add datanode design doc (#5206)

* add datanode design doc
Signed-off-by: yangxuan <xuan.yang@zilliz.com>

docs/design_docs/datanode_recovery_design_0513_2021.md

+# DataNode Recovery Design
+
+## Objectives
+
+DataNode is stateless. It does whatever DataService tells, so recovery is not a difficult thing for datanode.
+Once datanode subscribes certain vchannels, it starts working till crash. So the key to recovery is consuming
+vchannels at the right position. What's processed no longer need to be processed again, what's not processed is
+the key.
+
+What's the line between processed or not for DataNode? Wether the data is flushed into persistent storage, which's
+ the only job of DataNode. So recovering a DataNode needs the last positions of flushed data in every vchannels.
+Luckily, this information will be told by DataService, DataNode only worries about updating positions after flushing.
+
+There's more to fully recover a DataNode. DataNode replicates collection schema in memory to decode and encode
+data. Once it recovers to an older position of insert channels, it needs the collection schema snapshots from
+that exactly position. Luckily again, the snapshots will be provided via MasterService.
+
+So DataNode needs to achieve the following 3 objectives.
+
+### 1. Service Registration
+
+DataNode registers itself to Etcd after grpc server started, in *INITIALIZING* state.
+
+### 2. Service discovery
+
+DataNode discovers DataService and MasterService, in *HEALTHY* state.
+
+### 3. Recovery state
+
+After stage 1&2, DataNode is healthy but IDLE. DataNode starts to work until the following happens.
+
+- DataService info the vchannels and positions.
+
+- DataNode replicates the snapshots of collection schema at the positions to which these vchannel belongs.
+
+- DataNode initializes flowgraphs and subscribes to these vchannels
+
+There're some problems I haven't thought of.
+
+- What if DataService is unavaliable, by network failure, DataService crashing, etc.
+- What if MasterService is unavaliable, by network failure, MasterService crashing, etc.
+- What if MinIO is unavaliable, by network failure.
+
+## TODO
+
+### 1. DataNode no longer interacts with Etcd except service registering.
+
+   **O1-1**. DataService rather than DataNode saves binlog paths into Etcd. 1 Day
+    
+   ![datanode_design](graphs/datanode_design_01.jpg)
+    
+   **O1-2**. DataNode registers itself to Etcd when started. 1 Day
+    
+### 2. DataNode gets start and end MsgPositions of all channels, and report to DataService after flushing.
+
+   **O2-1**. Set start and end positions while publishing ddl messages. 0.5 Day
+
+   **O2-2**. [after **O4-1**] Get message positions in flowgraph and pass through nodes, report to DataService along with binlog paths. 1 Day
+
+   **O2-3**. [with **O1-1**] DataNode is no longer aware of whether if segment flushed, so SegmentFlushed messages should be sent by DataService. 1 Day
+
+### 3. DataNode recovery
+
+   **O3-1**. Flowgraph is initialized after DataService called WatchDmChannels, flowgraph is healthy if MasterService is available. 2 Day
+
+### 4. DataNode with collection with flowgraph with vchannel designs
+
+#### The winner
+  ![datanode_design](graphs/collection_flowgraph_relation.png)
+
+  ![datanode_design](graphs/collection_flowgraph_1_n.png)
+
+  **O4-1.** DataNode scales flowgraph 2 Day
+
+#### The boring design
+
+• If collection:flowgraph = 1 : 1, datanode must have ability to scale flowgraph.
+
+![datanode_design](graphs/collection_flowgraph_1_1.jpg)
+
+•** [Winner]** If collection:flowgraph = 1 : n, flowgraph:vchannel = 1:1
+
+![datanode_design](graphs/collection_flowgraph_1_n.png)
+
+• If collection:flowgraph = n : 1, in the blue cases, datanode must have ability to scale flowgraph. In the brown cases, flowgraph must be able to scale channels.
+
+![datanode_design](graphs/collection_flowgraph_n_1.jpg)
+
+• If collection:flowgraph = n : n  , load balancing on vchannels.
+
+![datanode_design](graphs/collection_flowgraph_n_n.jpg)
+
+
+
+
+
+
+