Docker Swarm代码分析笔记（6）——swarm driver的NewCluster函数

Swarm driver的NewCluster函数的核心功能如下（cluster/swarm/cluster.go）：

discoveryCh, errCh := cluster.discovery.Watch(nil)
go cluster.monitorDiscovery(discoveryCh, errCh)
go cluster.monitorPendingEngines()

cluster.discovery.Watch的定义如下：

Watch(stopCh <-chan struct{}) (<-chan Entries, <-chan error)

返回两个channel：第一个channel类型是Entries（type Entries []*Entry），用来传输cluster所包含的主机信息，第二个channel用来通知是否有错误发生。

Cluster.monitorDiscovery功能是整理Cluster.engines：

select {
    case entries := <-ch:
        added, removed := currentEntries.Diff(entries)
        currentEntries = entries

        // Remove engines first. `addEngine` will refuse to add an engine
        // if there's already an engine with the same ID.  If an engine
        // changes address, we have to first remove it then add it back.
        for _, entry := range removed {
            c.removeEngine(entry.String())
        }

        for _, entry := range added {
            c.addEngine(entry.String())
        }
    case err := <-errCh:
        log.Errorf("Discovery error: %v", err)
    }

而Cluster.monitorPendingEngines则是验证处于pending状态的Cluster.engines现在是否能够连接上：

for _, e := range pEngines {
        if e.TimeToValidate() {
            go c.validatePendingEngine(e)
        }
    }

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