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cloud-health

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A core Health Check module for use in building a health endpoint for Kubernetes

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# The `cloud-health` module is deprecated.

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

Cloud Health is used by [Cloud Health Connect](http://github.com/CloudNativeJS/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:
  ```js
  npm install @cloudnative/health
  ```
2. Set up a HealthChecker:
  ```js
  const health = require('@cloudnative/health');
  let healthcheck = new health.HealthChecker();
  ```
3. Register a startupCheck promise:
  ```js
  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:
  ```js
  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:
  ```js
  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:
  ```js
  healthcheck.getStatus()
  .then((result) => console.log('STATUS: ' + JSON.stringify(result)));
  ```
  Note that [Cloud Health Connect](http://github.com/CloudNativeJS/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)](http://github.com/CloudNativeJS/ModuleLTS) policy, with the following End Of Life (EOL) dates:

| Module Version   | Release Date | Minimum EOL | EOL With     | Status  |
|------------------|--------------|-------------|--------------|---------|
| 2.x.x	         | May 2019     | April 2021  |              | Current |
| 1.x.x	         | July 2018    | Dec 2019    |              | LTS |

## License

[Apache-2.0](LICENSE)

cloud-health

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