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// Copyright 2019 and All rights reserved.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to writing, software distributed
// under the License is distributed on a "AS IS" BASIS, WITHOUT WARRANTIES OR
// CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package supervisor
import (
type processFailure func()
// AlwaysRestart adjusts the supervisor to never halt in face of failures.
const AlwaysRestart = -1
type serviceType int
const (
permanent serviceType = iota
// ServiceSpecification defines how a service is executed by the supervisor.
type ServiceSpecification struct {
svc Service
svctype serviceType
// ServiceOption modifies the service specifications.
type ServiceOption func(*ServiceSpecification)
// Permanent services are always restarted
func Permanent(s *ServiceSpecification) {
s.svctype = permanent
// Transient services are restarted only when panic.
func Transient(s *ServiceSpecification) {
s.svctype = transient
// Temporary services are never restarted.
func Temporary(s *ServiceSpecification) {
s.svctype = temporary
// Service is the public interface expected by a Supervisor.
// This will be internally named after the result of fmt.Stringer, if available.
// Otherwise it is going to use an internal representation for the service
// name.
type Service interface {
// Serve is called by a Supervisor to start the service. It expects the
// service to honor the passed context and its lifetime. Observe
// <-ctx.Done() and ctx.Err(). If the service is stopped by anything
// but the Supervisor, it will get started again. Be careful with shared
// state among restarts.
Serve(ctx context.Context)
// Supervisor is the basic datastructure responsible for offering a supervisor
// tree. It implements Service, therefore it can be nested if necessary. When
// passing the Supervisor around, remind to do it as reference (&supervisor).
// Once the supervisor is started, its attributes are frozen.
type Supervisor struct {
// Name for this supervisor tree, used for logging.
Name string
name string
// MaxRestarts is the number of maximum restarts given MaxTime. If more
// than MaxRestarts occur in the last MaxTime, then the supervisor
// stops all services and halts. Set this to AlwaysRestart to prevent
// supervisor halt.
MaxRestarts int
maxrestarts int
// MaxTime is the time period on which the internal restart count will
// be reset.
MaxTime time.Duration
maxtime time.Duration
// Log is a replaceable function used for overall logging.
// Default: log.Printf.
Log func(interface{})
log func(interface{})
// indicates that supervisor is ready for use.
prepared sync.Once
// signals that a new service has just been added, so the started
// supervisor picks it up.
added chan struct{}
// indicates that supervisor has running services.
running sync.Mutex
runningServices sync.WaitGroup
mu sync.Mutex
svcorder []string // order in which services must be started
services map[string]ServiceSpecification // added services
cancelations map[string]context.CancelFunc // each service cancelation
terminations map[string]context.CancelFunc // each service termination call
lastRestart time.Time
restarts int
func (s *Supervisor) prepare() {
func (s *Supervisor) reset() {
if s.Name == "" {
s.Name = "supervisor"
if s.MaxRestarts == 0 {
s.MaxRestarts = 5
if s.MaxTime == 0 {
s.MaxTime = 15 * time.Second
if s.Log == nil {
s.Log = func(msg interface{}) {
log.Printf("%s: %v", s.Name, msg)
} = s.Name
s.maxrestarts = s.MaxRestarts
s.maxtime = s.MaxTime
s.log = s.Log
s.added = make(chan struct{})
s.cancelations = make(map[string]context.CancelFunc) = make(map[string]ServiceSpecification)
s.terminations = make(map[string]context.CancelFunc)
func (s *Supervisor) shouldRestart() bool {
if s.maxrestarts == AlwaysRestart {
return true
if time.Since(s.lastRestart) > s.maxtime {
s.restarts = 0
s.lastRestart = time.Now()
return s.restarts < s.maxrestarts
// Cancelations return a list of services names and their cancelation calls.
// These calls be used to force a service restart.
func (s *Supervisor) Cancelations() map[string]context.CancelFunc {
svclist := make(map[string]context.CancelFunc)
for k, v := range s.cancelations {
svclist[k] = v
return svclist
// Add inserts into the Supervisor tree a new permanent service. If the
// Supervisor is already started, it will start it automatically.
func (s *Supervisor) Add(service Service, opts ...ServiceOption) {
s.addService(service, opts...)
// AddFunc inserts into the Supervisor tree a new permanent anonymous service.
// If the Supervisor is already started, it will start it automatically.
func (s *Supervisor) AddFunc(f func(context.Context), opts ...ServiceOption) string {
svc := &funcsvc{
id: funcSvcID(),
f: f,
s.addService(svc, opts...)
return svc.String()
func (s *Supervisor) addService(svc Service, opts ...ServiceOption) {
name := fmt.Sprintf("%s", svc)
newsvc := ServiceSpecification{
svc: svc,
for _, opt := range opts {
}[name] = newsvc
s.svcorder = append(s.svcorder, name)
go func() {
s.added <- struct{}{}
// Remove stops the service in the Supervisor tree and remove from it.
func (s *Supervisor) Remove(name string) {
if _, ok :=[name]; !ok {
delete(, name)
for i, n := range s.svcorder {
if name == n {
s.svcorder = append(s.svcorder[:i], s.svcorder[i+1:]...)
if c, ok := s.terminations[name]; ok {
delete(s.terminations, name)
if _, ok := s.cancelations[name]; ok {
delete(s.cancelations, name)
// Serve starts the Supervisor tree. It can be started only once at a time. If
// stopped (canceled), it can be restarted. In case of concurrent calls, it will
// hang until the current call is completed.
func (s *Supervisor) Serve(ctx context.Context) {
restartCtx, cancel := context.WithCancel(ctx)
processFailure := func() {
restart := s.shouldRestart()
if !restart {
serve(s, restartCtx, processFailure)
// Services return a list of services
func (s *Supervisor) Services() map[string]Service {
svclist := make(map[string]Service)
for k, v := range {
svclist[k] = v.svc
return svclist
func (s *Supervisor) String() string {
func (s *Supervisor) logf(format string, a ...interface{}) {
s.log(fmt.Sprintf(format, a...))