Health Monitor

src/health-monitor.js

@@ -0,0 +1,115 @@
+'use strict';
+
+/**
+ * Module dependencies.
+ */
+
+const log = require('debugnyan')('process-manager:health-monitor');
+const utils = require('./utils');
+
+/**
+ * `HealthMonitor`.
+ */
+
+class HealthMonitor {
+  /**
+   * Constructor.
+   */
+
+  constructor() {
+    this.checks = {};
+    this.globalState = HealthMonitor.states.UNKNOWN;
+    this.states = {};
+  }
+
+  /**
+   * Add health check.
+   */
+
+  addCheck({ handler, id, interval = 5000 }) {
+    if (this.states[id]) {
+      throw new Error('Cannot add handler since it would overwrite an existing one');
+    }
+
+    this.states[id] = HealthMonitor.states.UNKNOWN;
+
+    const check = async () => {
+      let state;
+
+      try {
+        state = (await Promise.race([handler(), utils.timeout(5000, false)]))
+          ? HealthMonitor.states.HEALTHY
+          : HealthMonitor.states.UNHEALTHY;
+      } catch (e) {
+        state = HealthMonitor.states.UNHEALTHY;
+      }
+
+      this.updateState({ id, state });
+
+      this.checks[id] = setTimeout(check, interval);
+    };
+
+    this.checks[id] = setTimeout(check, 0);
+
+    log.info(`New health monitor check added with id '${id}'`);
+  }
+
+  /**
+   * Cleanup health monitor by clearing all timers and resetting the internal state.
+   */
+
+  cleanup() {
+    Object.values(this.checks).forEach(clearTimeout);
+
+    this.checks = {};
+    this.globalState = HealthMonitor.states.UNKNOWN;
+    this.states = {};
+  }
+
+  /**
+   * Handles state changes.
+   */
+
+  updateState({ id, state }) {
+    if (this.states[id] === state) {
+      return;
+    }
+
+    log.info({ id, newState: state, oldState: this.states[id] }, 'Component health status has changed');
+
+    this.states[id] = state;
+
+    // The sorted states array makes it so that the state at the end of the array is the relevant one.
+    // The global state is:
+    // - UNKNOWN if one exists.
+    // - UNHEALTHY if one exists and there are no UNKNOWN states.
+    // - HEALTHY if there are no UNKNOWN and UNHEALTHY states.
+    const [globalState] = Object.values(this.states).sort((left, right) => {
+      return left < right ? 1 : -1;
+    });
+
+    if (this.globalState === globalState) {
+      return;
+    }
+
+    log.info({ newState: globalState, oldState: this.globalState }, 'Global health status has changed');
+
+    this.globalState = globalState;
+  }
+}
+
+/**
+ * Health states.
+ */
+
+HealthMonitor.states = {
+  HEALTHY: 'healthy',
+  UNHEALTHY: 'unhealthy',
+  UNKNOWN: 'unknown'
+};
+
+/**
+ * Export `HealthMonitor` class.
+ */
+
+module.exports = HealthMonitor;

src/index.js

@@ -4,6 +4,7 @@
  * Module dependencies.
  */
 
+const HealthMonitor = require('./health-monitor');
 const utils = require('./utils');
 
 /**
@@ -24,13 +25,22 @@ class ProcessManager {
   constructor() {
     this.errors = [];
     this.forceShutdown = utils.deferred();
+    this.healthMonitor = new HealthMonitor();
     this.hooks = [];
     this.log = utils.getDefaultLogger();
     this.running = [];
     this.terminating = false;
     this.timeout = 30000;
   }
 
+  /**
+   * Add health monitor check.
+   */
+
+  addHealthCheck(...args) {
+    this.healthMonitor.addCheck(...args);
+  }
+
   /**
    * Add hook.
    */
@@ -79,6 +89,17 @@ class ProcessManager {
     process.exit();
   }
 
+  /**
+   * Get health monitor status.
+   */
+
+  getHealthStatus() {
+    return {
+      global: this.healthMonitor.globalState,
+      individual: this.healthMonitor.states
+    };
+  }
+
   /**
    * Call all handlers for a hook.
    */
@@ -190,6 +211,7 @@ class ProcessManager {
       .then(() => this.log.info('All running instances have stopped'))
       .then(() => this.hook('drain'))
       .then(() => this.log.info(`${this.hooks.filter(hook => hook.type === 'drain').length} server(s) drained`))
+      .then(() => this.healthMonitor.cleanup())
       .then(() => this.hook('disconnect'))
       .then(() =>
         this.log.info(`${this.hooks.filter(hook => hook.type === 'disconnect').length} service(s) disconnected`)