Cloud Health

A core library to provide application lifecycle handling and liveness checks for Node.js applications.

Cloud Health is used by Cloud Health Connect to provide a Connect Middleware for use in Express.js, Loopback and other frameworks that provides:

• Startup checks
• Readiness checks
• Liveness checks
• Shutdown handling

for use with Kubernetes and Cloud Foundry based clouds.

Using Cloud Health

Cloud Health allows you to register promises which are executed during the three phases of your application, and allows you to call getLivenessStatus(), getReadinessStatus(), or a combined getStatus() to return a promise which resolves to whether the application is STARTING, UP, DOWN, STOPPING or STOPPED.

1. At startup for "startup"
   Promises that are created as part of a StartupCheck and registered using registerStartupCheck are executed at startup and can be used to execute any code that must complete before your application is ready. If the startup promises are still running, calls to getLivenessStatus(), getReadinessStatus(), and getStatus(), return STARTING. Once the promises complete, DOWN is reported if there were any failures, or the "liveness" and/or "readiness" promises are then executed.

2. At runtime for "liveness"
   Promises that are created as part of a LivenessCheck and registered using registerLivenessCheck are executed on calls to getLiveness() and getStatus(). These can be used to ensure that the application is still running correctly. If no promises are registered, or the complete successfully, UP is reported. If there are any failures, DOWN is reported.

3. At runtime for "readiness"
   Promises that are created as part of a ReadinessCheck and registered using registerReadinessCheck are executed on calls to getReadinessStatus() and getStatus(). These can be used to ensure that the application is still running correctly. If no promises are registered, or the complete successfully, UP is reported. If there are any failures, DOWN is reported.

4. On a SIGTERM signal for shutdown
   Promises that are created as part of a ShutdownCheck and registered using registerShutdownCheck are executed when the process receives a SIGTERM making it possible to clean up any resources used by the application. If the shutdown promises are still running, calls to getReadinessStatus(), getLivenessStatus() and getStatus() return STOPPING. Once the promises complete, STOPPED is reported.

Readiness vs. Liveness

Liveness and readiness checks are executed in the same way but are executed independently (based on calls to getLivenessStatus() or getReadinessStatus()) or together (based on calls to getStatus()).

The difference between liveness and readiness is intended to be purpose: readiness should be used to denote whether an application is "ready" to receive requests, and liveness should be used to denote whether an application is "live" (vs. in a state where it should be restarted).

Using Cloud Health with Node.js

1. Installation:

npm install @cloudnative/health

2. Set up a HealthChecker:

const health = require('@cloudnative/health');
let healthcheck = new health.HealthChecker();

3. Register a startupCheck promise:

const startPromise = () => new Promise(function (resolve, _reject) {
  setTimeout(function () {
    console.log('STARTED!');
    resolve();
  }, 10);
});
let startCheck = new health.StartupCheck("startCheck", startPromise);
healthcheck.registerStartupCheck(startCheck);

Note that registerStartupCheck() also returns a promise which can be used to wait until the promise is resolved.

4. Register a livenessCheck promise:

const livePromise = () => new Promise(function (resolve, _reject) {
  setTimeout(function () {
    console.log('ALIVE!');
    resolve();
  }, 10);
});
let liveCheck = new health.LivenessCheck("liveCheck", livePromise);
healthcheck.registerLivenessCheck(liveCheck);

5. Register a shutdownCheck promise:

const shutdownPromise = () => new Promise(function (resolve, _reject) {
  setTimeout(function () {
    console.log('DONE!');
    resolve();
  }, 10);
});
let shutdownCheck = new health.ShutdownCheck("shutdownCheck", shutdownPromise);
healthcheck.registerShutdownCheck(shutdownCheck);

6. Check the applications status:

healthcheck.getStatus()
.then((result) => console.log('STATUS: ' + JSON.stringify(result)));

Note that Cloud Health Connect provides a Connect Middleware for use in Express.js, Loopback and other frameworks that exposes the results as an endpoint for us in Cloud Foundry and Kubernetes based clouds.

Using Cloud Health with Typescript

The Cloud Health module is created in TypeScript and as such provides out of the box TypeScript support.

Module Long Term Support Policy

This module adopts the Module Long Term Support (LTS) policy, with the following End Of Life (EOL) dates:

License

Apache-2.0