Current state: Under Discussion
ISSUE: https://github.com/milvus-io/milvus/issues/4812
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Keywords: Collection Alias, Collection Hot Reload
Released: TODO
MEP: https://wiki.lfaidata.foundation/display/MIL/MEP+10+--+Support+Collection+Alias
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
andCollectionAlias
are equal.
e.g.MetaTable.GetCollectionByName(collectionName string, ts typeutil.Timestamp)
can receiveCollectionAlias
and return correspondingCollectionInfo.
CollectionAlias
∧CollectionName
= ∅ .CollectionAlias
cannot collide with existingCollectionName
s.
...
upsert
all the items.- It will be problematic 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.
- In a distributed system, it is pretty costly(
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
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Code Block |
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// milvus.proto message CreateAliasRequest { common.MsgBase base = 1; string collection_name = 2; string alias = 3; } message DropAliasRequest { common.MsgBase base = 1; string collection_namealias = 2; string alias = 3; } 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) {} } |
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- Users can't drop the
collection
if thecollection
is referenced by analias
. DescribeCollection
now returnsaliases
Code Block |
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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;
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// NEW |
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repeated string aliases = 8; |
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} service MilvusService { |
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// Users are required to drop the aliases first before dropping the collection. |
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rpc DropCollection(DropCollectionRequest) returns (common.Status) {} |
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// DescribeCollectionResponse containes `aliases` that refer to this collection.
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rpc DescribeCollection(DescribeCollectionRequest) returns (DescribeCollectionResponse) {} |
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} |
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Design Details
Changes to the MetaTable
Code Block |
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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
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collAlias2ID map[string]typeutil.UniqueID |
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} |
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As CollectionAlias
& CollectionName
are equal, GetCollectionByName
also checks metaTable.collAlias2ID
when getting the collection by name.
Code Block |
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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
|
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if vid, ok = mt.collAlias2ID[collectionName]; !ok { |
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return nil, fmt.Errorf("can't find collection: " + collectionName)
}
}
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} |
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CollectionAlias
also have to be persisted in the etcd
.
Code Block |
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const (
ComponentPrefix = "root-coord"
TenantMetaPrefix = ComponentPrefix + "/tenant"
ProxyMetaPrefix = ComponentPrefix + "/proxy"
CollectionMetaPrefix = ComponentPrefix + "/collection"
SegmentIndexMetaPrefix = ComponentPrefix + "/segment-index"
IndexMetaPrefix = ComponentPrefix + "/index"
// NEW Additions
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CollectionAliasMetaPrefix = ComponentPrefix + "/collection-alias" |
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) |
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For persistence in etcd
, the key will be fmt.Sprintf("%s/%s", CollectionAliasMetaPrefix, CollectionAlias)
and the value be CollectionID
.
Changes to the RootCoordinator
Code Block |
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// root_coord.proto
service RootCoord {
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// NEW |
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// 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) {}
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// NEW |
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// 1. DropAlias // 1.1 Removes existing |
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mapping from the metaTable // 1.2 Removes existing |
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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) {} |
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// NEW |
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// 1. AlterAlias // 1.1 Existing |
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mapping will be updated in metaTable. // 1.2 |
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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) {} |
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// UPDATED REQUIRED |
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// Collection can't be dropped if it is referenced by an `alias`. rpc DropCollection(DropCollectionRequest) returns (common.Status) {} |
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// UPDATED REQUIRED |
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// 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.
Code Block |
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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
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