health

README

Health

A simple and flexible health check library for Go.

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Table of Contents

1. Features
2. Getting Started
3. Caching
4. Periodic Checks
5. Failure Tolerance
6. Listening to Status Changes
7. Listening to Lifecycle Events
8. License

Features

This library allows you to build health checks that do not simply return HTTP status code 200 but actually check if all necessary components are healthy.

This library provides the following features:

• Request based and fixed-schedule health checks.
• Global and check-based timeout management.
• Caching
• Lifecycle hooks and status change listeners.
• Failure tolerance based on fail count and/or time thresholds.
• Provides an http.Handler and http.HandlerFunc that are fully compatible with net/http.

Getting Started

package main

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

func main() {
	db, _ := sql.Open("sqlite3", "simple.sqlite")
	defer db.Close()

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

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

		// A simple check to see if database connection is up.
		health.WithCheck(health.Check{
			Name:    "database",
			Timeout: 2 * time.Second, // A a check specific timeout.
			Check:   db.PingContext,
		}),

		// The following check will be executed periodically every 30 seconds.
		health.WithPeriodicCheck(30*time.Second, health.Check{
			Name: "search",
			Check: func(ctx context.Context) error {
				return fmt.Errorf("this makes the check fail")
			},
		}),
	)

	// We Create a new http.Handler that provides health check information
	// serialized as a JSON string via HTTP.
	http.Handle("/health", health.NewHandler(checker))
	http.ListenAndServe(":3000", nil)
}

Because our search component is down, the request curl -u username:password 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"
    }
  }
}

Caching

Health check responses 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.

Periodic Checks

When executing health check functions synchronously (i.e. for every HTTP request), the overall response delay will be at least as high as the one of your slowest check function. This is usually OK for smaller applications with a low number of quickly checkable dependencies and enabled caching. This approach, however, will likely be problematic for more involved applications that either have many dependencies and/or some relatively slow check functions.

Rather than executing a health check function on every request that is received over the health endpoint, periodic checks execute the check function on a fixed schedule. With this approach, the health status is always read from a local cache. It allows responding to HTTP requests instantly without waiting for the check function to complete.

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

Failure Tolerance

This library lets you configure failure tolerant checks that allow some degree of failure. The check is only considered failed, when predefined tolerance thresholds are crossed.

Example

Let's assume that your app provides a REST API but also consumes messages from a Kafka topic. If the connection to Kafka is down, your app can still serve API requests, but it will not process any messages during this time. If the Kafka health check is configured without any failure tolerance, your whole application will become unhealthy. This is most likely not what you want. However, if Kafka is down for too long, there may indeed be a problem that requires attention. In this case, you still may want to flag your app unhealthy by returning a failing health check, so that it can be automatically restarted by your infrastructure.

Failure tolerant health checks let you configure this kind of behaviour.

health.WithCheck(health.Check{
    Name:    "unreliable-service",
    // Check is allowed to fail up to 4 times until considered unavailable
    MaxConsecutiveFails: 4,
    // Check is allowed to be in an erroneous state for up to 1 minute until considered unavailable.
    MaxTimeInError:      1 * time.Minute,
    Check: myCheckFunc,
}),

Listening to Status Changes

It can be useful to react to health status changes. For example, you might want to log status changes, 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 either when the overall system health status, or a component status changes.

Example

The example below shows a configuration that adds the following two listeners:

• a status listener to a check that will be called whenever the status of the check changes (e.g., from "up" to "down"),
• an overall system status listener, that will be called whenever the overall system status changes.

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

health.WithStatusListener(func (status health.AvailabilityStatus, state map[string]health.CheckState) {
    log.Printf("overall system health status changed to %s", status)
}),

Listening to Lifecycle Events

It can be useful to hook into the checking lifecycle to do some pre- and postprocessing. For example, you might want to add tracing (such as Jaeger traces and spans), or some logging functionality that requires you to perform some actions before and after a check function is executed.

This library allows you to add listeners for both,

• a BeforeCheckListener and AfterCheckListener for each individual component, and
• a BeforeSystemCheckListener and AfterSystemCheckListener that are triggered before/after a full system check is executed.

Please refer to the documentation for more information.

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.

Documentation

Index

• func NewHandler(checker Checker) http.Handler
• func NewHandlerFunc(checker Checker) http.HandlerFunc
• func NewHandlerFuncWithConfig(checker Checker, cfg HandlerConfig) http.HandlerFunc
• func NewHandlerWithConfig(checker Checker, cfg HandlerConfig) http.Handler
• func WithAfterCheckListener(listener AfterCheckListener) option
• func WithBeforeCheckListener(listener BeforeCheckListener) option
• func WithCacheDuration(duration time.Duration) option
• func WithCheck(check Check) option
• func WithDisabledCache() option
• func WithDisabledDetails() option
• func WithMaxErrorMessageLength(length uint) option
• func WithPeriodicCheck(refreshPeriod time.Duration, check Check) option
• func WithStatusListener(listener StatusListener) option
• func WithTimeout(timeout time.Duration) option
• type AfterCheckListener
• type AfterComponentCheckListener
• type AvailabilityStatus
• type BeforeCheckListener
• type BeforeComponentCheckListener
• type Check
• type CheckState
• type CheckStatus
• type Checker
  • func NewChecker(options ...option) Checker
• type ComponentStatusListener
• type HandlerConfig
• type StatusListener
• type SystemStatus

Functions

func NewHandler

func NewHandler(checker Checker) http.Handler

NewHandler creates a new health check http.Handler. The Checker will be started automatically (see Checker.Start).

func NewHandlerFunc

func NewHandlerFunc(checker Checker) http.HandlerFunc

NewHandlerFunc creates a new health check http.Handler. The Checker will be started automatically (see Checker.Start).

func NewHandlerFuncWithConfig

func NewHandlerFuncWithConfig(checker Checker, cfg HandlerConfig) http.HandlerFunc

NewHandlerFuncWithConfig creates a new health check http.Handler. If HandlerConfig.DisableCheckerAutostart is not true, the Checker will be started automatically (see Checker.Start).

func NewHandlerWithConfig

func NewHandlerWithConfig(checker Checker, cfg HandlerConfig) http.Handler

NewHandlerWithConfig creates a new health check http.Handler. If HandlerConfig.DisableCheckerAutostart is not true, the Checker will be started automatically (see Checker.Start).

func WithAfterCheckListener

func WithAfterCheckListener(listener AfterCheckListener) option

WithAfterCheckListener registers a handler function that will be called whenever the overall system health AvailabilityStatus changes.

func WithBeforeCheckListener

func WithBeforeCheckListener(listener BeforeCheckListener) option

WithBeforeCheckListener registers a handler function that will be called whenever the overall system health AvailabilityStatus changes.

func WithCacheDuration

func WithCacheDuration(duration time.Duration) option

WithCacheDuration sets the duration for how long the aggregated health check result will be cached. By default, the cache TTL (i.e, the duration for how long responses will be cached) is set to 1 second. Caching will prevent that each incoming HTTP request triggers a new health check. A duration of 0 will effectively disable the cache and has the same effect as WithDisabledCache.

func WithCheck

func WithCheck(check Check) option

WithCheck adds a new health check that contributes to the overall service availability status. This check will be triggered each time Checker.Check is called (i.e., for each HTTP request). If health checks are expensive or you expect a bigger amount of requests on your the health endpoint, consider using WithPeriodicCheck instead.

func WithDisabledCache

func WithDisabledCache() option

WithDisabledCache disabled the check cache. This is not recommended in most cases. This will effectively lead to a health endpoint that initiates a new health check for each incoming HTTP request. This may have an impact on the systems that are being checked (especially if health checks are expensive). Caching also mitigates "denial of service" attacks. Caching is enabled by default.

func WithDisabledDetails

func WithDisabledDetails() option

WithDisabledDetails disables all data in the JSON response body. The AvailabilityStatus will be the only content. Example: { "status":"down" }. Enabled by default.

func WithMaxErrorMessageLength

func WithMaxErrorMessageLength(length uint) option

WithMaxErrorMessageLength limits maximum number of characters in error messages. Default is 500.

func WithPeriodicCheck

func WithPeriodicCheck(refreshPeriod time.Duration, check Check) option

WithPeriodicCheck adds a new health check that contributes to the overall service availability status. The health check will be performed on a fixed schedule and will not be executed for each HTTP request (as in contrast to WithCheck). This allows to process a much higher number of HTTP requests without actually calling the checked services too often or to execute long running checks. This way Checker.Check (and the health endpoint) always returns the last result of the periodic check.

func WithStatusListener

func WithStatusListener(listener StatusListener) option

WithStatusListener registers a handler function that will be called whenever the overall system health AvailabilityStatus changes.

func WithTimeout

func WithTimeout(timeout time.Duration) option

WithTimeout defines a timeout duration for all checks. You can override this timeout by using the timeout value in the Check configuration. Default value is 30 seconds.

Types

type AfterCheckListener

type AfterCheckListener func(ctx context.Context, state map[string]CheckState) context.Context

AfterCheckListener is a callback function that will be called right after a the availability status of the system was checked. The listener is allowed to add or remove values to/from the context in parameter ctx. The new context is expected in the return value of the function. If you do not want to extend the context, just return the passed ctx parameter. Attention: This listener will only be invoked when the Checker.Check function is executed (i.e., for every HTTP request). It will not be invoked before a periodic check function is executed!

type AfterComponentCheckListener

type AfterComponentCheckListener func(ctx context.Context, state CheckState) context.Context

AfterComponentCheckListener is a callback function that will be called right after a components availability status will be checked. The listener is allowed to add or remove values to/from the context in parameter ctx. The new context is expected in the return value of the function. If you do not want to extend the context, just return the passed ctx parameter.

type AvailabilityStatus

type AvailabilityStatus string

AvailabilityStatus expresses the availability of either a component or the whole system.

const (
	// StatusUnknown holds the information that the availability
	// status is not known, because not all checks were executed yet.
	StatusUnknown AvailabilityStatus = "unknown"
	// StatusUp holds the information that the system or a component
	// is up and running.
	StatusUp AvailabilityStatus = "up"
	// StatusDown holds the information that the system or a component
	// down and not available.
	StatusDown AvailabilityStatus = "down"
)

type BeforeCheckListener

type BeforeCheckListener func(ctx context.Context, state map[string]CheckState) context.Context

BeforeCheckListener is a callback function that will be called right before a the availability status of the system will be checked. The listener is allowed to add/remove values to the context in parameter ctx. The new context is expected in the return value of the function. If you do not want to extend the context, just return the passed ctx parameter. Attention: This listener will only be invoked when the Checker.Check function is executed (i.e., for every HTTP request). It will not be invoked before a periodic check function is executed!

type BeforeComponentCheckListener

type BeforeComponentCheckListener func(ctx context.Context, state CheckState) context.Context

BeforeComponentCheckListener is a callback function that will be called right before a components availability status will be checked. The listener is allowed to add/remove values to the context in parameter ctx. The new context is expected in the return value of the function. If you do not want to extend the context, just return the passed ctx parameter.

type Check

type Check struct {
	// The Name must be unique among all checks. Name is a required attribute.
	Name string // Required
	// Check is the check function that will be executed to check availability.
	// This function must return an error if the checked service is considered
	// not available. Check is a required attribute.
	Check func(ctx context.Context) error // Required
	// Timeout will override the global timeout value, if it is smaller than
	// the global timeout (see WithTimeout).
	Timeout time.Duration // Optional
	// MaxTimeInError will set a duration for how long a service must be
	// in an error state until it is considered down/unavailable.
	MaxTimeInError time.Duration // Optional
	// MaxConsecutiveFails will set a maximum number of consecutive
	// check fails until the service is considered down/unavailable.
	MaxConsecutiveFails uint // Optional
	// StatusListener allows to set a listener that will be called
	// whenever the AvailabilityStatus of the check changes.
	StatusListener ComponentStatusListener // Optional
	// BeforeCheckListener is a callback function that will be called
	// right before a components availability status will be checked.
	BeforeCheckListener BeforeComponentCheckListener // Optional
	// AfterCheckListener is a callback function that will be called
	// right after a components availability status was checked.
	AfterCheckListener AfterComponentCheckListener // Optional
	// contains filtered or unexported fields
}

Check allows to configure health checks.

type CheckState

type CheckState struct {
	// LastCheckedAt holds the time of when the check was last executed.
	LastCheckedAt *time.Time
	// LastCheckedAt holds the last time of when the check did not return an error.
	LastSuccessAt *time.Time
	// LastFailureAt holds the last time of when the check did return an error.
	LastFailureAt *time.Time
	// FirstCheckStartedAt holds the time of when the first check was started.
	FirstCheckStartedAt time.Time
	// LastResult holds the error of the last check (nil if successful).
	LastResult error
	// ConsecutiveFails holds the number of how often the check failed in a row.
	ConsecutiveFails uint
	// The current availability status of the check.
	Status AvailabilityStatus
}

CheckState represents the current state of a component check.

type CheckStatus

type CheckStatus struct {
	// Status is the availability status of a component.
	Status AvailabilityStatus `json:"status"`
	// Timestamp holds the time when the check was executed.
	Timestamp *time.Time `json:"timestamp,omitempty"`
	// Error contains the check error message, if the check failed.
	Error *string `json:"error,omitempty"`
}

CheckStatus holds a components health information.

type Checker

type Checker interface {
	// Start will start all necessary background workers and prepare
	// the checker for further usage.
	Start()
	// Stop stops will stop the checker.
	Stop()
	// Check performs a health check. It expects a context, that
	// may contain deadlines to which will be adhered to. The context
	// will be passed to downstream calls.
	Check(ctx context.Context) SystemStatus
	// GetRunningPeriodicCheckCount returns the number of currently
	// running periodic checks.
	GetRunningPeriodicCheckCount() int
}

Checker is the main checker interface. It provides all health checking logic.

func NewChecker

func NewChecker(options ...option) Checker

NewChecker creates a new Checker. The provided options will be used to modify its configuration.

type ComponentStatusListener

type ComponentStatusListener func(ctx context.Context, state CheckState)

ComponentStatusListener is a callback function that will be called when a components availability status changes (e.g. from "up" to "down").

type HandlerConfig

type HandlerConfig struct {
	StatusCodeUp            int
	StatusCodeDown          int
	DisableCheckerAutostart bool
}

type StatusListener

type StatusListener func(ctx context.Context, status AvailabilityStatus, state map[string]CheckState)

StatusListener is a callback function that will be called when the system availability status changes (e.g. from "up" to "down").

type SystemStatus

type SystemStatus struct {
	// Status is the aggregated system availability status.
	Status AvailabilityStatus `json:"status"`
	// Details contains health information for all checked components.
	Details *map[string]CheckStatus `json:"details,omitempty"`
}

SystemStatus holds the aggregated system availability status and detailed information about the individual checks.