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Etcd key：channel / [nodeID] / [channelName]，value: ChannelInfo
ChanelInfo contains State,StartTime, SeekPosition. State is a enum whose values are Unwatched, Watched. This means whether datanode watch it successfully.
StartTime is the watch event start time.
If there is a new channel registration, datacoord updates channel / [nodeid] / [channelname]
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- When the datacoord is started, the channels of offline datanodes are assigned to current online nodes.
- When DataNode comes online, DataCoord may move some channels to the node and change the channels of different nodes through etcd transactions operation.
- When DataNode goes offline, DataCoord reassigns the channels to other nodes, changing them through the etcd transaction.
- Specially, if the last DataNode goes offline and there is no living DataNode at this time, record the channel in channel/remaining/[channelName].
- Start a background goroutine to check states of channels. If a channel's state has't changed to Watched for a long time, maybe we should reallocated it to another node atomically.
DataNode:
- When DataNode starts, the channel of this Node on etcd must be empty, because the nodeID is incremented.
- When DataNode receives an Add event, execute WatchChannel, and transactionlly change state of channel on etcd to Watched.
- When DataNode receives Delete event, execute ReleaseChannel.
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