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186 lines
7.7 KiB
Markdown
186 lines
7.7 KiB
Markdown
# Drop Collection
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`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`.
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The execution flow of `Drop Collection` is shown in the following figure:
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1. Firstly, `SDK` starts a `DropCollection` request to `Proxy` via `Grpc`, the `proto` is defined as follows:
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```proto
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service MilvusService {
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...
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rpc DropCollection(DropCollectionRequest) returns (common.Status) {}
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...
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}
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message DropCollectionRequest {
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common.MsgBase base = 1; // must
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string db_name = 2;
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string collection_name = 3; // must
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}
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```
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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.
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```go
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type task interface {
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TraceCtx() context.Context
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ID() UniqueID // return ReqID
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SetID(uid UniqueID) // set ReqID
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Name() string
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Type() commonpb.MsgType
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BeginTs() Timestamp
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EndTs() Timestamp
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SetTs(ts Timestamp)
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OnEnqueue() error
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PreExecute(ctx context.Context) error
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Execute(ctx context.Context) error
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PostExecute(ctx context.Context) error
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WaitToFinish() error
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Notify(err error)
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}
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type DropCollectionTask struct {
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Condition
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*milvuspb.DropCollectionRequest
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ctx context.Context
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rootCoord types.RootCoord
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result *commonpb.Status
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chMgr channelsMgr
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chTicker channelsTimeTicker
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}
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```
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3. There is a background service in `Proxy`, this service would get the `DropCollectionTask` from `DdTaskQueue`, and execute it in three phases:
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- `PreExecute`, do some static checking at this phase, such as check if `Collection Name` is legal etc.
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- `Execute`, at this phase, `Proxy` would send `DropCollection` request to `RootCoord` via `Grpc`, and wait the response, the `proto` is defined as below:
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```proto
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service RootCoord {
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...
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rpc DropCollection(milvus.DropCollectionRequest) returns (common.Status) {}
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...
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}
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```
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- `PostExecute`, `Proxy` would delete `Collection`'s meta from global meta table at this phase.
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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.
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```go
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type reqTask interface {
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Ctx() context.Context
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Type() commonpb.MsgType
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Execute(ctx context.Context) error
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Core() *Core
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}
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type DropCollectionReqTask struct {
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baseReqTask
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Req *milvuspb.DropCollectionRequest
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}
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```
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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.
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6. After `Collection`'s meta has been deleted from `metaTable`, `Milvus` would consider this collection has been deleted successfully.
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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.
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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:
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```proto
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message DropCollectionRequest {
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common.MsgBase base = 1;
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string db_name = 2;
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string collectionName = 3;
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int64 dbID = 4;
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int64 collectionID = 5;
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}
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```
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9. After these operations, `RootCoord` would update internal timestamp.
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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:
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```proto
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service QueryCoord {
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...
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rpc ReleaseCollection(ReleaseCollectionRequest) returns (common.Status) {}
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...
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}
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message ReleaseCollectionRequest {
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common.MsgBase base = 1;
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int64 dbID = 2;
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int64 collectionID = 3;
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int64 nodeID = 4;
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}
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```
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11. At last, `RootCoord` would send `InvalidateCollectionMetaCache` request to each `Proxy`, notify `Proxy` to remove `Collection`'s meta. The `proto` is defined as follows:
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```proto
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service Proxy {
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...
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rpc InvalidateCollectionMetaCache(InvalidateCollMetaCacheRequest) returns (common.Status) {}
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...
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}
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message InvalidateCollMetaCacheRequest {
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common.MsgBase base = 1;
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string db_name = 2;
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string collection_name = 3;
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}
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```
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12. The execution flow of `QueryCoord.ReleaseCollection` is shown in the following figure:
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13. `QueryCoord` would wrap `ReleaseCollection` into `ReleaseCollectionTask`, and push the task into `TaskScheduler`
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14. There is a background service in `QueryCoord`. This service would get the `ReleaseCollectionTask` from `TaskScheduler`, and execute it in three phases:
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- `PreExecute`, `ReleaseCollectionTask` would only print debug log at this phase.
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- `Execute`, there are two jobs at this phase:
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- 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:
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```proto
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message ReleaseDQLMessageStreamRequest {
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common.MsgBase base = 1;
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int64 dbID = 2;
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int64 collectionID = 3;
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}
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```
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- 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
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```proto
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service QueryNode {
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...
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rpc ReleaseCollection(ReleaseCollectionRequest) returns (common.Status) {}
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...
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}
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message ReleaseCollectionRequest {
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common.MsgBase base = 1;
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int64 dbID = 2;
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int64 collectionID = 3;
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int64 nodeID = 4;
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}
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```
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- `PostExecute`, `ReleaseCollectionTask` would only print debug log at this phase.
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15. After these operations, `QueryCoord` would send `ReleaseCollection`'s response to `RootCoord`.
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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.
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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.
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*Notes*:
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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.
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2. Currently, the `IndexCoord` doesn't have response to the `DropCollection`. So the `Collection`'s `index file` still exists in the persistent storage.
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