if(!check.id){
 				logger.warn({
@@ -86,6 +65,4 @@ module.exports = (app, options, meta) => {
 			);
 		}
 	);
-
-	return tickingMetricChecks;
 };

src/lib/instrument-listen.js

@@ -1,6 +1,3 @@
-/**
- * @typedef {import("../../typings/metrics").TickingMetric} TickingMetric
- */
 // @ts-nocheck
 
 const metrics = require('next-metrics');
@@ -18,8 +15,6 @@ const readFile = denodeify(fs.readFile);
 module.exports = class InstrumentListen {
 	constructor (app, meta, initPromises) {
 		this.app = app;
-		/** @type {TickingMetric[]} */
-		this.tickingMetrics = [];
 		this.server = null;
 		this.initApp(meta, initPromises);
 	}
@@ -118,15 +113,9 @@ module.exports = class InstrumentListen {
 		 * Attempts to clean up the ticking checks and close the server
 		 */
 		this.app.close = (callback) => {
-			this.tickingMetrics.forEach(check => check.stop());
 			if (this.server) {
 				this.server.close(() => callback && callback());
 			}
 		};
 	}
-
-	addMetrics (item) {
-		const items = (Array.isArray(item) ? item : [item]);
-		this.tickingMetrics.push(...items);
-	}
 };

test/app/clear-interval.test.js

@@ -2,26 +2,9 @@ const express = require('express');
 const nextExpress = require('../../main');
 const InstrumentListen = require('../../src/lib/instrument-listen');
 const expect = require('chai').expect;
-const healthChecks = require('../../src/lib/health-checks');
 const sinon = require('sinon');
 
 describe('clears intervals', () => {
-	it('should return an array of objects with stop functions in health-checks', () => {
-		const app = express();
-
-		const checks = healthChecks(app, {healthChecks: []}, {});
-		expect(checks).to.be.an('array');
-		for (const check of checks) {
-			expect(check.stop).to.be.a('function');
-		}
-
-		//ensure the start() function is called before the stop(),
-		//because healthChecks(...) eventually calls n-health runCheck(), but runCheck() doesn't await on start()
-		setTimeout(() => {
-			checks.forEach(check => check.stop());
-		}, 0);
-	});
-
 	it('should close server in app.close if there is a live server', () => {
 		const app = express();
 		const instrumentListen = new InstrumentListen(app, {}, [Promise.resolve()]);

typings/metrics.d.ts

@@ -10,10 +10,6 @@ export interface HealthcheckStatus {
 	technicalSummary: string;
 }
 
-export type TickingMetric = {
-	stop: () => void;
-}
-
 export type Healthcheck = {
 	getStatus: () => HealthcheckStatus;
 };