express-physical

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.

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": "WARNING"
  }]
}

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 object{string,object}	Object of any depth with additional info about this check.	No
link string	A URL to where more information can be found	No

If your physical.response is invalid, an InvalidHealthcheckResponse error will be thrown. The original response data is available at InvalidHealthcheckResponse.responseData.

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

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

Contributing

Contributions are more than welcome. However, if your contribution goes beyond bug fixing, it is recommended that you first create an issue, in order to agree on a solution before investing time into implementing something that may not be merged. Changes that affect the schema are very unlikely to be accepted.

Testing

Contributions are expected to be tested to a reasonable degree.

• Run unit tests with npm run test:unit
• Run API tests with npm run test:integration
• Run the linter with npm run lint

Formatting

Formatting is done using Prettier. There is a pre-commit hook that should format your code automatically, but if you want to manually format it, run npm run format.

Benchmarks

If you are concerned with the performance implications of your change, there are benchmarks that you can run to find out. In order to run them, you need to have docker and docker-compose installed. Information on how to install Docker is available here.

To run the benchmarks, run npm run benchmark. The benchmarks will be run against your local branch and a reference version of physical. Some variance is to be expected, but if your change has a drastic impact, consider creating an issue to discuss it if you need help.

To change the reference version of physical that you want to run the benchmarks against, change the version in scripts/performance/benchmarks/package.json and run npm run benchmark:rebuild.

📣 Note that the benchmarks are a work in progress. Contributions for adding more benchmarks are very welcome!