milvus/docs/developer_guides/appendix_a_basic_components.md

## Appendix A. Basic Components

#### A.1 Watchdog

``` go
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

``` go
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

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

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

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



#### A.4 Time Ticked Flow Graph

###### A.4.1 Flow Graph States

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

###### A.4.2 Message

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

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

###### A.4.3 Node

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



```go
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

```go
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*.

```go
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

```go
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).

<img src="./figs/hlc.png" width=400>

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

```go
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

```go
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

```go
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

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

func NewEtcdKV(etcdAddr string, rootPath string) *EtcdKV
```

EtcdKV implements all *KVBase* interfaces.