Express Physical

Healthcheck middleware for Express.

Purpose

express-physical is intended to be more or less a port of physical. The intention is to have Dropwizard-style healthchecks for Node.

Installation

npm install --save express-physical
# or
yarn add express-physical

Usage

const express = require('express')
const physical = require('express-physical')

const app = express()

const passingCheck = () => physical.response({
  name: 'Sample passing check',
  actionable: false,
  healthy: true,
  type: physical.type.SELF
})

app.use('/healthcheck', physical([passingCheck]))

/healthcheck will respond with the result of all checks, with a status code of 200 if all are healthy, and 500 if one or more checks are unhealthy.

API

const physical = require('express-physical')

The physical function expects an array of Checks and returns a middleware.

physical([Check])

Returns a middleware that returns an object with the currently healthy and unhealthy checks:

const middleware = physical([alwaysPassing, alwaysFailing])

// Will send a 200 OK with the following payload:

{
  "healthy": [{
    "name": "Always passing",
    "actionable": false,
    "type": "SELF",
    "healthy": true
  }],
  "unhealthy": [{
    "name": "Always failing",
    "actionable": false,
    "type": "SELF",
    "healthy": false,
    "message": "What can you do...?",
    "severity": "WARN"
  }]
}

Check(fn)

Check is not a class provided by physical, it is simply a function with one of two forms:

//Synchronous check
const synchronousCheck = function () {
  return physical.response(...)
}

// Async Check
const asyncCheck = function (done) {
  done(physical.response(...))
}

The check must always return or call the callback with a physical.response.

If the check throws an error, it will be forwarded to the next middleware where it has to be handled by the application. It will not be caught by the physical middleware and treated as a failing check.

physical.response({ options })

Option	Description	Mandatory
name string	A descriptive name	Yes
healthy boolean		Yes
type physical.type	The type of this check	Yes
severity physical.severity		When the check is unhealthy
actionable boolean	Whether or not the owner can act on this check failing	Yes
message string		When the check is unhealthy
dependentOn string	The name of a service that this check relies on	When the type is one of INFRASTRUCTURE, INTERNAL_DEPENDENCY and EXTERNAL_DEPENDENCY. Excluded otherwise.
info string{}	Object of any depth with additional info about this check	No
link string	A URL to where more information can be found	No

physical.type

Constants representing the type of a check.

• SELF
• METRICS
• INFRASTRUCTURE
• INTERNAL_DEPENDENCY
• EXTERNAL_DEPENDENCY
• INTERNET_CONNECTIVITY

physical.severity

Constants representing the severity of a failing check.

• WARNING
• CRITICAL
• DOWN

Prior art

• express-healthcheck - A little too simple for my use case, where I can have multiple independently failing checks where I want to provide data regarding all of them. express-physical could be implemented within express-healthcheck, but I'm looking for something a little more structured.
• node-healthchecks - Checks the system by trying to request public resources, but couldn't, for example, tell you if you're unable to connect to the database.
• Heavily inspired by physical. The API contract is essentially the same.

Tips

Some of my healthchecks are more expensive than others to run

express-physical does not expose a way to configure the interval at which healthchecks are invoked. If one of your healthchecks is too expensive to be run every time your monitoring system polls the healthcheck endpoint, throttle your function call so that it returns a cached value if called multiple times within a certain interval.

I need to pass arguments into my healthcheck

Checks need to receive either one or zero arguments, where that argument is a callback that will be provided by express-physical. If you have to pass more arguments into it, pass them when you create the check rather than at time of invokation.

const (db) => (done) => db.isHealthy((err, res) => done(physical.response({ ... })))

Avoiding repeating data for responses

Use a higher-order function to avoid having to repeat certain elements when creating responses. Ex.

const physical = require('express-physical')

const createDatabaseHealthCheck = (data) => (db) => (done) => {
  if (db.connection.readyState === 1) {
    done(Object.assign({}, data, physical.response({
      healthy: true,
      actionable: false
    })))
  } else {
    done(Object.assign({}, data, physical.response({
      healthy: false,
      actionable: true,
      severity: physical.severity.CRITICAL,
      message: 'Failed to connect to db'
    })))
  }
}

const canConnectToDatabase = ({
  name: 'Database connection',
  type: physical.type.INTERNAL_DEPENDENCY,
  dependentOn: {
    serviceName: 'mongodb'
  }
})(db)

app.use('/healthcheck', physical([canConnectToDatabase]))