lightship

Lightship 🚢

Abstracts readiness/ liveness checks and graceful shutdown of Node.js services running in Kubernetes.

• Lightship 🚢
  • Behaviour
    • /health
    • /live
    • /ready
  • Usage
    • Kubernetes container probe configuration
    • Logging
  • Usage examples
    • Using with Express.js
  • FAQ
    • What is the reason for having separate /live and /ready endpoints?

Behaviour

Creates a HTTP service used to check container probes.

Refer to the following Kubernetes documentation for information about the readiness and liveness checks:

• Pod Lifecycle
• Configure Liveness and Readiness Probes

/health

/health endpoint describes the current state of a Node.js service.

The endpoint responds:

• 200 status code, message "SERVER_IS_READY" when server is accepting new connections.
• 500 status code, message "SERVER_IS_NOT_READY" when server is initialising.
• 500 status code, message "SERVER_IS_SHUTTING_DOWN" when server is shutting down.

Used for human inspection.

/live

The endpoint responds:

• 200 status code, message "SERVER_IS_NOT_SHUTTING_DOWN".
• 500 status code, message "SERVER_IS_SHUTTING_DOWN".

Used to configure liveness probe.

/ready

The endpoint responds:

• 200 status code, message "SERVER_IS_READY".
• 500 status code, message "SERVER_IS_NOT_READY".

Used to configure readiness probe.

Usage

Use createLightship to create an instance of Lightship.

import {
  createLightship
} from 'lightship';

const configuration: LightshipConfigurationType = {};

const lightship: LightshipType = createLightship(configuration);

The following types describe the configuration shape and the resulting Lightship instance interface.

/**
 * A teardown function called when shutdown is initialized.
 */
type ShutdownHandlerType = () => Promise<void> | void;

/**
 * @property port The port on which the Lightship service listens. This port must be different than your main service port, if any. The default port is 9000.
 * @property signals An a array of [signal events]{@link https://nodejs.org/api/process.html#process_signal_events}. Default: [SIGTERM].
 */
type LightshipConfigurationType = {|
  +port?: number,
  +signals?: $ReadOnlyArray<string>
|};

/**
 * @property registerShutdownHandler Registers teardown functions that are called when shutdown is initialized. All registered shutdown handlers are executed in the order they have been registered. After all shutdown handlers have been executed, Lightship asks `process.exit()` to terminate the process synchronously.
 * @property shutdown Changes server state to SERVER_IS_SHUTTING_DOWN and initialises the shutdown of the application.
 * @property signalNotReady Changes server state to SERVER_IS_NOT_READY.
 * @property signalReady Changes server state to SERVER_IS_READY.
 */
type LightshipType = {|
  +isServerReady: () => boolean,
  +isServerShuttingDown: () => boolean,
  +registerShutdownHandler: (shutdownHandler: ShutdownHandlerType) => void,
  +shutdown: () => Promise<void>,
  +signalNotReady: () => void,
  +signalReady: () => void
|};

Kubernetes container probe configuration

This is an example of a reasonable container probe configuration to use with Lightship.

readinessProbe:
  httpGet:
    path: /ready
    port: 9000
  initialDelaySeconds: 5
  periodSeconds: 5
  failureThreshold: 1
  successThreshold: 1
livenessProbe:
  httpGet:
    path: /live
    port: 9000
  initialDelaySeconds: 10
  # Allow sufficient amount of time (180 seconds = periodSeconds * failureThreshold)
  # for the registered shutdown handlers to run to completion.
  periodSeconds: 30
  failureThreshold: 3
  successThreshold: 1

Logging

lightship is using Roarr to implement logging.

Set ROARR_LOG=true environment variable to enable logging.

Usage examples

Using with Express.js

Suppose that you have Express.js application that simply respond "Hello, World!".

import express from 'express';

const app = express();

app.get('/', (req, res) => {
  res.send('Hello, World!');
});

app.listen(8080);

To create a liveness and readiness check, simply create an instance of Lightship and use registerShutdownHandler to register a server shutdown handler, e.g.

import express from 'express';
import {
  createLightship
} from 'lightship';

const app = express();

app.get('/', (req, res) => {
  res.send('Hello, World!');
});

const server = app.listen(8080);

const lightship = createLightship();

lightship.registerShutdownHandler(() => {
  server.close();
});

// Lightship default state is "SERVER_IS_NOT_READY". Therefore, you must signal
// that the server is now ready to accept connections.
lightship.signalReady();

Suppose that a requirement has been added that you need to ensure that you do not say "Hello, World!" more often than 100 times per minute.

Use signalNotReady method to change server state to "SERVER_IS_NOT_READY" and use signalReady to revert the server state to "SERVER_IS_READY".

import express from 'express';
import {
  createLightship
} from 'lightship';

const app = express();

const minute = 60 * 1000;

let runningTotal = 0;

app.get('/', (req, res) => {
  runningTotal++;

  setTimeout(() => {
    runningTotal--;

    if (runningTotal < 100) {
      lightship.signalReady();
    } else {
      lightship.signalNotReady();
    }
  }, minute);

  res.send('Hello, World!');
});

const server = app.listen(8080);

const lightship = createLightship();

lightship.registerShutdownHandler(() => {
  server.close();
});

// Lightship default state is "SERVER_IS_NOT_READY". Therefore, you must signal
// that the server is now ready to accept connections.
lightship.signalReady();

How quick Kubernetes observes that the server state has changed depends on the probe configuration, specifically periodSeconds, successThreshold and failureThreshold, i.e. expect requests to continue coming through for a while after the server state has changed.

Suppose that a requirement has been added that the server must shutdown after saying "Hello, World!" 1000 times.

Use shutdown method to change server state to "SERVER_IS_SHUTTING_DOWN", e.g.

import express from 'express';
import delay from 'delay';
import {
  createLightship
} from 'lightship';

const app = express();

const minute = 60 * 1000;

let total = 0;
let runningTotal = 0;

app.get('/', (req, res) => {
  total++;
  runningTotal++;

  if (total === 1000) {
    lightship.shutdown();
  }

  setTimeout(() => {
    runningTotal--;

    if (runningTotal < 100) {
      lightship.signalReady();
    } else {
      lightship.signalNotReady();
    }
  }, minute);

  res.send('Hello, World!');
});

const server = app.listen(8080);

const lightship = createLightship();

lightship.registerShutdownHandler(async () => {
  // Allow sufficient amount of time to allow all of the existing
  // HTTP requests to finish before terminating the service.
  await delay(minute);

  server.close();
});

// Lightship default state is "SERVER_IS_NOT_READY". Therefore, you must signal
// that the server is now ready to accept connections.
lightship.signalReady();

Do not call process.exit() in a shutdown handler – Lighthouse calls process.exit() after all registered shutdown handlers have run to completion.

If for whatever reason a registered shutdown handler hangs, then (subject to the Pod's restart policy) Kubernetes will forcefully restart the Container after the livenessProbe deems the service to be failed.

FAQ

What is the reason for having separate /live and /ready endpoints?

Distinct endpoints are needed if you want your Container to be able to take itself down for maintenance (as done in the Using with Express.js usage example). Otherwise, you can use /health.