MEP 13 -- Support Arrow Data Frame

Current state: "Under Discussion"

ISSUE: #7210

PRs: 

Keywords: arrow

Released: Milvus 2.0

Summary(required)

What are we going to do?

Motivation(required)

From data inspect, Milvus includes 2 data flows mainly: 1). Insert data flow 2). Search data flow

Insert Data Flow

Insert data flow includes following steps:

1.  pymilvus creates a data insert request with type milvuspb.InsertRequest (client/prepare.py::bulk_insert_param)

   // grpc-proto/milvus.proto
   message InsertRequest {
     common.MsgBase base = 1;
     string db_name = 2;
     string collection_name = 3;
     string partition_name = 4;
     repeated schema.FieldData fields_data = 5;	// fields' data
     repeated uint32 hash_keys = 6;
     uint32 num_rows = 7;
   }

   Data is inserted into fields_data by column, schemapb.FieldData is defined as following:

   // grpc-proto/schema.proto
   message ScalarField {
     oneof data {
       BoolArray bool_data = 1;
       IntArray int_data = 2;
       LongArray long_data = 3;
       FloatArray float_data = 4;
       DoubleArray double_data = 5;
       StringArray string_data = 6;
       BytesArray bytes_data = 7;
     }
   }
   ​
   message VectorField {
     int64 dim = 1;
     oneof data {
       FloatArray float_vector = 2;
       bytes binary_vector = 3;
     }
   }
   ​
   message FieldData {
     DataType type = 1;
     string field_name = 2;
     oneof field {
       ScalarField scalars = 3;
       VectorField vectors = 4;
     }
     int64 field_id = 5;
   }

2.  milvuspb.InsertRequest is serialized and send via gRPC

3.  Proxy receives milvuspb.InsertRequest, creates InsertTask for it, and adds this task into execution queue (internal/proxy/impl.go::Insert)

4.  InsertTask is executed, the column-based data stored in InsertTask.req is converted to row-based format, and saved into another internal message with type internalpb.InsertRequest (internal/proxy/task.go::transferColumnBasedRequestToRowBasedData)

   // internal/proto/internal.proto
   message InsertRequest {
     common.MsgBase base = 1;
     string db_name = 2;
     string collection_name = 3;
     string partition_name = 4;
     int64 dbID = 5;
     int64 collectionID = 6;
     int64 partitionID = 7;
     int64 segmentID = 8;
     string channelID = 9;
     repeated uint64 timestamps = 10;
     repeated int64 rowIDs = 11;
     repeated common.Blob row_data = 12;  // row-based data
   }

    rowID and timestamp are added for each row data

5.  Proxy encapsulates internalpb.InsertRequest into InsertMsg, and send it to pulsar channel

6.  Datanode receives InsertMsg from pulsar channel, restore data to column-based into structure InsertData (internal/datanode/flow_graph_insert_buffer_node.go::insertBufferNode::Operate)

   type InsertData struct {
       Data  map[FieldID]FieldData // field id to field data
       Infos []BlobInfo
   }

7.  InsertData is written into Minio with parquet format (internal/datanode/flow_graph_insert_buffer_node.go::flushSegment)

Search Data Flow

1. querynode 在收到 LoadSegments 请求后，会把 segment 对应的所有 binlog 文件从 minio 加载到内存，并放入内存数据结构 Blob 中 (internal/querynode/segment_loader.go::loadSegmentFieldsData)

   type Blob struct {
       Key   string    // binlog file path
       Value []byte    // binlog file data
   }

   通常 querynode 只加载标量数据，不加载向量数据，除非该向量列未建 index。

   Blob 里的数据经过反序列化后，提取出原始数据，存入 InsertData 数据结构中

2. querynode 执行 search 请求，通过 CGO 调用 knowhere 搜索引擎，得到 SearchResult (internal/query_node/query_collection.go::search)

   // internal/core/src/common/Types.h
   struct SearchResult {
    ...
    public:
       int64_t num_queries_;
       int64_t topk_;
       std::vector<float> result_distances_;
   ​
    public:
       void* segment_;
       std::vector<int64_t> internal_seg_offsets_;
       std::vector<int64_t> result_offsets_;
       std::vector<std::vector<char>> row_data_;
   };

   knowhere 返回的 SearchResult 中只有 result_distances_ 和 internal_seg_offsets_ 被填入了数据。

   querynode 在得到所有 segment 返回的 SearchResult 后，对结果做归并，并通过 internal_seg_offsets_ 得到其它输出列数据，并按 行存 格式写入 row_data_ 中 (internal/query_node/query_collection.go::reduceSearchResultsAndFillData)

3. querynode 对 SearchResult 数据再次整理，存入数据结构 milvus.Hits 中

   // internal/proto/milvus.proto
   message Hits {
     repeated int64 IDs = 1;
     repeated bytes row_data = 2;
     repeated float scores = 3;
   }

4. milvus.Hits 中的数据通过函数 translateHits 转为 列存 数据 schemapb.SearchResultData (internal/query_node/query_collection.go::translateHits)

   // internal/proto/schema.proto
   message SearchResultData {
     int64 num_queries = 1;
     int64 top_k = 2;
     repeated FieldData fields_data = 3;
     repeated float scores = 4;
     IDs ids = 5;
     repeated int64 topks = 6;
   }

5. schemapb.SearchResultData 被序列化后，封装为 internalpb.SearchResults，并放入 msgstream.SearchResultMsg，通过 pulsar channel 发送 (internal/query_node/query_collection.go::search)

   // internal/proto/internal.proto
   message SearchResults {
     common.MsgBase base = 1;
     common.Status status = 2;
     string result_channelID = 3;
     string metric_type = 4;
     repeated bytes hits = 5;  // search result data
   ​
     // schema.SearchResultsData inside
     bytes sliced_blob = 9;
     int64 sliced_num_count = 10;
     int64 sliced_offset = 11;
   ​
     repeated int64 sealed_segmentIDs_searched = 6;
     repeated string channelIDs_searched = 7;
     repeated int64 global_sealed_segmentIDs = 8;
   }

6. proxy 从 pulsar channel 中收集到所有 querynode 发送过来的 msgstream.SearchResultMsg，反序列化得到 schemapb.SearchResultData，再做一次归并，数据放入 milvuspb.SearchResults，通过 gRPC 传回 SDK (internal/proxy/task.go::SearchTask::PostExecute)

   // internal/proto/milvus.proto
   message SearchResults {
     common.Status status = 1;
     schema.SearchResultData results = 2;
   }

7. SDK 收到 milvuspb.SearchResults，返回

Public Interfaces(optional)

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Design Details(required)

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## Compatibility, Deprecation, and Migration Plan(optional)
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- If we are changing behavior how will we phase out the older behavior?
- If we need special migration tools, describe them here.
- When will we remove the existing behavior?

Test Plan(required)

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Rejected Alternatives(optional)

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