design detail of flush collection (#6821)

Signed-off-by: Nyefu.chen <yefu.chen@zilliz.com>

docs/design_docs/milvus_flush_collections_en.md

+# Flush Collection 
+`Flush` operation is used to make sure that the data has been writen into the persistent storage, this document introduce how `Flush` operation works in `Milvus 2.0`. The following figure shows the execution flow of `Flush`
+
+![flush_collections](./graphs/flush_data_coord.png)
+
+1. Firstly, `SDK` starts a `Flush` request to `Proxy` via `Grpc`, the `proto` is defined as follows:
+```proto
+service MilvusService {
+  ...
+
+  rpc Flush(FlushRequest) returns (FlushResponse) {}
+
+  ...
+}
+
+message FlushRequest {
+  common.MsgBase base = 1;
+  string db_name = 2;
+  repeated string collection_names = 3;
+}
+
+message FlushResponse{
+  common.Status status = 1;
+  string db_name = 2;
+  map<string, schema.LongArray> coll_segIDs = 3;
+}
+```
+
+
+2. When received the `Flush` request, the `Proxy` would wraps this request into `FlushTask`, and pushs this task into `DdTaskQueue` queue. After that, `Proxy` would call method of `WatiToFinish` to wait until the task finished.
+```go
+type task interface {
+	TraceCtx() context.Context
+	ID() UniqueID       // return ReqID
+	SetID(uid UniqueID) // set ReqID
+	Name() string
+	Type() commonpb.MsgType
+	BeginTs() Timestamp
+	EndTs() Timestamp
+	SetTs(ts Timestamp)
+	OnEnqueue() error
+	PreExecute(ctx context.Context) error
+	Execute(ctx context.Context) error
+	PostExecute(ctx context.Context) error
+	WaitToFinish() error
+	Notify(err error)
+}
+
+type FlushTask struct {
+	Condition
+	*milvuspb.FlushRequest
+	ctx       context.Context
+	dataCoord types.DataCoord
+	result    *milvuspb.FlushResponse
+}
+```
+
+3. There is a backgroud service in `Proxy`, this service would get the `FlushTask` from `DdTaskQueue`, and executes it in three phases.
+    - `PreExecute`,`FlushTask` does nothing at this phase, and return directly
+    - `Execute`, at this phase, `Proxy` would send `Flush` request to `DataCoord` via `Grpc`,and wait for the reponse, the `proto` is defined as follow:
+    ```proto
+        service DataCoord {
+          ...
+
+          rpc Flush(FlushRequest) returns (FlushResponse) {}
+
+          ...
+        }
+
+        message FlushRequest {
+          common.MsgBase base = 1;
+          int64 dbID = 2;
+          int64 collectionID = 4;
+        }
+
+        message FlushResponse {
+          common.Status status = 1;
+          int64 dbID = 2;
+          int64 collectionID = 3;
+          repeated int64 segmentIDs = 4;
+    ```
+    - `PostExecute`, `FlushTask` does nothing at this phase, and return directly
+
+4. After receiving `Flush` request from `Proxy`, `DataCoord` would call `SealAllSegments` to seal all the growing segments that belong to this `Collection`, and no longer allocate new `ID`s for these segments. After that, `DataCoord` would send response to `Proxy`, and the response should contain all the sealed segment ID.
+
+5. In `Milvus 2.0`, the `Flush` is an asynchronous operation. So when `SDK` receives the response of `Flush`, it only means that the `DataCoord` has sealed these segments, and there are 2 problem that we have to soluved.
+    - The sealed segments might still in the memory, and not have been writen into persistent storage yet.
+    - `DataCoord` would no longer allocate new `ID`s for these sealed segments, but how to make sure all the allocated `ID`s have been consumed by `DataNode`.
+
+
+6. For the first problem, `SDK` should send `GetSegmentInfo` request to `DataCoord` periodically, until all the sealed segment are in state of `Flushed`. the `proto` is defined as following.
+```proto
+service DataCoord {
+  ...
+
+  rpc GetSegmentInfo(GetSegmentInfoRequest) returns (GetSegmentInfoResponse) {}
+
+  ...
+}
+
+message GetSegmentInfoRequest {
+  common.MsgBase base = 1;
+  repeated int64 segmentIDs = 2;
+}
+
+message GetSegmentInfoResponse {
+  common.Status status = 1;
+  repeated SegmentInfo infos = 2;
+}
+
+message SegmentInfo {
+  int64 ID = 1;
+  int64 collectionID = 2;
+  int64 partitionID = 3;
+  string insert_channel = 4;
+  int64 num_of_rows = 5;
+  common.SegmentState state = 6;
+  internal.MsgPosition dml_position = 7;
+  int64 max_row_num = 8;
+  uint64 last_expire_time = 9;
+  internal.MsgPosition start_position = 10;
+}
+
+enum SegmentState {
+    SegmentStateNone = 0;
+    NotExist = 1;
+    Growing = 2;
+    Sealed = 3;
+    Flushed = 4;
+    Flushing = 5;
+}
+
+```
+
+7. For second problem, `DataNode` would report a timestamp to `DataCoord` every time it consumes a package from `MsgStream`,the Proto is define as follow.
+
+ ```proto
+ message DataNodeTtMsg {
+    common.MsgBase base =1;
+    string channel_name = 2;
+    uint64 timestamp = 3;
+}
+```
+
+8. There is a backgroud service, `startDataNodeTsLoop`, in `DataCoord` to process the message of `DataNodeTtMsg`.
+    - Firstly, `DataCoord` would extract `channel_name` from `DataNodeTtMsg`, and filter out all the sealed segments that attached on this `channel_name`
+    - Compare the timestamp when the segment enters into state of `Sealed` with the `DataNodeTtMsg.timestamp`, if `DataNodeTtMsg.timestamp` is greater, it means that all the `ID`s belong to that segment have been consumed by `DataNode`,so it's safe to notify `DataNode` to write that segment into persistent storage. The `proto` is defined as follow.
+```proto
+service DataNode {
+  ...
+
+  rpc FlushSegments(FlushSegmentsRequest) returns(common.Status) {}
+
+  ...
+}
+
+message FlushSegmentsRequest {
+  common.MsgBase base = 1;
+  int64 dbID = 2;
+  int64 collectionID = 3;
+  repeated int64 segmentIDs = 4;
+}
+
+```
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