A simple and flexible health check library for Go.

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Table of Contents

1. Getting started
2. Synchronous vs. Asynchronous Checks
3. Caching
4. Listening to Status Changes
5. Middleware and Interceptors
6. Compatibility With Other Libraries
7. License

Getting Started

This library provides a http.Handler that acts as a health endpoint. It can be used by cloud infrastructure or other services to determine the availability of an application.

Rather than simply returning a response with HTTP status code 200, this library allows building health checks that test the availability of all required dependencies. The HTTP response contains the aggregated health result and details about the health status of each component.

Example

package main

import (
	"context"
	"database/sql"
	"fmt"
	"github.com/alexliesenfeld/health"
	_ "github.com/mattn/go-sqlite3"
	"log"
	"net/http"
	"time"
)

// This is a very simple example that shows the basic features of this library.
func main() {
	db, _ := sql.Open("sqlite3", "simple.sqlite")
	defer db.Close()

	// Create a new Checker.
	checker := health.NewChecker(

		// Set the time-to-live for our cache to 1 second (default).
		health.WithCacheDuration(1*time.Second),

		// Configure a global timeout that will be applied to all checks.
		health.WithTimeout(10*time.Second),

		// A check configuration to see if our database connection is up.
		// The check function will be executed for each HTTP request.
		health.WithCheck(health.Check{
			Name:    "database",      // A unique check name.
			Timeout: 2 * time.Second, // A check specific timeout.
			Check:   db.PingContext,
		}),

		// The following check will be executed periodically every 15 seconds
		// started with an initial delay of 3 seconds. The check function will NOT
		// be executed for each HTTP request.
		health.WithPeriodicCheck(15*time.Second, 3*time.Second, health.Check{
			Name: "search",
			// The check function checks the health of a component. If an error is
			// returned, the component is considered unavailable (or "down").
			// The context contains a deadline according to the configured timeouts.
			Check: func(ctx context.Context) error {
				return fmt.Errorf("this makes the check fail")
			},
		}),

		// Set a status listener that will be invoked when the health status changes.
		// More powerful hooks are also available (see docs).
		health.WithStatusListener(func(ctx context.Context, state health.CheckerState) {
			log.Println(fmt.Sprintf("health status changed to %s", state.Status))
		}),
	)

	// Create a new health check http.Handler that returns the health status
	// serialized as a JSON string. You can pass pass further configuration
	// options to NewHandler to modify default configuration.
	http.Handle("/health", health.NewHandler(checker))
	log.Fatalln(http.ListenAndServe(":3000", nil))
}

Because our search component is down, the request curl http://localhost:3000/health would yield a response with HTTP status code 503 (Service Unavailable), and the following JSON response body:

{
  "status": "down",
  "details": {
    "database": {
      "status": "up",
      "timestamp": "2021-07-01T08:05:14.603364Z"
    },
    "search": {
      "status": "down",
      "timestamp": "2021-07-01T08:05:08.522685Z",
      "error": "this makes the check fail"
    }
  }
}

This example shows all features of this library.

Synchronous vs. Asynchronous Checks

With "synchronous" health checks we mean that every HTTP request initiates a health check and waits until all check functions complete before returning an aggregated health result. You can configure synchronous checks using the WithCheck configuration option (see example above).

Synchronous checks can be sufficient for smaller applications but might not scale well for more involved applications. Sometimes an application needs to read a large amount of data, can experience latency issues or make an expensive calculation to tell something about its health. With synchronous health checks the application will not be able to respond quickly to health check requests (see here why this is necessary to avoid service disruptions in modern cloud infrastructure).

Rather than executing health check functions on every HTTP request, periodic (or "asynchronous") health checks execute the check function on a fixed schedule. With this approach, the health status is always read from a local cache that is regularly updated in the background. This allows responding to HTTP requests instantly without waiting for check functions to complete.

Periodic checks can be configured using the WithPeriodicCheck configuration option (see example above).

This library allows you to mix synchronous and asynchronous check functions, so you can start out simple and easily transition into a more scalable and robust health check implementation later.

Caching

Health check results are cached to avoid sending too many request to the services that your program checks and to mitigate "denial of service" attacks. The TTL is set to 1 second by default. If you do not want to use caching altogether, you can disable it using the health.WithDisabledCache() configuration option.

Listening to Status Changes

It can be useful to react to health status changes. For example, you might want to log status changes or adjust some metrics, so you can easier correlate logs during root cause analysis or perform actions to mitigate the impact of an unhealthy component.

This library allows you to configure listener functions that will be called when either the overall/aggregated health status changes, or that of a specific component.

Example

health.WithPeriodicCheck(5*time.Second, 0, health.Check{
    Name:   "search",
    Check:  myCheckFunc,
    StatusListener: func (ctx context.Context, name string, state CheckState) ) {
	    log.Printf("status of component '%s' changed to %s", name, state.Status)
    },
}),

health.WithStatusListener(func (ctx context.Context, state CheckerState)) {
    log.Printf("overall system health status changed to %s", state.Status)
}),

Middleware and Interceptors

It can be useful to hook into the checking lifecycle to do some processing before and after a health check. For example, you might want to add some tracing information to the Context before the check function executes, do some logging or modify the check result before sending the HTTP response (e.g., removing details on failed authentication).

This library provides two mechanisms that allow you to hook into processing:

• Middleware gives you the possibility to intercept all calls of Checker.Check, which corresponds to every incoming HTTP request. In contrary to the usually used middleware pattern, this middleware allows you to access check related information and post-process a check result before sending it in an HTTP response.

  Middleware	Description
  BasicAuth	Reduces exposed health details based on authentication success. Uses basic auth for authentication.
  CustomAuth	Same as BasicAuth middleware, but allows using an arbitrary function for authentication.
  FullDetailsOnQueryParam	Disables health details unless the request contains a previously configured query parameter name.
  BasicLogger	Basic request-oriented logging functionality.

• Interceptors make it possible to intercept all calls to a check function. This is useful if you have cross-functional code that needs to be reusable and should have access to check state information.

  Interceptor	Description
  BasicLogger	Basic component check function logging functionality

Compatibility With Other Libraries

Most existing Go health check libraries come with their own implementations of tool specific check functions (such as for Redis, memcached, Postgres, etc.). Rather than reinventing the wheel and come up with yet another library specific implementation of check functions, the goal was to design this library in a way that makes it easy to reuse existing solutions. The following (non-exhaustive) list of health check implementations should work with this library without or minimal adjustments:

• github.com/hellofresh/health-go ( see full example here)

  import httpCheck "github.com/hellofresh/health-go/v4/checks/http"
  ...
  health.WithCheck(health.Check{
     Name:    "google",
     Check:   httpCheck.New(httpCheck.Config{
        URL: "https://www.google.com",
     }),
  }),

• github.com/etherlabsio/healthcheck ( see full example here)

  import "github.com/etherlabsio/healthcheck/v2/checkers"
  ...
  health.WithCheck(health.Check{
    Name:    "database",
    Check:   checkers.DiskSpace("/var/log", 90).Check,
  })

• github.com/heptiolabs/healthcheck ( see full example here)

  import "github.com/heptiolabs/healthcheck"
  ...
  health.WithCheck(health.Check{
      Name: "google",
      Check: func(ctx context.Context) error {
         deadline, _ := ctx.Deadline()
         timeout := time.Now().Sub(deadline)
         return healthcheck.HTTPGetCheck("https://www.google.com", timeout)()
      },
  }),

• github.com/InVisionApp/go-health ( see full example here)

    import "github.com/InVisionApp/go-health/checkers"
    ...
    // Create check as usual (no error checking for brevity)
    googleURL, err := url.Parse("https://www.google.com")
    check, err := checkers.NewHTTP(&checkers.HTTPConfig{
        URL: googleURL,
    })
    ...
    // Add the check in the Checker configuration.
    health.WithCheck(health.Check{
        Name: "google",
        Check: func(_ context.Context) error {
            _, err := check.Status() 
            return err
        },
    })

License

health is free software: you can redistribute it and/or modify it under the terms of the MIT Public License.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the MIT Public License for more details.