在分析prometheus中的ruler代码时，看到一个很经典的Manager模式，用于周期性更新规则信息，代码简单实用，特地总结一下：

Manager模式

首先，定义一个manager，其中需要包含维护状态的字段，比如记录上次的配置，同时也需要互斥锁，用于并发访问；

然后，NewManager生成一个新的manager；

最后，调用mangaer.start manager.run manager.stop维护生命周期。

 // NewManager returns an implementation of Manager, ready to be started
// by calling the Run method.
func NewManager(o *ManagerOptions) *Manager {
	if o.Metrics == nil {
		o.Metrics = NewGroupMetrics(o.Registerer)
	}

	if o.GroupLoader == nil {
		o.GroupLoader = FileLoader{}
	}

	m := &Manager{
		groups: map[string]*Group{},
		opts:   o,
		block:  make(chan struct{}),
		done:   make(chan struct{}),
		logger: o.Logger,
	}

	return m
}

// Run starts processing of the rule manager. It is blocking.
func (m *Manager) Run() {
	m.start()
	<-m.done
}

func (m *Manager) start() {
	close(m.block)
}

// Stop the rule manager's rule evaluation cycles.
func (m *Manager) Stop() {
	m.mtx.Lock()
	defer m.mtx.Unlock()

	level.Info(m.logger).Log("msg", "Stopping rule manager...")

	for _, eg := range m.groups {
		eg.stop()
	}

	// Shut down the groups waiting multiple evaluation intervals to write
	// staleness markers.
	close(m.done)

	level.Info(m.logger).Log("msg", "Rule manager stopped")
}  

Load and Update

首先，加载所有新的配置，并为每个配置生成hash key，存入map中；

然后，与Manager维护的状态，进行比较

• 相同则跳过
• 没有或者不相同则更新
• 最后删除旧的数据

实现要点：

• 用sync.WaitGroup保证goroutine并发运行结束
• 用<-m.block信号作为同步运行信号
• 用delete(key)删除map中已经处理过的数据，最后直接删除map中剩余的多余数据

代码如下：

 // Update the rule manager's state as the config requires. If
// loading the new rules failed the old rule set is restored.
func (m *Manager) Update(interval time.Duration, files []string, externalLabels labels.Labels, externalURL string, ruleGroupPostProcessFunc RuleGroupPostProcessFunc) error {
	m.mtx.Lock()
	defer m.mtx.Unlock()

	groups, errs := m.LoadGroups(interval, externalLabels, externalURL, ruleGroupPostProcessFunc, files...)

	if errs != nil {
		for _, e := range errs {
			level.Error(m.logger).Log("msg", "loading groups failed", "err", e)
		}
		return errors.New("error loading rules, previous rule set restored")
	}
	m.restored = true

	var wg sync.WaitGroup
	for _, newg := range groups {
		// If there is an old group with the same identifier,
		// check if new group equals with the old group, if yes then skip it.
		// If not equals, stop it and wait for it to finish the current iteration.
		// Then copy it into the new group.
		gn := GroupKey(newg.file, newg.name)
		oldg, ok := m.groups[gn]
		delete(m.groups, gn)

		if ok && oldg.Equals(newg) {
			groups[gn] = oldg
			continue
		}

		wg.Add(1)
		go func(newg *Group) {
			if ok {
				oldg.stop()
				newg.CopyState(oldg)
			}
			wg.Done()
			// Wait with starting evaluation until the rule manager
			// is told to run. This is necessary to avoid running
			// queries against a bootstrapping storage.
			<-m.block
			newg.run(m.opts.Context)
		}(newg)
	}

	// Stop remaining old groups.
	wg.Add(len(m.groups))
	for n, oldg := range m.groups {
		go func(n string, g *Group) {
			g.markStale = true
			g.stop()
			if m := g.metrics; m != nil {
				m.IterationsMissed.DeleteLabelValues(n)
				m.IterationsScheduled.DeleteLabelValues(n)
				m.EvalTotal.DeleteLabelValues(n)
				m.EvalFailures.DeleteLabelValues(n)
				m.GroupInterval.DeleteLabelValues(n)
				m.GroupLastEvalTime.DeleteLabelValues(n)
				m.GroupLastDuration.DeleteLabelValues(n)
				m.GroupRules.DeleteLabelValues(n)
				m.GroupSamples.DeleteLabelValues((n))
			}
			wg.Done()
		}(n, oldg)
	}

	wg.Wait()
	m.groups = groups

	return nil
}