milvus/docs/developer_guides/appendix_a_basic_components.md

Appendix A. Basic Components

A.1 Watchdog

type ActiveComponent interface {
  Id() string
  Status() Status
  Clean() Status
  Restart() Status
}

type ComponentHeartbeat interface {
  Id() string
  Status() Status
  Serialize() string
}

type Watchdog struct {
  targets [] *ActiveComponent
  heartbeats ComponentHeartbeat chan
}

// register ActiveComponent
func (dog *Watchdog) Register(target *ActiveComponent)

// called by ActiveComponents
func (dog *Watchdog) PutHeartbeat(heartbeat *ComponentHeartbeat)

// dump heatbeats as log stream
func (dog *Watchdog) dumpHeartbeat(heartbeat *ComponentHeartbeat)

A.2 Global Parameter Table

type GlobalParamsTable struct {
  params memoryKV
}

func (gparams *GlobalParamsTable) Save(key, value string) error
func (gparams *GlobalParamsTable) Load(key string) (string, error)
func (gparams *GlobalParamsTable) LoadRange(key, endKey string, limit int) ([]string, []string, error)
func (gparams *GlobalParamsTable) Remove(key string) error
func (gparams *GlobalParamsTable) LoadYaml(filePath string) error

• LoadYaml(filePath string) turns a YAML file into multiple key-value pairs. For example, given the following YAML

etcd:
  address: localhost
  port: 12379
  rootpath: milvus/etcd

GlobalParamsTable.LoadYaml will insert three key-value pairs into params

"etcd.address" -> "localhost"
"etcd.port" -> "12379"
"etcd.rootpath" -> "milvus/etcd"

A.3 Message Stream

type MsgType uint32
const {
  kInsert MsgType = 400
  kDelete MsgType = 401
  kSearch MsgType = 500
  KSearchResult MsgType = 1000
  
  kSegStatistics MsgType = 1100
  
  kTimeTick MsgType = 1200
  kTimeSync MsgType = 1201
}

type TsMsg interface {
  SetTs(ts Timestamp)
  BeginTs() Timestamp
  EndTs() Timestamp
  Type() MsgType
  Marshal(*TsMsg) []byte
  Unmarshal([]byte) *TsMsg
}

type MsgPack struct {
  BeginTs Timestamp
  EndTs Timestamp
  Msgs []*TsMsg
}


type MsgStream interface {
  Produce(*MsgPack) error
  Broadcast(*MsgPack) error
  Consume() *MsgPack // message can be consumed exactly once
}

type RepackFunc(msgs []* TsMsg, hashKeys [][]int32) map[int32] *MsgPack

type PulsarMsgStream struct {
  client *pulsar.Client
  repackFunc RepackFunc
  producers []*pulsar.Producer
  consumers []*pulsar.Consumer
  unmarshal *UnmarshalDispatcher
}

func (ms *PulsarMsgStream) CreatePulsarProducers(topics []string)
func (ms *PulsarMsgStream) CreatePulsarConsumers(subname string, topics []string, unmarshal *UnmarshalDispatcher)
func (ms *PulsarMsgStream) SetRepackFunc(repackFunc RepackFunc)
func (ms *PulsarMsgStream) Produce(msgs *MsgPack) error
func (ms *PulsarMsgStream) Broadcast(msgs *MsgPack) error
func (ms *PulsarMsgStream) Consume() (*MsgPack, error)
func (ms *PulsarMsgStream) Start() error
func (ms *PulsarMsgStream) Close() error

func NewPulsarMsgStream(ctx context.Context, pulsarAddr string) *PulsarMsgStream


type PulsarTtMsgStream struct {
  client *pulsar.Client
  repackFunc RepackFunc
  producers []*pulsar.Producer
  consumers []*pulsar.Consumer
  unmarshal *UnmarshalDispatcher
  inputBuf []*TsMsg
  unsolvedBuf []*TsMsg
  msgPacks []*MsgPack
}

func (ms *PulsarTtMsgStream) CreatePulsarProducers(topics []string)
func (ms *PulsarTtMsgStream) CreatePulsarConsumers(subname string, topics []string, unmarshal *UnmarshalDispatcher)
func (ms *PulsarTtMsgStream) SetRepackFunc(repackFunc RepackFunc)
func (ms *PulsarTtMsgStream) Produce(msgs *MsgPack) error
func (ms *PulsarTtMsgStream) Broadcast(msgs *MsgPack) error
func (ms *PulsarTtMsgStream) Consume() *MsgPack //return messages in one time tick
func (ms *PulsarTtMsgStream) Start() error
func (ms *PulsarTtMsgStream) Close() error

func NewPulsarTtMsgStream(ctx context.Context, pulsarAddr string) *PulsarTtMsgStream

type MarshalFunc func(*TsMsg) []byte
type UnmarshalFunc func([]byte) *TsMsg


type UnmarshalDispatcher struct {
	tempMap map[ReqType]UnmarshalFunc 
}

func (dispatcher *MarshalDispatcher) Unmarshal([]byte) *TsMsg
func (dispatcher *MarshalDispatcher) AddMsgTemplate(msgType MsgType, marshal MarshalFunc)
func (dispatcher *MarshalDispatcher) addDefaultMsgTemplates()

func NewUnmarshalDispatcher() *UnmarshalDispatcher

A.4 Time Ticked Flow Graph

A.4.1 Flow Graph States

type flowGraphStates struct {
  startTick Timestamp
  numActiveTasks map[string]int32
  numCompletedTasks map[string]int64
}

A.4.2 Message

type Msg interface {
  TimeTick() Timestamp
  SkipThisTick() bool
  DownStreamNodeIdx() int32
}

type SkipTickMsg struct {}
func (msg *SkipTickMsg) SkipThisTick() bool // always return true

A.4.3 Node

type Node interface {
  Name() string
  MaxQueueLength() int32
  MaxParallelism() int32
  SetPipelineStates(states *flowGraphStates)
  Operate([]*Msg) []*Msg
}

type baseNode struct {
  Name string
  maxQueueLength int32
  maxParallelism int32
  graphStates *flowGraphStates
}
func (node *baseNode) MaxQueueLength() int32
func (node *baseNode) MaxParallelism() int32
func (node *baseNode) SetMaxQueueLength(n int32)
func (node *baseNode) SetMaxParallelism(n int32)
func (node *baseNode) SetPipelineStates(states *flowGraphStates)

A.4.4 Flow Graph

type nodeCtx struct {
  node *Node
  inputChans [](*chan *Msg)
  outputChans [](*chan *Msg)
  inputMsgs [](*Msg List)
  downstreams []*nodeCtx
  skippedTick Timestamp
}

func (nodeCtx *nodeCtx) Start(ctx context.Context) error

Start() will enter a loop. In each iteration, it tries to collect input messges from inputChan, then prepare node's input. When input is ready, it will trigger node.Operate. When node.Operate returns, it sends the returned Msg to outputChans, which connects to the downstreams' inputChans.

type TimeTickedFlowGraph struct {
  states *flowGraphStates
  nodeCtx map[string]*nodeCtx
}

func (*pipeline TimeTickedFlowGraph) AddNode(node *Node)
func (*pipeline TimeTickedFlowGraph) SetEdges(nodeName string, in []string, out []string)
func (*pipeline TimeTickedFlowGraph) Start() error
func (*pipeline TimeTickedFlowGraph) Close() error

func NewTimeTickedFlowGraph(ctx context.Context) *TimeTickedFlowGraph

A.5 ID Allocator

type IdAllocator struct {
}

func (allocator *IdAllocator) Start() error
func (allocator *IdAllocator) Close() error
func (allocator *IdAllocator) Alloc(count uint32) ([]int64, error)

func NewIdAllocator(ctx context.Context) *IdAllocator

A.6 Timestamp Allocator

A.6.1 Timestamp

Let's take a brief review of Hybrid Logical Clock (HLC). HLC uses 64bits timestamps which are composed of a 46-bits physical component (thought of as and always close to local wall time) and a 18-bits logical component (used to distinguish between events with the same physical component).

HLC's logical part is advanced on each request. The phsical part can be increased in two cases:

A. when the local wall time is greater than HLC's physical part,

B. or the logical part overflows.

In either cases, the physical part will be updated, and the logical part will be set to 0.

Keep the physical part close to local wall time may face non-monotonic problems such as updates to POSIX time that could turn time backward. HLC avoids such problems, since if 'local wall time < HLC's physical part' holds, only case B is satisfied, thus montonicity is guaranteed.

Milvus does not support transaction, but it should gurantee the deterministic execution of the multi-way WAL. The timestamp attached to each request should

• have its physical part close to wall time (has an acceptable bounded error, a.k.a. uncertainty interval in transaction senarios),
• and be globally unique.

HLC leverages on physical clocks at nodes that are synchronized using the NTP. NTP usually maintain time to within tens of milliseconds over local networks in datacenter. Asymmetric routes and network congestion occasionally cause errors of hundreds of milliseconds. Both the normal time error and the spike are acceptable for Milvus use cases.

The interface of Timestamp is as follows.

type timestamp struct {
  physical uint64 // 18-63 bits
  logical uint64  // 0-17 bits
}

type Timestamp uint64

A.6.2 Timestamp Oracle

type timestampOracle struct {
  client *etcd.Client // client of a reliable meta service, i.e. etcd client
  rootPath string // this timestampOracle's working root path on the reliable kv service
  saveInterval uint64
  lastSavedTime uint64
  tso Timestamp // monotonically increasing timestamp
}

func (tso *timestampOracle) GetTimestamp(count uint32) ([]Timestamp, error)

func (tso *timestampOracle) saveTimestamp() error
func (tso *timestampOracle) loadTimestamp() error

A.6.3 Timestamp Allocator

type TimestampAllocator struct {}

func (allocator *TimestampAllocator) Start() error
func (allocator *TimestampAllocator) Close() error
func (allocator *TimestampAllocator) Alloc(count uint32) ([]Timestamp, error)

func NewTimestampAllocator() *TimestampAllocator

• Batch Allocation of Timestamps

• Expiration of Timestamps

A.7 KV

A.7.1 KV Base

type KVBase interface {
	Load(key string) (string, error)
    MultiLoad(keys []string) ([]string, error)
	Save(key, value string) error
    MultiSave(kvs map[string]string) error
	Remove(key string) error

    MultiRemove(keys []string) error
    MultiSaveAndRemove(saves map[string]string, removals []string) error

	Watch(key string) clientv3.WatchChan
	WatchWithPrefix(key string) clientv3.WatchChan
	LoadWithPrefix(key string) ( []string, []string, error)
}

• MultiLoad(keys []string) Load multiple kv pairs. Loads are done transactional.
• MultiSave(kvs map[string]string) Save multiple kv pairs. Saves are done transactional.
• MultiRemove(keys []string) Remove multiple kv pairs. Removals are done transactional.

A.7.2 Etcd KV

type EtcdKV struct {
	client   *clientv3.Client
	rootPath string
}

func NewEtcdKV(etcdAddr string, rootPath string) *EtcdKV

EtcdKV implements all KVBase interfaces.