缘起

最近阅读<> (刘金亮, 2021.1)
本系列笔记拟采用golang练习之

Saga模式

saga模式将分布式长事务切分为一系列独立短事务
每个短事务是可通过补偿动作进行撤销的
事务动作和补动作偿都是幂等的, 允许重复执行而不会有副作用

Saga由一系列的子事务“Ti”组成，
每个Ti都有对应的补偿“Ci”，
当Ti出现问题时Ci用于处理Ti执行带来的问题。

可以通过下面的两个公式理解Saga模式。
T = T1 T2 … Tn
T = TCT

Saga模式的核心理念是避免使用长期持有锁（如14.2.2节介绍的两阶段提交）的长事务，
而应该将事务切分为一组按序依次提交的短事务，
Saga模式满足ACD（原子性、一致性、持久性）特征。

摘自 <> 刘金亮, 2021.1

目标

为实现saga模式的分布式事务, 先撸一个pub/sub事务消息队列服务
事务消息队列服务的功能性要求
- 消息不会丢失: 消息的持久化
- 消息的唯一性: 要求每个消息有全局ID和子事务ID
- 确保投递成功: 投递队列持久化, 投递状态持久化, 失败重试

子目标(Day 5)

重构和完善消息投递机制
- iMsgHeap: 使用待投递消息堆缓存消息. 总是优先投递创建时间最小的消息
- iMsgSource: 定义消息来源接口. 有两种消息来源, 1-数据库；2-eventbus
- iMsgHistoryRing: 使用ring buffer记录近期已投递成功的消息, 防止重复投递
- tConcurrentMsgHeap: 最小CreateTime优先的消息堆, 实现iMsgHeap接口, 并且是线程安全的.
- tDBMsgSource: 从数据库拉取待投递消息, 实现iMsgSource接口
- tLiveMsgSource: 监听eventbus即时推送的投递消息, 实现iMsgSource接口
- tMsgHistoryRing: 历史消息的固定大小环形队列, 实现iMsgHistoryRing接口, 缓存近期已投递成功的消息
- tDeliveryWorker:
  - 初始化时, 优先从数据库加载待投递消息
  - 使用iMsgHeap缓存待投递消息, 并确保有序
  - 使用iMsgSource接口, 分别从db和eventbus接收投递消息
  - 使用iMsgHistoryRing, 缓存已投递成功的消息, 防止重复投递

iMsgHeap.go

使用待投递消息堆缓存消息. 总是优先投递创建时间最小的消息

package delivery

import "learning/gooop/saga/mqs/models"

type iMsgHeap interface {
    Size() int
    IsEmpty() bool
    IsNotEmpty() bool

    Push(msg *models.QueuedMsg)
    Peek() *models.QueuedMsg
    Pop() *models.QueuedMsg
}

iMsgSource.go

定义消息来源接口. 有两种消息来源, 1-数据库；2-eventbus

package delivery

import "learning/gooop/saga/mqs/models"

type iMsgSource interface {
    MsgChan() <- chan *models.QueuedMsg
}

type tSourceExpireFunc func() bool

iMsgHistoryRing.go

使用ring buffer记录近期已投递成功的消息, 防止重复投递

package delivery

import "learning/gooop/saga/mqs/models"

type iMsgHistoryRing interface {
    Push(msg *models.QueuedMsg)
    Has(id int) bool
}

tConcurrentMsgHeap.go

最小CreateTime优先的消息堆, 实现iMsgHeap接口, 并且是线程安全的.

package delivery

import (
    "learning/gooop/saga/mqs/models"
    "sync"
)

type tConcurrentMsgHeap struct {
    items []*models.QueuedMsg
    size int
    mutex *sync.Mutex
}

func newMsgHeap() iMsgHeap {
    it := new(tConcurrentMsgHeap)
    it.init()
    return it
}

func (me *tConcurrentMsgHeap) init() {
    me.items = make([]*models.QueuedMsg, 0)
    me.size = 0
    me.mutex = new(sync.Mutex)
}

func (me *tConcurrentMsgHeap) Size() int {
    return me.size
}

func (me *tConcurrentMsgHeap) IsEmpty() bool {
    return me.size <= 0
}

func (me *tConcurrentMsgHeap) IsNotEmpty() bool {
    return !me.IsEmpty()
}


func (me *tConcurrentMsgHeap) has(msgID int) bool {
    for _,it := range me.items {
        if it.MsgID == msgID {
            return true
        }
    }
    return false
}

func (me *tConcurrentMsgHeap) Push(msg *models.QueuedMsg) {
    me.mutex.Lock()
    defer me.mutex.Unlock()

    if me.has(msg.MsgID) {
        return
    }

    me.ensureSize(me.size + 1)
    me.items[me.size] = msg
    me.size++

    me.shiftUp(me.size - 1)
}


func (me *tConcurrentMsgHeap) ensureSize(size int) {
    for ;len(me.items) < size; {
        me.items = append(me.items, nil)
    }
}

func (me *tConcurrentMsgHeap) parentOf(i int) int {
    return (i - 1) / 2
}

func (me *tConcurrentMsgHeap) leftChildOf(i int) int {
    return i*2 + 1
}

func (me *tConcurrentMsgHeap) rightChildOf(i int) int {
    return me.leftChildOf(i) + 1
}

func (me *tConcurrentMsgHeap) last() (i int, v *models.QueuedMsg) {
    if me.IsEmpty() {
        return -1, nil
    }

    i = me.size - 1
    v = me.items[i]
    return i,v
}

func (me *tConcurrentMsgHeap) shiftUp(i int) {
    if i <= 0 {
        return
    }
    v := me.items[i]

    pi := me.parentOf(i)
    pv := me.items[pi]

    if me.less(v, pv) {
        me.items[pi], me.items[i] = v, pv
        me.shiftUp(pi)
    }
}

func (me *tConcurrentMsgHeap) less(a, b *models.QueuedMsg) bool {
    return a.CreateTime < b.CreateTime
}

func (me *tConcurrentMsgHeap) Pop() *models.QueuedMsg {
    me.mutex.Lock()
    defer me.mutex.Unlock()

    if me.IsEmpty() {
        return nil
    }


    top := me.items[0]
    li, lv := me.last()
    me.items[0] = nil
    me.size--

    if me.IsEmpty() {
        return top
    }

    me.items[0] = lv
    me.items[li] = nil

    me.shiftDown(0)
    return top
}


func (me *tConcurrentMsgHeap) Peek() *models.QueuedMsg {
    me.mutex.Lock()
    defer me.mutex.Unlock()

    if me.IsEmpty() {
        return nil
    }

    return me.items[0]
}

func (me *tConcurrentMsgHeap) shiftDown(i int) {
    pv := me.items[i]
    ok, ci, cv := me.minChildOf(i)
    if ok && me.less(cv, pv) {
        me.items[i], me.items[ci] = cv, pv
        me.shiftDown(ci)
    }
}

func (me *tConcurrentMsgHeap) minChildOf(p int) (ok bool, i int, v *models.QueuedMsg) {
    li := me.leftChildOf(p)
    if li >= me.size {
        return false, 0, nil
    }
    lv := me.items[li]

    ri := me.rightChildOf(p)
    if ri >= me.size {
        return true, li, lv
    }
    rv := me.items[ri]

    if me.less(lv, rv) {
        return true, li, lv
    } else {
        return true, ri, rv
    }
}

tDBMsgSource.go

从数据库拉取待投递消息, 实现iMsgSource接口

package delivery

import (
    "github.com/jmoiron/sqlx"
    "learning/gooop/saga/mqs/database"
    "learning/gooop/saga/mqs/models"
    "time"
)

type tDBMsgSource struct {
    clientID string
    expireFunc tSourceExpireFunc
    msgChan chan *models.QueuedMsg
}

func newDBMsgSource(clientID string, expireFunc tSourceExpireFunc) iMsgSource {
    it := new(tDBMsgSource)
    it.init(clientID, expireFunc)
    return it
}

func (me *tDBMsgSource) init(clientID string, expireFunc tSourceExpireFunc) {
    me.clientID = clientID
    me.expireFunc = expireFunc
    me.msgChan = make(chan *models.QueuedMsg, 1)
    go me.beginPollDB()
}

func (me *tDBMsgSource) MsgChan() <- chan *models.QueuedMsg {
    return me.msgChan
}

func (me *tDBMsgSource) beginPollDB() {
    interval := time.Duration(1) * time.Second
    for !me.expireFunc() {
        if len(me.msgChan) <= 0 {
            ok, msg := me.poll()
            if ok {
                me.msgChan <- msg
                continue
            }
        }

        // poll failed, or chan full
        time.Sleep(interval)
    }

    close(me.msgChan)
}

func (me *tDBMsgSource) poll() (bool, *models.QueuedMsg) {
    msg := &models.QueuedMsg{}
    e := database.DB(func(db *sqlx.DB) error {
        rows, err := db.Queryx(
            "select * from delivery_queue where client_id=? order by create_time asc limit 1",
            me.clientID,
        )
        if err != nil {
            return err
        }

        if rows.Next() {
            err = rows.StructScan(msg)
            if err != nil {
                return err
            }
            return nil

        } else {
            return gEmptyRowsErr
        }
    })

    if e != nil {
        return false, nil
    } else {
        return true, msg
    }
}

tLiveMsgSource.go

监听eventbus即时推送的投递消息, 实现iMsgSource接口

package delivery

import (
    "fmt"
    "learning/gooop/saga/mqs/eventbus"
    "learning/gooop/saga/mqs/logger"
    "learning/gooop/saga/mqs/models"
    "learning/gooop/saga/mqs/models/events"
    "time"
)

type tLiveMsgSource struct {
    clientID string
    expireFunc tSourceExpireFunc
    msgChan chan *models.QueuedMsg
}

func newLiveMsgSource(clientID string, expireFunc tSourceExpireFunc) iMsgSource {
    it := new(tLiveMsgSource)
    it.init(clientID, expireFunc)
    return it
}

func (me *tLiveMsgSource) init(clientID string, expireFunc tSourceExpireFunc) {
    me.clientID = clientID
    me.expireFunc = expireFunc
    me.msgChan = make(chan *models.QueuedMsg, 1)

    eventbus.GlobalEventBus.Sub(events.MsgPublishedEvent,
        me.id(),
        me.handleMsgPublished)
    go me.beginWatchExpire()
}


func (me *tLiveMsgSource) id() string {
    return fmt.Sprintf("tLiveMsgSource.%s", me.clientID)
}

func (me *tLiveMsgSource) beginWatchExpire() {
    for range time.Tick(1 * time.Second) {
        if me.expireFunc() {
            me.afterExpired()
            return
        }
    }
}

func (me *tLiveMsgSource) afterExpired() {
    eventbus.GlobalEventBus.Unsub(events.MsgPublishedEvent, me.id())
    close(me.msgChan)
}

func (me *tLiveMsgSource) handleMsgPublished(_ string, args interface{}) {
    msg, ok := args.(*models.QueuedMsg)
    if !ok {
        return
    }

    if msg.ClientID != me.clientID {
        return
    }

    if len(me.msgChan) >= 0 {
        return
    }

    logger.Logf(
        "tLiveMsgSource.handleMsgPublished, clientID=%s, msg=%s/%s/%s",
        me.clientID, msg.GlobalID, msg.SubID, msg.Topic )
    me.msgChan <- msg
}

func (me *tLiveMsgSource) MsgChan() <- chan *models.QueuedMsg {
    return me.msgChan
}

tMsgHistoryRing.go

历史消息的固定大小环形队列, 实现iMsgHistoryRing接口, 缓存近期已投递成功的消息

package delivery

import "learning/gooop/saga/mqs/models"


type tMsgHistoryRing struct {
    items []*models.QueuedMsg
    capacity int
    index int
}

func newMsgHistoryRing(capacity int) iMsgHistoryRing {
    it := new(tMsgHistoryRing)
    it.init(capacity)
    return it
}

func (me *tMsgHistoryRing) init(capacity int) {
    me.items = make([]*models.QueuedMsg, capacity)
    me.capacity = capacity
    me.index = 0
}


func (me *tMsgHistoryRing) Has(id int) bool {
    for _,it := range me.items {
        if it != nil && it.ID == id {
            return true
        }
    }

    return false
}

func (me *tMsgHistoryRing) Push(msg *models.QueuedMsg) {
    me.items[me.index] = msg

    me.index++
    if me.index >= me.capacity {
        me.index = 0
    }
}

tDeliveryWorker.go

初始化时, 优先从数据库加载待投递消息
使用iMsgHeap缓存待投递消息, 并确保有序
使用iMsgSource接口, 分别从db和eventbus接收投递消息
使用iMsgHistoryRing, 缓存已投递成功的消息, 防止重复投递

package delivery

import (
    "bytes"
    "encoding/json"
    "errors"
    "github.com/jmoiron/sqlx"
    "io/ioutil"
    "learning/gooop/saga/mqs/database"
    "learning/gooop/saga/mqs/logger"
    "learning/gooop/saga/mqs/models"
    "net/http"
    "time"
)

type tDeliveryWorker struct {
    info        *tWorkerInfo
    successRing iMsgHistoryRing
    dbSource iMsgSource
    liveSource iMsgSource
    msgHeap iMsgHeap
}

func newDeliveryWorker(info *tWorkerInfo) *tDeliveryWorker {
    it := new(tDeliveryWorker)
    it.init(info)
    return it
}

// init: do initialization, and start initial load
func (me *tDeliveryWorker) init(info *tWorkerInfo) {
    me.info = info
    me.successRing = newMsgHistoryRing(64)

    me.dbSource = newDBMsgSource(info.ClientID, me.isExpired)
    me.liveSource = newLiveMsgSource(info.ClientID, me.isExpired)
    me.msgHeap = newMsgHeap()

    go me.beginInitialLoadFromDB()
}

// beginInitialLoadFromDB: initially, load queued msg from database
func (me *tDeliveryWorker) beginInitialLoadFromDB() {
    buf := [][]*models.QueuedMsg{ nil }
    for !me.isExpired() {
        err := database.DB(func(db *sqlx.DB) error {
            e, rows := me.loadFromDB(db)
            if e != nil {
                return e
            }

            buf[0] = rows
            return nil
        })

        if err != nil {
            logger.Logf("tDeliveryWorker.initialLoadFromDB, clientID=%s, err=%s", me.info.ClientID, err.Error())
            time.Sleep(3 * time.Second)
        } else {
            me.afterInitialLoad(buf[0])
        }
    }
}

// loadFromDB: load queued msg from database
func (me *tDeliveryWorker) loadFromDB(db *sqlx.DB) (error, []*models.QueuedMsg) {
    rows, err := db.Queryx(
        "select * from delivery_queue where client_id=? order by create_time asc limit ?",
        me.info.ClientID,
        gInitialLoadRows,
    )
    if err != nil {
        return err, nil
    }

    msgList := []*models.QueuedMsg{}
    for rows.Next() {
        msg := &models.QueuedMsg{}
        err = rows.StructScan(msg)
        if err != nil {
            return err, nil
        }
        msgList = append(msgList, msg)
    }

    return nil, msgList
}

// afterInitialLoad: after initial load done, push msgs into heap, and start delivery loop
func (me *tDeliveryWorker) afterInitialLoad(msgList []*models.QueuedMsg) {
    logger.Logf("tDeliveryWorker.afterInitialLoad, clientID=%s, rows=%d", me.info.ClientID, len(msgList))
    for _,it := range msgList {
        me.msgHeap.Push(it)
    }

    go me.beginPollAndDeliver()
}

// beginPollAndDeliver: poll msg from heap, and then deliver it
func (me *tDeliveryWorker) beginPollAndDeliver() {
    for !me.isExpired() {
        select {
        case msg := <- me.dbSource.MsgChan():
            me.msgHeap.Push(msg)
            break

        case msg := <- me.liveSource.MsgChan():
            me.msgHeap.Push(msg)
            break
        }

        if me.msgHeap.IsEmpty() {
            continue
        }

        msg := me.msgHeap.Pop()
        if msg == nil {
            continue
        }

        switch msg.StatusFlag {
        case 0:
            // 未处理的消息
            me.handleUndeliveredMsg(msg)
            break

        case 1:
            // 处理中的消息
            me.handleDeliveringMsg(msg)
            break
        }
    }
}

// isExpired: is me expired?
func (me *tDeliveryWorker) isExpired() bool {
    return time.Now().UnixNano() >= me.info.ExpireTime
}

// handleUndeliveredMsg: if msg unhandled, then try to deliver it
func (me *tDeliveryWorker) handleUndeliveredMsg(msg *models.QueuedMsg) {
    err := database.DB(func(db *sqlx.DB) error {
        now := time.Now().UnixNano()
        r,e := db.Exec(
            "update delivery_queue set status_flag=1, update_time=? where id=? and status_flag=0 and update_time=?",
            now,
            msg.ID,
            msg.UpdateTime,
        )
        if e != nil {
            return e
        }

        rows, e := r.RowsAffected()
        if e != nil {
            return e
        }
        if rows != 1 {
            return gOneRowsErr
        }

        msg.StatusFlag = 1
        msg.UpdateTime = now

        return nil
    })

    if err != nil {
        logger.Logf("tDeliveryWorker.handleNewMsg, id=%v, msg=%s/%s, err=%s", me.info.ClientID, msg.GlobalID, msg.SubID, err.Error())
        return
    }

    if me.deliver(msg) {
        me.afterDeliverySuccess(msg)

    } else {
        me.afterDeliveryFailed(msg)
    }
}

// deliver: use http.Post function to delivery msg
func (me *tDeliveryWorker) deliver(msg *models.QueuedMsg) bool {
    if me.successRing.Has(msg.ID) {
        return true
    }

    t := &models.TxMsg{
        GlobalID: msg.GlobalID,
        SubID: msg.SubID,
        Topic: msg.Topic,
        CreateTime: msg.CreateTime,
        Content: msg.Content,
    }
    j,e := json.Marshal(t)
    if e != nil {
        logger.Logf("tDeliveryWorker.deliver, failed json.Marshal, id=%v, msg=%s/%s", me.info.ClientID, msg.GlobalID, msg.SubID)
        return false
    }

    r, e := http.Post(me.info.NotifyURL, "application/json", bytes.NewReader(j))
    if e != nil {
        logger.Logf("tDeliveryWorker.deliver, failed http.Post, id=%v, msg=%s/%s", me.info.ClientID, msg.GlobalID, msg.SubID)
        return false
    }

    defer r.Body.Close()
    rep, e := ioutil.ReadAll(r.Body)
    if e != nil {
        logger.Logf("tDeliveryWorker.deliver, failed ioutil.ReadAll, id=%v, msg=%s/%s", me.info.ClientID, msg.GlobalID, msg.SubID)
        return false
    }

    m := &models.OkMsg{}
    e = json.Unmarshal(rep, m)
    if e != nil {
        logger.Logf("tDeliveryWorker.deliver, failed json.Unmarshal, id=%v, msg=%s/%s", me.info.ClientID, msg.GlobalID, msg.SubID)
        return false
    }

    if m.OK {
        return true
    } else {
        logger.Logf("tDeliveryWorker.deliver, failed OkMsg.OK, id=%v, msg=%s/%s", me.info.ClientID, msg.GlobalID, msg.SubID)
        return false
    }
}

// handleDeliveringMsg: if delivery timeout, then retry delivery
func (me *tDeliveryWorker) handleDeliveringMsg(msg *models.QueuedMsg) {
    now := time.Now().UnixNano()
    if msg.UpdateTime + gDeliveryTimeoutNanos > now {
        return
    }

    // delivery timeout
    me.afterDeliveryTimeout(msg)
}

// afterDeliverySuccess: if done, move msg to success queue
func (me *tDeliveryWorker) afterDeliverySuccess(msg *models.QueuedMsg) {
    if me.successRing.Has(msg.ID) {
        return
    }
    me.successRing.Push(msg)

    err := database.TX(func(db *sqlx.DB, tx *sqlx.Tx) error {
        r,e := db.Exec(
    "delete from delivery_queue where id=? and update_time=? and status_flag=1",
            msg.ID,
            msg.UpdateTime,
        )
        if e != nil {
            return e
        }

        rows, e := r.RowsAffected()
        if e != nil {
            return e
        }
        if rows != 1 {
            return gOneRowsErr
        }

        r, e = db.Exec(
            "insert into success_queue (msg_id, client_id, create_time) values(?, ?, ?)",
            msg.ID,
            msg.ClientID,
            time.Now().UnixNano(),
        )
        if e != nil {
            return e
        }

        rows, e = r.RowsAffected()
        if e != nil {
            return e
        }
        if rows != 1 {
            return gOneRowsErr
        }

        return nil
    })

    if err != nil {
        logger.Logf("tDeliveryWorker.afterDeliverySuccess, failed, id=%v, msg=%s/%s, err=%s", me.info.ClientID, msg.GlobalID, msg.SubID, err.Error())
    } else {
        logger.Logf("tDeliveryWorker.afterDeliverySuccess, done, id=%v, msg=%s/%s", me.info.ClientID, msg.GlobalID, msg.SubID)
    }
}


// afterDeliveryFailed: if failed, do nothing but just log it
func (me *tDeliveryWorker) afterDeliveryFailed(msg *models.QueuedMsg) {
    logger.Logf("tDeliveryWorker.afterDeliveryFailed, id=%v, msg=%s/%s", me.info.ClientID, msg.GlobalID, msg.SubID)
}

// afterDeliveryTimeout: if timeout, then reset status and retry
func (me *tDeliveryWorker) afterDeliveryTimeout(msg *models.QueuedMsg) {
    err := database.DB(func(db *sqlx.DB) error {
        r,e := db.Exec(
            "update delivery_queue set status_flag=0 where id=? and status_flag=1 and update_time=?",
            msg.ID,
            msg.UpdateTime,
        )
        if e != nil {
            return e
        }

        rows,e := r.RowsAffected()
        if e != nil {
            return e
        }

        if rows != 1 {
            return gOneRowsErr
        }

        return nil
    })

    if err != nil {
        logger.Logf("tDeliveryWorker.afterDeliveryTimeout, failed, id=%v, msg=%s/%s, err=%s", me.info.ClientID, msg.GlobalID, msg.SubID, err.Error())
    } else {
        logger.Logf("tDeliveryWorker.afterDeliveryTimeout, done, id=%v, msg=%s/%s", me.info.ClientID, msg.GlobalID, msg.SubID)
    }
}

var gEmptyRowsErr = errors.New("empty rows")
var gOneRowsErr = errors.New("expecting one row affected")
var gDeliveryTimeoutNanos = int64(10 * (time.Second / time.Nanosecond))
var gInitialLoadRows = 100

(未完待续)

手撸golang GO与微服务 Saga模式之5

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