# Drop Collection `Milvus 2.0` uses `Collection` to represent a set of data, like `Table` in traditional database. Users can create or drop `Collection`. Altering the `Schema` of `Collection` is not supported yet. This article introduces the execution path of `Drop Collection`. At the end of this article, you should know which components are involved in `Drop Collection`. The execution flow of `Drop Collection` is shown in the following figure: ![drop_collection](./graphs/dml_drop_collection.png) 1. Firstly, `SDK` starts a `DropCollection` request to `Proxy` via `Grpc`, the `proto` is defined as follows: ```proto service MilvusService { ... rpc DropCollection(DropCollectionRequest) returns (common.Status) {} ... } message DropCollectionRequest { common.MsgBase base = 1; // must string db_name = 2; string collection_name = 3; // must } ``` 2. Once the `DropCollection` request is received, the `Proxy` would wrap this request into `DropCollectionTask`, and push this task into `DdTaskQueue` queue. After that, `Proxy` would call method of `WatiToFinish` to wait until the task is 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 DropCollectionTask struct { Condition *milvuspb.DropCollectionRequest ctx context.Context rootCoord types.RootCoord result *commonpb.Status chMgr channelsMgr chTicker channelsTimeTicker } ``` 3. There is a background service in `Proxy`, this service would get the `DropCollectionTask` from `DdTaskQueue`, and execute it in three phases: - `PreExecute`, do some static checking at this phase, such as check if `Collection Name` is legal etc. - `Execute`, at this phase, `Proxy` would send `DropCollection` request to `RootCoord` via `Grpc`, and wait the response, the `proto` is defined as below: ```proto service RootCoord { ... rpc DropCollection(milvus.DropCollectionRequest) returns (common.Status) {} ... } ``` - `PostExecute`, `Proxy` would delete `Collection`'s meta from global meta table at this phase. 4. `RootCoord` would wrap the `DropCollection` request into `DropCollectionReqTask`, and then call function `executeTask`. `executeTask` would return until the `context` is done or `DropCollectionReqTask.Execute` is returned. ```go type reqTask interface { Ctx() context.Context Type() commonpb.MsgType Execute(ctx context.Context) error Core() *Core } type DropCollectionReqTask struct { baseReqTask Req *milvuspb.DropCollectionRequest } ``` 5. Firstly, `RootCoord` would delete `Collection`'s meta from `metaTable`, including `schema`,`partition`, `segment`,`index`. All of these delete operations are committed in one transaction. 6. After `Collection`'s meta has been deleted from `metaTable`, `Milvus` would consider this collection has been deleted successfully. 7. `RootCoord` would alloc a timestamp from `TSO` before deleting `Collection`'s meta from `metaTable`. This timestamp is considered as the point when the collection was deleted. 8. `RootCoord` would send a message of `DropCollectionRequest` into `MsgStream`. Thus other components, who have subscribed to the `MsgStream`, would be notified. The `Proto` of `DropCollectionRequest` is defined as below: ```proto message DropCollectionRequest { common.MsgBase base = 1; string db_name = 2; string collectionName = 3; int64 dbID = 4; int64 collectionID = 5; } ``` 9. After these operations, `RootCoord` would update internal timestamp. 10. Then `RootCoord` would start a `ReleaseCollection` request to `QueryCoord` via `Grpc` , notify `QueryCoord` to release all resources that related to this `Collection`. This `Grpc` request is done in another `goroutine`, so it would not block the main thread. The `proto` is defined as follows: ```proto service QueryCoord { ... rpc ReleaseCollection(ReleaseCollectionRequest) returns (common.Status) {} ... } message ReleaseCollectionRequest { common.MsgBase base = 1; int64 dbID = 2; int64 collectionID = 3; int64 nodeID = 4; } ``` 11. At last, `RootCoord` would send `InvalidateCollectionMetaCache` request to each `Proxy`, notify `Proxy` to remove `Collection`'s meta. The `proto` is defined as follows: ```proto service Proxy { ... rpc InvalidateCollectionMetaCache(InvalidateCollMetaCacheRequest) returns (common.Status) {} ... } message InvalidateCollMetaCacheRequest { common.MsgBase base = 1; string db_name = 2; string collection_name = 3; } ``` 12. The execution flow of `QueryCoord.ReleaseCollection` is shown in the following figure: ![release_collection](./graphs/dml_release_collection.png) 13. `QueryCoord` would wrap `ReleaseCollection` into `ReleaseCollectionTask`, and push the task into `TaskScheduler` 14. There is a background service in `QueryCoord`. This service would get the `ReleaseCollectionTask` from `TaskScheduler`, and execute it in three phases: - `PreExecute`, `ReleaseCollectionTask` would only print debug log at this phase. - `Execute`, there are two jobs at this phase: - send a `ReleaseDQLMessageStream` request to `RootCoord` via `Grpc`, `RootCoord` would redirect the `ReleaseDQLMessageStream` request to each `Proxy`, and notify the `Proxy` that stop processing any message of this `Collection` anymore. The `proto` is defined as follows: ```proto message ReleaseDQLMessageStreamRequest { common.MsgBase base = 1; int64 dbID = 2; int64 collectionID = 3; } ``` - send a `ReleaseCollection` request to each `QueryNode` via `Grpc`, and notify the `QueryNode` to release all the resources related to this `Collection`, including `Index`, `Segment`, `FlowGraph`, etc. `QueryNode` would no longer read any message from this `Collection`'s `MsgStream` anymore ```proto service QueryNode { ... rpc ReleaseCollection(ReleaseCollectionRequest) returns (common.Status) {} ... } message ReleaseCollectionRequest { common.MsgBase base = 1; int64 dbID = 2; int64 collectionID = 3; int64 nodeID = 4; } ``` - `PostExecute`, `ReleaseCollectionTask` would only print debug log at this phase. 15. After these operations, `QueryCoord` would send `ReleaseCollection`'s response to `RootCoord`. 16. At `Step 8`, `RootCoord` has sent a message of `DropCollectionRequest` into `MsgStream`. `DataNode` would subscribe this `MsgStream`, so that it would be notified to release related resources. The execution flow is shown in the following figure. ![release_collection](./graphs/dml_release_flow_graph_on_data_node.png) 17. In `DataNode`, each `MsgStream` will have a `FlowGraph`, which processes all messages. When the `DataNode` receives the message of `DropCollectionRequest`, `DataNode` would notify `BackGroundGC`, which is a background service on `DataNode`, to release resources. *Notes*: 1. Currently, the `DataCoord` doesn't have response to the `DropCollection`. So the `Collection`'s `segment meta` still exists in the `DataCoord`'s `metaTable`, and the `Binlog` files belonging to this `Collection` still exist in the persistent storage. 2. Currently, the `IndexCoord` doesn't have response to the `DropCollection`. So the `Collection`'s `index file` still exists in the persistent storage.