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1. Proxy receives a delete request
   1. parse `expr` to get primary keys, then split DeleteMsg into insert channels by primary keys (done)
2. DataNode watches insert channel
   1. receive DeleteMsg and persist delete data
      1. when DataNode start up, load all segments info from Minio into memory (done)
      2. update datanode flowgraph
         1. when DDNode receive DeleteMsg, save it into FlowGraphMsg structure, and send FlowGraphMsg to next node InsertBufferNode (done)
         2. when InsertBufferNode receive FlowGraphMsg, process InsertMsg in it, then wrapper DeleteMsg into another FlowGraphMsg and sent it to next node DeleteNode (done)
         3. when DeleteNode receive FlowGraphMsg, process DeleteMsg in it, save deleted ids and timestamps into delBuf (done)
      3. update DeleteNode to handle flushMsg
         1. add another flushChan for DeleteNode (done)
         2. when DeleteNode receive receives flushMsg, save all data in delBuf into MinioMinIO 
            1. deleted ids and timestamps are saved into two separated Minio file with name "/by-dev/deltalog/collectionID/partitionID/segmentID/xxx" (done)
   2.  DataNode forwards ttMsg and DeleteMsg to delta channel (doing)

Proposal delete operation serving search(sealed+growing)

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