Full Code of docker/go-healthcheck for AI

Repository: docker/go-healthcheck
Branch: master
Commit: a4f20681d8a1
Files: 11
Total size: 42.8 KB

Directory structure:
gitextract_cyo82iev/

├── CONTRIBUTING.md
├── LICENSE
├── MAINTAINERS
├── README.md
├── api/
│   ├── api.go
│   └── api_test.go
├── checks/
│   ├── checks.go
│   └── checks_test.go
├── doc.go
├── health.go
└── health_test.go

================================================
FILE CONTENTS
================================================

================================================
FILE: CONTRIBUTING.md
================================================
# Contributing to Docker open source projects

Want to hack on this project? Awesome! Here are instructions to get you started.

This project is a part of the [Docker](https://www.docker.com) project, and follows
the same rules and principles. If you're already familiar with the way
Docker does things, you'll feel right at home.

Otherwise, go read Docker's
[contributions guidelines](https://github.com/docker/docker/blob/master/CONTRIBUTING.md),
[issue triaging](https://github.com/docker/docker/blob/master/project/ISSUE-TRIAGE.md),
[review process](https://github.com/docker/docker/blob/master/project/REVIEWING.md) and
[branches and tags](https://github.com/docker/docker/blob/master/project/BRANCHES-AND-TAGS.md).

For an in-depth description of our contribution process, visit the
contributors guide: [Understand how to contribute](https://docs.docker.com/opensource/workflow/make-a-contribution/)

### Sign your work

The sign-off is a simple line at the end of the explanation for the patch. Your
signature certifies that you wrote the patch or otherwise have the right to pass
it on as an open-source patch. The rules are pretty simple: if you can certify
the below (from [developercertificate.org](http://developercertificate.org/)):

```
Developer Certificate of Origin
Version 1.1

Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
1 Letterman Drive
Suite D4700
San Francisco, CA, 94129

Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.


Developer's Certificate of Origin 1.1

By making a contribution to this project, I certify that:

(a) The contribution was created in whole or in part by me and I
    have the right to submit it under the open source license
    indicated in the file; or

(b) The contribution is based upon previous work that, to the best
    of my knowledge, is covered under an appropriate open source
    license and I have the right under that license to submit that
    work with modifications, whether created in whole or in part
    by me, under the same open source license (unless I am
    permitted to submit under a different license), as indicated
    in the file; or

(c) The contribution was provided directly to me by some other
    person who certified (a), (b) or (c) and I have not modified
    it.

(d) I understand and agree that this project and the contribution
    are public and that a record of the contribution (including all
    personal information I submit with it, including my sign-off) is
    maintained indefinitely and may be redistributed consistent with
    this project or the open source license(s) involved.
```

Then you just add a line to every git commit message:

    Signed-off-by: Joe Smith <joe.smith@email.com>

Use your real name (sorry, no pseudonyms or anonymous contributions.)

If you set your `user.name` and `user.email` git configs, you can sign your
commit automatically with `git commit -s`.


================================================
FILE: LICENSE
================================================

                                 Apache License
                           Version 2.0, January 2004
                        http://www.apache.org/licenses/

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================================================
FILE: MAINTAINERS
================================================
# Project maintainers file
#
# This file describes who runs the docker/go-healthcheck project and how.
# This is a living document - if you see something out of date or missing, speak up!
#
# It is structured to be consumable by both humans and programs.
# To extract its contents programmatically, use any TOML-compliant parser.
#
# This file is compiled into the MAINTAINERS file in docker/opensource.
#
[Org]
	[Org."Core maintainers"]
		people = [
			"diogomonica",
			"nathanmccauley",
			"stevvooe",
		]

[people]

# A reference list of all people associated with the project.
# All other sections should refer to people by their canonical key
# in the people section.

	# ADD YOURSELF HERE IN ALPHABETICAL ORDER

	[people.diogomonica]
	Name = "Diogo Monica"
	Email = "diogo@docker.com"
	GitHub = "diogomonica"

	[people.nathanmccauley]
	Name = "Nathan McCauley"
	Email = "nathan.mccauley@docker.com"
	GitHub = "nathanmccauley"

	[people.stevvooe]
	Name = "Stephen Day"
	Email = "stephen.day@docker.com"
	GitHub = "stevvooe"


================================================
FILE: README.md
================================================
# go-healthcheck

Go Healthcheck provides a generic health checking framework. The health package works expvar style. By importing the package the debug server is getting a `/debug/health` endpoint that returns the current status of the application. If there are no errors, `/debug/health` will return a HTTP 200 status, together with an empty JSON reply `{}`. If there are any checks with errors, the JSON reply will include all the failed checks, and the response will be have a HTTP 500 status.

A Check can either be run synchronously, or asynchronously. We recommend that most checks are registered as an asynchronous check, so a call to the `/debug/health` endpoint always returns
immediately. This pattern is particularly useful for checks that verify upstream connectivity or database status, since they might take a long time to return/timeout.

## Installing

To install go-healthcheck, just import it in your application:

`import "github.com/docker/go-healthcheck/health"`

You can also (optionally) import `health/api` that will add two convenience endpoints: `/debug/health/down` and `/debug/health/up`. These endpoints add "manual" checks that allow the service to quickly be brought in/out of rotation.

`import _ "github.com/docker/go-healthcheck/health/api"`

```bash
# curl localhost:5001/debug/health
{}
# curl -X POST localhost:5001/debug/health/down
# curl localhost:5001/debug/health
{"manual_http_status":"Manual Check"}
```
After importing these packages to your main application, you can start registering checks.

## Registering Checks

The recommended way of registering checks is using a periodic Check. PeriodicChecks run on a certain schedule and asynchronously update the status of the check. This allows `CheckStatus()` to return without blocking on an expensive check.

A trivial example of a check that runs every 5 seconds and shuts down our server if the current minute is even, could be added as follows:

```go
 func currentMinuteEvenCheck() error {
   m := time.Now().Minute()
   if m%2 == 0 {
     return errors.New("Current minute is even!")
   }
   return nil
 }

 health.RegisterPeriodicFunc("minute_even", currentMinuteEvenCheck, time.Second*5)
```

Alternatively, you can also make use of `RegisterPeriodicThresholdFunc` to implement the exact same check, but add a threshold of failures after which the check will be unhealthy. This is particularly useful for flaky Checks, ensuring some stability of the service when handling them.

```go
health.RegisterPeriodicThresholdFunc("minute_even", currentMinuteEvenCheck, time.Second*5, 4)
```

The lowest-level way to interact with the health package is calling `Register` directly. Register allows you to pass in an arbitrary string and something that implements `Checker` and runs your check. If your method returns an error with nil, it is considered a healthy check, otherwise it will make the health check endpoint `/debug/health` start returning a 500 and list the specific check that failed.

Assuming you wish to register a method called `currentMinuteEvenCheck() error` you could do that by doing:

```go
health.Register("even_minute", health.CheckFunc(currentMinuteEvenCheck))
```

CheckFunc is a convenience type that implements Checker.

Another way of registering a check could be by using an anonymous function and the convenience method RegisterFunc. An example that makes the status endpoint always return an error:

```go
 health.RegisterFunc("my_check", func() error {
  return Errors.new("This is an error!")
}))
```

## Examples

You could also use the health checker mechanism to ensure your application only comes up if certain conditions are met, or to allow the developer to take the service out of rotation immediately. An example that checks database connectivity and immediately takes the server out of rotation on err:

```go
 updater = health.NewStatusUpdater()
  health.RegisterFunc("database_check", func() error {
   return updater.Check()
 }))

 conn, err := Connect(...) // database call here
 if err != nil {
   updater.Update(errors.New("Error connecting to the database: " + err.Error()))
 }
 ```

You can also use the predefined Checkers that come included with the health package. First, import the checks:

`import "github.com/docker/distribution/health/checks`

After that you can make use of any of the provided checks. An example of using a `FileChecker` to take the application out of rotation if a certain file exists can be done as follows:

```go
health.Register("fileChecker", health.PeriodicChecker(checks.FileChecker("/tmp/disable"), time.Second*5))
```
After registering the check, it is trivial to take an application out of rotation from the console:
```bash
# curl localhost:5001/debug/health
{}
# touch /tmp/disable
# curl localhost:5001/debug/health
{"fileChecker":"file exists"}
```
You could also test the connectivity to a downstream service by using a `HTTPChecker`, but ensure that you only mark the test unhealthy if there are a minimum of two failures in a row:

```go
health.Register("httpChecker", health.PeriodicThresholdChecker(checks.HTTPChecker("https://www.google.pt"), time.Second*5, 2))
```

## Reporting security issues

The maintainers take security seriously. If you discover a security issue,
please bring it to their attention right away!

Please **DO NOT** file a public issue, instead send your report privately to
[security@docker.com](mailto:security@docker.com).

Security reports are greatly appreciated and we will publicly thank you for it.
We also like to send gifts—if you're into Docker schwag, make sure to let
us know. We currently do not offer a paid security bounty program, but are not
ruling it out in the future.

## Copyright and license

Copyright © 2015 Docker, Inc.
Licensed under the [Apache License, Version 2.0](LICENSE).


================================================
FILE: api/api.go
================================================
package api

import (
	"errors"
	"net/http"

	"github.com/docker/distribution/health"
)

var (
	updater = health.NewStatusUpdater()
)

// DownHandler registers a manual_http_status that always returns an Error
func DownHandler(w http.ResponseWriter, r *http.Request) {
	if r.Method == "POST" {
		updater.Update(errors.New("Manual Check"))
	} else {
		w.WriteHeader(http.StatusNotFound)
	}
}

// UpHandler registers a manual_http_status that always returns nil
func UpHandler(w http.ResponseWriter, r *http.Request) {
	if r.Method == "POST" {
		updater.Update(nil)
	} else {
		w.WriteHeader(http.StatusNotFound)
	}
}

// init sets up the two endpoints to bring the service up and down
func init() {
	health.Register("manual_http_status", updater)
	http.HandleFunc("/debug/health/down", DownHandler)
	http.HandleFunc("/debug/health/up", UpHandler)
}


================================================
FILE: api/api_test.go
================================================
package api

import (
	"net/http"
	"net/http/httptest"
	"testing"

	"github.com/docker/distribution/health"
)

// TestGETDownHandlerDoesNotChangeStatus ensures that calling the endpoint
// /debug/health/down with METHOD GET returns a 404
func TestGETDownHandlerDoesNotChangeStatus(t *testing.T) {
	recorder := httptest.NewRecorder()

	req, err := http.NewRequest("GET", "https://fakeurl.com/debug/health/down", nil)
	if err != nil {
		t.Errorf("Failed to create request.")
	}

	DownHandler(recorder, req)

	if recorder.Code != 404 {
		t.Errorf("Did not get a 404.")
	}
}

// TestGETUpHandlerDoesNotChangeStatus ensures that calling the endpoint
// /debug/health/down with METHOD GET returns a 404
func TestGETUpHandlerDoesNotChangeStatus(t *testing.T) {
	recorder := httptest.NewRecorder()

	req, err := http.NewRequest("GET", "https://fakeurl.com/debug/health/up", nil)
	if err != nil {
		t.Errorf("Failed to create request.")
	}

	DownHandler(recorder, req)

	if recorder.Code != 404 {
		t.Errorf("Did not get a 404.")
	}
}

// TestPOSTDownHandlerChangeStatus ensures the endpoint /debug/health/down changes
// the status code of the response to 503
// This test is order dependent, and should come before TestPOSTUpHandlerChangeStatus
func TestPOSTDownHandlerChangeStatus(t *testing.T) {
	recorder := httptest.NewRecorder()

	req, err := http.NewRequest("POST", "https://fakeurl.com/debug/health/down", nil)
	if err != nil {
		t.Errorf("Failed to create request.")
	}

	DownHandler(recorder, req)

	if recorder.Code != 200 {
		t.Errorf("Did not get a 200.")
	}

	if len(health.CheckStatus()) != 1 {
		t.Errorf("DownHandler didn't add an error check.")
	}
}

// TestPOSTUpHandlerChangeStatus ensures the endpoint /debug/health/up changes
// the status code of the response to 200
func TestPOSTUpHandlerChangeStatus(t *testing.T) {
	recorder := httptest.NewRecorder()

	req, err := http.NewRequest("POST", "https://fakeurl.com/debug/health/up", nil)
	if err != nil {
		t.Errorf("Failed to create request.")
	}

	UpHandler(recorder, req)

	if recorder.Code != 200 {
		t.Errorf("Did not get a 200.")
	}

	if len(health.CheckStatus()) != 0 {
		t.Errorf("UpHandler didn't remove the error check.")
	}
}


================================================
FILE: checks/checks.go
================================================
package checks

import (
	"errors"
	"net"
	"net/http"
	"os"
	"strconv"
	"time"

	"github.com/docker/distribution/health"
)

// FileChecker checks the existence of a file and returns an error
// if the file exists.
func FileChecker(f string) health.Checker {
	return health.CheckFunc(func() error {
		if _, err := os.Stat(f); err == nil {
			return errors.New("file exists")
		}
		return nil
	})
}

// HTTPChecker does a HEAD request and verifies that the HTTP status code
// returned matches statusCode.
func HTTPChecker(r string, statusCode int, timeout time.Duration, headers http.Header) health.Checker {
	return health.CheckFunc(func() error {
		client := http.Client{
			Timeout: timeout,
		}
		req, err := http.NewRequest("HEAD", r, nil)
		if err != nil {
			return errors.New("error creating request: " + r)
		}
		for headerName, headerValues := range headers {
			for _, headerValue := range headerValues {
				req.Header.Add(headerName, headerValue)
			}
		}
		response, err := client.Do(req)
		if err != nil {
			return errors.New("error while checking: " + r)
		}
		if response.StatusCode != statusCode {
			return errors.New("downstream service returned unexpected status: " + strconv.Itoa(response.StatusCode))
		}
		return nil
	})
}

// TCPChecker attempts to open a TCP connection.
func TCPChecker(addr string, timeout time.Duration) health.Checker {
	return health.CheckFunc(func() error {
		conn, err := net.DialTimeout("tcp", addr, timeout)
		if err != nil {
			return errors.New("connection to " + addr + " failed")
		}
		conn.Close()
		return nil
	})
}


================================================
FILE: checks/checks_test.go
================================================
package checks

import (
	"testing"
)

func TestFileChecker(t *testing.T) {
	if err := FileChecker("/tmp").Check(); err == nil {
		t.Errorf("/tmp was expected as exists")
	}

	if err := FileChecker("NoSuchFileFromMoon").Check(); err != nil {
		t.Errorf("NoSuchFileFromMoon was expected as not exists, error:%v", err)
	}
}

func TestHTTPChecker(t *testing.T) {
	if err := HTTPChecker("https://www.google.cybertron", 200, 0, nil).Check(); err == nil {
		t.Errorf("Google on Cybertron was expected as not exists")
	}

	if err := HTTPChecker("https://www.google.pt", 200, 0, nil).Check(); err != nil {
		t.Errorf("Google at Portugal was expected as exists, error:%v", err)
	}
}


================================================
FILE: doc.go
================================================
// Package health provides a generic health checking framework.
// The health package works expvar style. By importing the package the debug
// server is getting a "/debug/health" endpoint that returns the current
// status of the application.
// If there are no errors, "/debug/health" will return a HTTP 200 status,
// together with an empty JSON reply "{}". If there are any checks
// with errors, the JSON reply will include all the failed checks, and the
// response will be have an HTTP 503 status.
//
// A Check can either be run synchronously, or asynchronously. We recommend
// that most checks are registered as an asynchronous check, so a call to the
// "/debug/health" endpoint always returns immediately. This pattern is
// particularly useful for checks that verify upstream connectivity or
// database status, since they might take a long time to return/timeout.
//
// Installing
//
// To install health, just import it in your application:
//
//  import "github.com/docker/distribution/health"
//
// You can also (optionally) import "health/api" that will add two convenience
// endpoints: "/debug/health/down" and "/debug/health/up". These endpoints add
// "manual" checks that allow the service to quickly be brought in/out of
// rotation.
//
//  import _ "github.com/docker/distribution/registry/health/api"
//
//  # curl localhost:5001/debug/health
//  {}
//  # curl -X POST localhost:5001/debug/health/down
//  # curl localhost:5001/debug/health
//  {"manual_http_status":"Manual Check"}
//
// After importing these packages to your main application, you can start
// registering checks.
//
// Registering Checks
//
// The recommended way of registering checks is using a periodic Check.
// PeriodicChecks run on a certain schedule and asynchronously update the
// status of the check. This allows CheckStatus to return without blocking
// on an expensive check.
//
// A trivial example of a check that runs every 5 seconds and shuts down our
// server if the current minute is even, could be added as follows:
//
//  func currentMinuteEvenCheck() error {
//    m := time.Now().Minute()
//    if m%2 == 0 {
//      return errors.New("Current minute is even!")
//    }
//    return nil
//  }
//
//  health.RegisterPeriodicFunc("minute_even", currentMinuteEvenCheck, time.Second*5)
//
// Alternatively, you can also make use of "RegisterPeriodicThresholdFunc" to
// implement the exact same check, but add a threshold of failures after which
// the check will be unhealthy. This is particularly useful for flaky Checks,
// ensuring some stability of the service when handling them.
//
//  health.RegisterPeriodicThresholdFunc("minute_even", currentMinuteEvenCheck, time.Second*5, 4)
//
// The lowest-level way to interact with the health package is calling
// "Register" directly. Register allows you to pass in an arbitrary string and
// something that implements "Checker" and runs your check. If your method
// returns an error with nil, it is considered a healthy check, otherwise it
// will make the health check endpoint "/debug/health" start returning a 503
// and list the specific check that failed.
//
// Assuming you wish to register a method called "currentMinuteEvenCheck()
// error" you could do that by doing:
//
//  health.Register("even_minute", health.CheckFunc(currentMinuteEvenCheck))
//
// CheckFunc is a convenience type that implements Checker.
//
// Another way of registering a check could be by using an anonymous function
// and the convenience method RegisterFunc. An example that makes the status
// endpoint always return an error:
//
//  health.RegisterFunc("my_check", func() error {
//   return Errors.new("This is an error!")
//  }))
//
// Examples
//
// You could also use the health checker mechanism to ensure your application
// only comes up if certain conditions are met, or to allow the developer to
// take the service out of rotation immediately. An example that checks
// database connectivity and immediately takes the server out of rotation on
// err:
//
//  updater = health.NewStatusUpdater()
//   health.RegisterFunc("database_check", func() error {
//    return updater.Check()
//  }))
//
//  conn, err := Connect(...) // database call here
//  if err != nil {
//    updater.Update(errors.New("Error connecting to the database: " + err.Error()))
//  }
//
// You can also use the predefined Checkers that come included with the health
// package. First, import the checks:
//
//  import "github.com/docker/distribution/health/checks
//
// After that you can make use of any of the provided checks. An example of
// using a `FileChecker` to take the application out of rotation if a certain
// file exists can be done as follows:
//
//  health.Register("fileChecker", health.PeriodicChecker(checks.FileChecker("/tmp/disable"), time.Second*5))
//
// After registering the check, it is trivial to take an application out of
// rotation from the console:
//
//  # curl localhost:5001/debug/health
//  {}
//  # touch /tmp/disable
//  # curl localhost:5001/debug/health
//  {"fileChecker":"file exists"}
//
// You could also test the connectivity to a downstream service by using a
// "HTTPChecker", but ensure that you only mark the test unhealthy if there
// are a minimum of two failures in a row:
//
//  health.Register("httpChecker", health.PeriodicThresholdChecker(checks.HTTPChecker("https://www.google.pt"), time.Second*5, 2))
package health


================================================
FILE: health.go
================================================
package health

import (
	"encoding/json"
	"fmt"
	"net/http"
	"sync"
	"time"

	"github.com/docker/distribution/context"
	"github.com/docker/distribution/registry/api/errcode"
)

// A Registry is a collection of checks. Most applications will use the global
// registry defined in DefaultRegistry. However, unit tests may need to create
// separate registries to isolate themselves from other tests.
type Registry struct {
	mu               sync.RWMutex
	registeredChecks map[string]Checker
}

// NewRegistry creates a new registry. This isn't necessary for normal use of
// the package, but may be useful for unit tests so individual tests have their
// own set of checks.
func NewRegistry() *Registry {
	return &Registry{
		registeredChecks: make(map[string]Checker),
	}
}

// DefaultRegistry is the default registry where checks are registered. It is
// the registry used by the HTTP handler.
var DefaultRegistry *Registry

// Checker is the interface for a Health Checker
type Checker interface {
	// Check returns nil if the service is okay.
	Check() error
}

// CheckFunc is a convenience type to create functions that implement
// the Checker interface
type CheckFunc func() error

// Check Implements the Checker interface to allow for any func() error method
// to be passed as a Checker
func (cf CheckFunc) Check() error {
	return cf()
}

// Updater implements a health check that is explicitly set.
type Updater interface {
	Checker

	// Update updates the current status of the health check.
	Update(status error)
}

// updater implements Checker and Updater, providing an asynchronous Update
// method.
// This allows us to have a Checker that returns the Check() call immediately
// not blocking on a potentially expensive check.
type updater struct {
	mu     sync.Mutex
	status error
}

// Check implements the Checker interface
func (u *updater) Check() error {
	u.mu.Lock()
	defer u.mu.Unlock()

	return u.status
}

// Update implements the Updater interface, allowing asynchronous access to
// the status of a Checker.
func (u *updater) Update(status error) {
	u.mu.Lock()
	defer u.mu.Unlock()

	u.status = status
}

// NewStatusUpdater returns a new updater
func NewStatusUpdater() Updater {
	return &updater{}
}

// thresholdUpdater implements Checker and Updater, providing an asynchronous Update
// method.
// This allows us to have a Checker that returns the Check() call immediately
// not blocking on a potentially expensive check.
type thresholdUpdater struct {
	mu        sync.Mutex
	status    error
	threshold int
	count     int
}

// Check implements the Checker interface
func (tu *thresholdUpdater) Check() error {
	tu.mu.Lock()
	defer tu.mu.Unlock()

	if tu.count >= tu.threshold {
		return tu.status
	}

	return nil
}

// thresholdUpdater implements the Updater interface, allowing asynchronous
// access to the status of a Checker.
func (tu *thresholdUpdater) Update(status error) {
	tu.mu.Lock()
	defer tu.mu.Unlock()

	if status == nil {
		tu.count = 0
	} else if tu.count < tu.threshold {
		tu.count++
	}

	tu.status = status
}

// NewThresholdStatusUpdater returns a new thresholdUpdater
func NewThresholdStatusUpdater(t int) Updater {
	return &thresholdUpdater{threshold: t}
}

// PeriodicChecker wraps an updater to provide a periodic checker
func PeriodicChecker(check Checker, period time.Duration) Checker {
	u := NewStatusUpdater()
	go func() {
		t := time.NewTicker(period)
		for {
			<-t.C
			u.Update(check.Check())
		}
	}()

	return u
}

// PeriodicThresholdChecker wraps an updater to provide a periodic checker that
// uses a threshold before it changes status
func PeriodicThresholdChecker(check Checker, period time.Duration, threshold int) Checker {
	tu := NewThresholdStatusUpdater(threshold)
	go func() {
		t := time.NewTicker(period)
		for {
			<-t.C
			tu.Update(check.Check())
		}
	}()

	return tu
}

// CheckStatus returns a map with all the current health check errors
func (registry *Registry) CheckStatus() map[string]string { // TODO(stevvooe) this needs a proper type
	registry.mu.RLock()
	defer registry.mu.RUnlock()
	statusKeys := make(map[string]string)
	for k, v := range registry.registeredChecks {
		err := v.Check()
		if err != nil {
			statusKeys[k] = err.Error()
		}
	}

	return statusKeys
}

// CheckStatus returns a map with all the current health check errors from the
// default registry.
func CheckStatus() map[string]string {
	return DefaultRegistry.CheckStatus()
}

// Register associates the checker with the provided name.
func (registry *Registry) Register(name string, check Checker) {
	if registry == nil {
		registry = DefaultRegistry
	}
	registry.mu.Lock()
	defer registry.mu.Unlock()
	_, ok := registry.registeredChecks[name]
	if ok {
		panic("Check already exists: " + name)
	}
	registry.registeredChecks[name] = check
}

// Register associates the checker with the provided name in the default
// registry.
func Register(name string, check Checker) {
	DefaultRegistry.Register(name, check)
}

// RegisterFunc allows the convenience of registering a checker directly from
// an arbitrary func() error.
func (registry *Registry) RegisterFunc(name string, check func() error) {
	registry.Register(name, CheckFunc(check))
}

// RegisterFunc allows the convenience of registering a checker in the default
// registry directly from an arbitrary func() error.
func RegisterFunc(name string, check func() error) {
	DefaultRegistry.RegisterFunc(name, check)
}

// RegisterPeriodicFunc allows the convenience of registering a PeriodicChecker
// from an arbitrary func() error.
func (registry *Registry) RegisterPeriodicFunc(name string, period time.Duration, check CheckFunc) {
	registry.Register(name, PeriodicChecker(CheckFunc(check), period))
}

// RegisterPeriodicFunc allows the convenience of registering a PeriodicChecker
// in the default registry from an arbitrary func() error.
func RegisterPeriodicFunc(name string, period time.Duration, check CheckFunc) {
	DefaultRegistry.RegisterPeriodicFunc(name, period, check)
}

// RegisterPeriodicThresholdFunc allows the convenience of registering a
// PeriodicChecker from an arbitrary func() error.
func (registry *Registry) RegisterPeriodicThresholdFunc(name string, period time.Duration, threshold int, check CheckFunc) {
	registry.Register(name, PeriodicThresholdChecker(CheckFunc(check), period, threshold))
}

// RegisterPeriodicThresholdFunc allows the convenience of registering a
// PeriodicChecker in the default registry from an arbitrary func() error.
func RegisterPeriodicThresholdFunc(name string, period time.Duration, threshold int, check CheckFunc) {
	DefaultRegistry.RegisterPeriodicThresholdFunc(name, period, threshold, check)
}

// StatusHandler returns a JSON blob with all the currently registered Health Checks
// and their corresponding status.
// Returns 503 if any Error status exists, 200 otherwise
func StatusHandler(w http.ResponseWriter, r *http.Request) {
	if r.Method == "GET" {
		checks := CheckStatus()
		status := http.StatusOK

		// If there is an error, return 503
		if len(checks) != 0 {
			status = http.StatusServiceUnavailable
		}

		statusResponse(w, r, status, checks)
	} else {
		http.NotFound(w, r)
	}
}

// Handler returns a handler that will return 503 response code if the health
// checks have failed. If everything is okay with the health checks, the
// handler will pass through to the provided handler. Use this handler to
// disable a web application when the health checks fail.
func Handler(handler http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		checks := CheckStatus()
		if len(checks) != 0 {
			errcode.ServeJSON(w, errcode.ErrorCodeUnavailable.
				WithDetail("health check failed: please see /debug/health"))
			return
		}

		handler.ServeHTTP(w, r) // pass through
	})
}

// statusResponse completes the request with a response describing the health
// of the service.
func statusResponse(w http.ResponseWriter, r *http.Request, status int, checks map[string]string) {
	p, err := json.Marshal(checks)
	if err != nil {
		context.GetLogger(context.Background()).Errorf("error serializing health status: %v", err)
		p, err = json.Marshal(struct {
			ServerError string `json:"server_error"`
		}{
			ServerError: "Could not parse error message",
		})
		status = http.StatusInternalServerError

		if err != nil {
			context.GetLogger(context.Background()).Errorf("error serializing health status failure message: %v", err)
			return
		}
	}

	w.Header().Set("Content-Type", "application/json; charset=utf-8")
	w.Header().Set("Content-Length", fmt.Sprint(len(p)))
	w.WriteHeader(status)
	if _, err := w.Write(p); err != nil {
		context.GetLogger(context.Background()).Errorf("error writing health status response body: %v", err)
	}
}

// Registers global /debug/health api endpoint, creates default registry
func init() {
	DefaultRegistry = NewRegistry()
	http.HandleFunc("/debug/health", StatusHandler)
}


================================================
FILE: health_test.go
================================================
package health

import (
	"errors"
	"fmt"
	"net/http"
	"net/http/httptest"
	"testing"
)

// TestReturns200IfThereAreNoChecks ensures that the result code of the health
// endpoint is 200 if there are not currently registered checks.
func TestReturns200IfThereAreNoChecks(t *testing.T) {
	recorder := httptest.NewRecorder()

	req, err := http.NewRequest("GET", "https://fakeurl.com/debug/health", nil)
	if err != nil {
		t.Errorf("Failed to create request.")
	}

	StatusHandler(recorder, req)

	if recorder.Code != 200 {
		t.Errorf("Did not get a 200.")
	}
}

// TestReturns500IfThereAreErrorChecks ensures that the result code of the
// health endpoint is 500 if there are health checks with errors
func TestReturns503IfThereAreErrorChecks(t *testing.T) {
	recorder := httptest.NewRecorder()

	req, err := http.NewRequest("GET", "https://fakeurl.com/debug/health", nil)
	if err != nil {
		t.Errorf("Failed to create request.")
	}

	// Create a manual error
	Register("some_check", CheckFunc(func() error {
		return errors.New("This Check did not succeed")
	}))

	StatusHandler(recorder, req)

	if recorder.Code != 503 {
		t.Errorf("Did not get a 503.")
	}
}

// TestHealthHandler ensures that our handler implementation correct protects
// the web application when things aren't so healthy.
func TestHealthHandler(t *testing.T) {
	// clear out existing checks.
	DefaultRegistry = NewRegistry()

	// protect an http server
	handler := http.Handler(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.WriteHeader(http.StatusNoContent)
	}))

	// wrap it in our health handler
	handler = Handler(handler)

	// use this swap check status
	updater := NewStatusUpdater()
	Register("test_check", updater)

	// now, create a test server
	server := httptest.NewServer(handler)

	checkUp := func(t *testing.T, message string) {
		resp, err := http.Get(server.URL)
		if err != nil {
			t.Fatalf("error getting success status: %v", err)
		}
		defer resp.Body.Close()

		if resp.StatusCode != http.StatusNoContent {
			t.Fatalf("unexpected response code from server when %s: %d != %d", message, resp.StatusCode, http.StatusNoContent)
		}
		// NOTE(stevvooe): we really don't care about the body -- the format is
		// not standardized or supported, yet.
	}

	checkDown := func(t *testing.T, message string) {
		resp, err := http.Get(server.URL)
		if err != nil {
			t.Fatalf("error getting down status: %v", err)
		}
		defer resp.Body.Close()

		if resp.StatusCode != http.StatusServiceUnavailable {
			t.Fatalf("unexpected response code from server when %s: %d != %d", message, resp.StatusCode, http.StatusServiceUnavailable)
		}
	}

	// server should be up
	checkUp(t, "initial health check")

	// now, we fail the health check
	updater.Update(fmt.Errorf("the server is now out of commission"))
	checkDown(t, "server should be down") // should be down

	// bring server back up
	updater.Update(nil)
	checkUp(t, "when server is back up") // now we should be back up.
}