MEP 10 -- Support Collection Alias

Current state: Under Discussion

ISSUE: https://github.com/milvus-io/milvus/issues/4812

PRs: TODO

Keywords: Collection Alias, Collection Hot Reload

Released: TODO

Summary

As the name indicates, CollectionAlias is an alias to an existing collection.
The collection alias can be updated to a new collection.
Within RootCoordinator, Proxy, and all the key components, CollectionName and CollectionAlias are equal.
e.g. MetaTable.GetCollectionByName(collectionName string, ts typeutil.Timestamp) can receive CollectionAlias and return corresponding CollectionInfo.
CollectionAlias ∧ CollectionName = ∅ . CollectionAlias cannot collide with existing CollectionNames.

Motivation

In recommendation systems, there is a need to update the whole collection(e.g. User embedding) periodically(hourly, daily). To update the whole collection, we can:

upsert all the items.
- It will be slow if the collection is huge.
Insert new collection B data & rename the new collection B as A.
- In a distributed system, it is pretty costly(Performance, Availability, Complexity) to update the state globally.

As CollectionAlias works as an extra pointer to the existing collection in the RootCoordinator, we can implement collection hot reloading at a much lower cost compared to the 1, 2 approaches.

With CollectionAlias, we can implement collection hot reloading functionality as such:

Create Collection A & insert data.
Create CollectionAlias Z -> A.
Create Collection B & insert data.
Update CollectionAlias Z -> B.
Users can access new data with the Z alias.

Public Interfaces

New Public APIs

There will be CollectionAlias related 3 new APIs.

CreateAlias , DropAlias , AlterAlias

// milvus.proto
message CreateAliasRequest {
	common.MsgBase base = 1;
	string collection_name = 2;
	string alias = 3;
}

message DropAliasRequest {
	common.MsgBase base = 1;
	string alias = 2;
}

message AlterAliasRequest{
	common.MsgBase base = 1;
	string collection_name = 2;
	string alias = 3;
}

service MilvusService {
	// NEW
	rpc CreateAlias(SetAliasRequest) returns (common.Status) {}
	// NEW
	rpc DropAlias(DropAliasRequest) returns (common.Status) {}
	// NEW
	rpc AlterAlias(AlterAliasRequest) returns (common.Status) {}

}

Changes to Existing APIs

Users can't drop the collection if the collection is referenced by an alias .
DescribeCollection now returns aliases

message DescribeCollectionResponse {
  common.Status status = 1;
  schema.CollectionSchema schema = 2;
  int64 collectionID = 3;
  repeated string virtual_channel_names = 4;
  repeated string physical_channel_names = 5;
  uint64 created_timestamp = 6;
  uint64 created_utc_timestamp = 7;

  // NEW 
  repeated string aliases = 8;
}

service MilvusService {
  // Users are required to drop the aliases first before dropping the collection.
  rpc DropCollection(DropCollectionRequest) returns (common.Status) {}
  // DescribeCollectionResponse containes `aliases` that refer to this collection.
  rpc DescribeCollection(DescribeCollectionRequest) returns (DescribeCollectionResponse) {}
}

Design Details

Changes to the MetaTable

type metaTable struct {
	client          kv.SnapShotKV                                                   // client of a reliable kv service, i.e. etcd client
	tenantID2Meta   map[typeutil.UniqueID]pb.TenantMeta                             // tenant id to tenant meta
	proxyID2Meta    map[typeutil.UniqueID]pb.ProxyMeta                              // proxy id to proxy meta
	collID2Meta     map[typeutil.UniqueID]pb.CollectionInfo                         // collection_id -> meta
	collName2ID     map[string]typeutil.UniqueID                                    // collection name to collection id
	// NEW
	collAlias2ID    map[string]typeutil.UniqueID
	...
}

As CollectionAlias & CollectionName are equal, GetCollectionByName also checks metaTable.collAlias2ID when getting the collection by name.

func (mt *metaTable) GetCollectionByName(collectionName string, ts typeutil.Timestamp) (*pb.CollectionInfo, error) {
	mt.ddLock.RLock()
	defer mt.ddLock.RUnlock()

	if ts == 0 {
		vid, ok := mt.collName2ID[collectionName]
		if !ok {
			// NEW
			if vid, ok = mt.collAlias2ID[collectionName]; !ok {
				return nil, fmt.Errorf("can't find collection: " + collectionName)
			}
		}
	...
}

CollectionAlias also have to be persisted in the etcd .

const (
	ComponentPrefix        = "root-coord"
	TenantMetaPrefix       = ComponentPrefix + "/tenant"
	ProxyMetaPrefix        = ComponentPrefix + "/proxy"
	CollectionMetaPrefix   = ComponentPrefix + "/collection"
	SegmentIndexMetaPrefix = ComponentPrefix + "/segment-index"
	IndexMetaPrefix        = ComponentPrefix + "/index"
	// NEW Additions
	CollectionAliasMetaPrefix = ComponentPrefix + "/collection-alias"
)

For persistence in etcd, the key will be fmt.Sprintf("%s/%s", CollectionAliasMetaPrefix, CollectionAlias) and the value be CollectionID.

Changes to the RootCoordinator

// root_coord.proto
service RootCoord {
  // NEW
  // CreateAlias creates 1 to 1 mapping between `alias` and `collection_name`
  // 1. If there no `alias` in the metaTable:
  // 	1.1 new `alias` will be added to the metaTable
  // 	1.2 `alias` will be persisted in the `etcd`
  //	1.3 `dd_op` will sent to log broker.
  // 2. If there is `alias/collection` in the metaTable:
  // 	2.1 An `alias/collection already exists` error will be returned.
 rpc CreateAlias(milvus.CreateAliasRequest) returns (common.Status) {}

  // NEW
  // 1. DropAlias
  // 	1.1 Removes existing mapping from the metaTable
  //	1.2 Removes existing mapping from the `etcd`
  //	1.3 `dd_op` will be sent to the log broker.
	//  1.4 Invalidates proxy caches
 rpc DropAlias(milvus.DropAliasRequest) returns (common.Status) {}

 // NEW
 // 1. AlterAlias
 //  1.1 Existing mapping will be updated in metaTable.
 //  1.2 Existing mapping will be updated in `etcd`
 //  1.3 `dd_op` will be sent to log broker.
 //  1.4 Invalidates proxy caches.
 rpc AlterAlias(milvus.AlterAliasRequest) returns (common.Status) {}

 // UPDATED REQUIRED
 // Collection can't be dropped if it is referenced by an `alias`.
 rpc DropCollection(DropCollectionRequest) returns (common.Status) {}
 // UPDATED REQUIRED
 // DescribeCollection now returns `aliases`
 rpc DescribeCollection(DescribeCollectionRequest) returns (DescribeCollectionResponse) {}
}

There will be new tasks as `CreateAliasTask`, `DropAliasTask`, and `AlterAliasTask` that handles the actual implementation within the GRPC methods.

Recovery

The recovery process will handle CreateAliasDDType , DropAliasDDType, AlterAliasDDType types.

func (c *Core) reSendDdMsg(ctx context.Context) error {
	flag, err := c.MetaTable.client.Load(DDMsgSendPrefix, 0)
	if err != nil || flag == "true" {
		log.Debug("No un-successful DdMsg")
		return nil
	}

	ddOpStr, err := c.MetaTable.client.Load(DDOperationPrefix, 0)
	if err != nil {
		log.Debug("DdOperation key does not exist")
		return nil
	}
	var ddOp DdOperation
	if err = json.Unmarshal([]byte(ddOpStr), &ddOp); err != nil {
		return err
	}

	switch ddOp.Type {
	case CreateCollectionDDType:
		var ddReq = internalpb.CreateCollectionRequest{}
		if err = proto.UnmarshalText(ddOp.Body, &ddReq); err != nil {
			return err
		}
		collInfo, err := c.MetaTable.GetCollectionByName(ddReq.CollectionName, 0)
		if err != nil {
			return err
		}
		if err = c.SendDdCreateCollectionReq(ctx, &ddReq, collInfo.PhysicalChannelNames); err != nil {
			return err
		}
	// NEW
	case CreateAliasDDType:
		...
	// NEW
	case DropAliasDDType:
		...
	// NEW
	case AlterAliasDDType:
		...
	...
}

Compatibility, Deprecation, and Migration Plan

CollectionAlias design is not intrusive, there won't be compatibility issues.

Test Plan

Unit tests

Rejected Alternatives

Rejected Physical Collection Rename Design due to:
- high implementation complexity
- potential undesirable performance characteristics
Rejected SetAlias API
- SetAlias combines CreateAlias & UpdateAlias semantics, but it may have unexpected behaviors for the user. e.g. If the alias was already in the metaTable there is a risk that it may be overridden unexpectedly.

References

ElasticSearch provides similar abstraction https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-aliases.html