milvus/docs/developer_guides/chap06_master.md

10. Master

10.1 Interfaces (RPC)

RPC	description
CreateCollection	create a collection base on schema statement
DropCollection	drop a collection
HasCollection	whether or not a collection exists
DescribeCollection	show a collection's schema and its descriptive statistics
ShowCollections	list all collections
CreatePartition	create a partition
DropPartition	drop a partition
HasPartition	whether or not a partition exists
DescribePartition	show a partition's name and its descriptive statistics
ShowPartitions	list a collection's all partitions
AllocTimestamp	allocate a batch of consecutive timestamps
AllocID	allocate a batch of consecutive IDs
AssignSegmentID	assign segment id to insert rows (master determines which segment these rows belong to)
GetSysConfigs	get system configurations

10.2 Master Instance

type Master interface {
  tso timestampOracle	// timestamp oracle
  ddScheduler ddRequestScheduler // data definition request scheduler
  metaTable metaTable // in-memory system meta
  collManager collectionManager // collection & partition manager
  segManager segmentManager // segment manager
}

• Timestamp allocation

Master serves as a centrol clock of the whole system. Other components (i.e. Proxy) allocates timestamps from master via RPC AllocTimestamp. All the timestamp allocation requests will be handled by the timestampOracle singleton. See section 4.2 for the details about timestampOracle.

• Request Scheduling

• System Meta

• Collection Management

• Segment Management

10.3 Data definition Request Scheduler

10.2.1 Task

Master receives data definition requests via grpc. Each request (described by a proto) will be wrapped as a task for further scheduling. The task interface is

type task interface {
  Type() ReqType
  Ts() Timestamp
  Execute() error
  WaitToFinish() error
  Notify() error
}

A task example is as follows. In this example, we wrap a CreateCollectionRequest (a proto) as a createCollectionTask. The wrapper need to implement task interfaces.

type createCollectionTask struct {
  req *CreateCollectionRequest
  cv int chan
}

// Task interfaces
func (task *createCollectionTask) Type() ReqType
func (task *createCollectionTask) Ts() Timestamp
func (task *createCollectionTask) Execute() error
func (task *createCollectionTask) Notify() error
func (task *createCollectionTask) WaitToFinish() error

10.2.3 Scheduler

type ddRequestScheduler struct {
  reqQueue *task chan
  ddStream *MsgStream
}

func (rs *ddRequestScheduler) Enqueue(task *task) error
func (rs *ddRequestScheduler) schedule() *task // implement scheduling policy

In most cases, a data definition task need to

• update system's meta data (via metaTable),
• and synchronize the data definition request to other related system components so that the quest can take effect system wide.

Master

10.4 Meta Table

10.4.1 Meta

• Tenant Meta

message TenantMeta {
  uint64 id = 1;
  uint64 num_query_nodes = 2;
  repeated string insert_channel_ids = 3;
  string query_channel_id = 4;
}

• Proxy Meta

message ProxyMeta {
  uint64 id = 1;
  common.Address address = 2;
  repeated string result_channel_ids = 3;
}

• Collection Meta

message CollectionMeta {
  uint64 id=1;
  schema.CollectionSchema schema=2;
  uint64 create_time=3;
  repeated uint64 segment_ids=4;
  repeated string partition_tags=5;
}

• Segment Meta

message SegmentMeta {
  uint64 segment_id=1;
  uint64 collection_id =2;
  string partition_tag=3;
  int32 channel_start=4;
  int32 channel_end=5;
  uint64 open_time=6;
  uint64 close_time=7;
  int64 num_rows=8;
}

10.4.2 KV pairs in EtcdKV

"tenant/$tenantId" string -> tenantMetaBlob string
"proxy/$proxyId" string -> proxyMetaBlob string
"collection/$collectionId" string -> collectionMetaBlob string
"segment/$segmentId" string -> segmentMetaBlob string

Note that tenantId, proxyId, collectionId, segmentId are unique strings converted from int64.

tenantMeta, proxyMeta, collectionMeta, segmentMeta are serialized protos.

10.4.3 Meta Table

type metaTable struct {
  kv kv.Base // client of a reliable kv service, i.e. etcd client
  tenantId2Meta map[UniqueId]TenantMeta // tenant id to tenant meta
  proxyId2Meta map[UniqueId]ProxyMeta // proxy id to proxy meta
  collId2Meta map[UniqueId]CollectionMeta // collection id to collection meta
  collName2Id map[string]UniqueId // collection name to collection id
  segId2Meta map[UniqueId]SegmentMeta // segment id to segment meta
  
  tenantLock sync.RWMutex
  proxyLock sync.RWMutex
  ddLock sync.RWMutex
}

func (meta *metaTable) AddTenant(tenant *TenantMeta) error
func (meta *metaTable) DeleteTenant(tenantId UniqueId) error

func (meta *metaTable) AddProxy(proxy *ProxyMeta) error
func (meta *metaTable) DeleteProxy(proxyId UniqueId) error

func (meta *metaTable) AddCollection(coll *CollectionMeta) error
func (meta *metaTable) DeleteCollection(collId UniqueId) error
func (meta *metaTable) HasCollection(collId UniqueId) bool
func (meta *metaTable) GetCollectionByName(collName string) (*CollectionMeta, error)

func (meta *metaTable) AddPartition(collId UniqueId, tag string) error
func (meta *metaTable) HasPartition(collId UniqueId, tag string) bool
func (meta *metaTable) DeletePartition(collId UniqueId, tag string) error

func (meta *metaTable) AddSegment(seg *SegmentMeta) error
func (meta *metaTable) GetSegmentById(segId UniqueId)(*SegmentMeta, error)
func (meta *metaTable) DeleteSegment(segId UniqueId) error
func (meta *metaTable) CloseSegment(segId UniqueId, closeTs Timestamp, num_rows int64) error

func NewMetaTable(kv kv.Base) (*metaTable,error)

metaTable maintains meta both in memory and etcdKV. It keeps meta's consistency in both sides. All its member functions may be called concurrently.

• AddSegment(seg *SegmentMeta) first update CollectionMeta by adding the segment id, then it adds a new SegmentMeta to kv. All the modifications are done transactionally.

10.5 System Time Synchronization

10.5.1 Time Tick Barrier

• Soft Time Tick Barrier

type softTimeTickBarrier struct {
  peer2LastTt map[UniqueId]Timestamp
  minTtInterval Timestamp
  lastTt Timestamp
  outTt chan Timestamp
  ttStream *MsgStream
  ctx context.Context
}

func (ttBarrier *softTimeTickBarrier) GetTimeTick() (Timestamp,error)
func (ttBarrier *softTimeTickBarrier) Start() error

func newSoftTimeTickBarrier(ctx context.Context, ttStream *MsgStream, peerIds []UniqueId, minTtInterval Timestamp) *softTimeTickBarrier

• Hard Time Tick Barrier

type hardTimeTickBarrier struct {
  peer2Tt map[UniqueId]List
  outTt chan Timestamp
  ttStream *MsgStream
  ctx context.Context
}

func (ttBarrier *hardTimeTickBarrier) GetTimeTick() (Timestamp,error)
func (ttBarrier *hardTimeTickBarrier) Start() error

func newHardTimeTickBarrier(ctx context.Context, ttStream *MsgStream, peerIds []UniqueId) *softTimeTickBarrier

10.5.1 Time Synchronization Message Producer

type TimeTickBarrier interface {
   GetTimeTick() (Timestamp,error)
   Start() error
}

type timeSyncMsgProducer struct {
 proxyTtBarrier TimeTickBarrier // softTimeTickBarrier
 WriteNodeTtBarrier TimeTickBarrier //hardTimeTickBarrier
 
 dmSyncStream *MsgStream // insert & delete
 k2sSyncStream *MsgStream
 
 ctx context.Context
}

func (syncMsgProducer *timeSyncMsgProducer) SetProxyTtStreams(proxyTt *MsgStream, proxyIds []UniqueId)
func (syncMsgProducer *timeSyncMsgProducer) SetWriteNodeTtStreams(WriteNodeTt *MsgStream, writeNodeIds []UniqueId)

func (syncMsgProducer *timeSyncMsgProducer) SetDmSyncStream(dmSyncStream *MsgStream)
func (syncMsgProducer *timeSyncMsgProducer) SetK2sSyncStream(k2sSyncStream *MsgStream)

func (syncMsgProducer *timeSyncMsgProducer) Start() error
func (syncMsgProducer *timeSyncMsgProducer) Close() error

func newTimeSyncMsgProducer(ctx context.Context) *timeSyncMsgProducer error

10.6 System Statistics

10.6.1 Query Node Statistics

message SegmentStats {
  int64 segment_id = 1;
  int64 memory_size = 2;
  int64 num_rows = 3;
  bool recently_modified = 4;
}

message QueryNodeStats {
    int64 id = 1;
    uint64 timestamp = 2;
    repeated SegmentStats seg_stats = 3;
}

10.7 Segment Management

type assignment struct {
	MemSize    int64
	AssignTime time.Time
}

type segmentStatus struct {
  assignments []*assignment
}

type collectionStatus struct {
  openedSegment []UniqueID
}

type SegmentManagement struct {
  segStatus map[UniqueID]*SegmentStatus
  collStatus map[UniqueID]*collectionStatus
}

func NewSegmentManagement(ctx context.Context) *SegmentManagement

10.7.1 Assign Segment ID to Inserted Rows

Master receives AssignSegIDRequest which contains a list of SegIDRequest(count, channelID, collectionName, partitionTag) from Proxy. Segment Manager will assign the opened segments or open a new segment if there is no enough space, and Segment Manager will record the allocated space which can be reallocated after a expire duration.

func (segMgr *SegmentManager) AssignSegmentID(segIDReq []*internalpb.SegIDRequest) ([]*internalpb.SegIDAssignment, error)

10.8 System Config

// examples of keys:
// "/pulsar/ip"
// "/pulsar/port"
// examples of key_prefixes:
// "/proxy"
// "/msg_stream/insert"

message SysConfigRequest {
    MsgType msg_type = 1;
    int64 reqID = 2;
    int64 proxyID = 3;
    uint64 timestamp = 4;
	repeated string keys = 5;
	repeated string key_prefixes = 6;
}

message SysConfigResponse {
    common.Status status = 1;
	repeated string keys = 2;
	repeated string values = 3;
}

type SysConfig struct {
	kv *kv.EtcdKV
}

func (conf *SysConfig) InitFromFile(filePath string) (error)
func (conf *SysConfig) GetByPrefix(keyPrefix string) (keys []string, values []string, err error)
func (conf *SysConfig) Get(keys []string) ([]string, error)

configuration examples in etcd:

key: root_path/config/master/address
value: "localhost"

key: root_path/config/proxy/timezone
value: "UTC+8"