利用mongo的findAndModify原子性操作实现auto increment ID

摘要：实际情况默认情况下，使用自动生成，而且在自带的命令里，无法指定一个自增字段。自增字段在多线程时必须是原子性的，这在大数据情况下很难实现伸缩性。而且，在里面，有一个命令是原子性的。代码如下无法打开创建个模拟多线程环境输出查询结果

默认情况下，mongo使用_id自动生成uniq id，而且在mongo自带的命令里，无法指定一个自增字段。自增字段在多线程时必须是原子性的，这在大数据情况下很难实现伸缩性(scalability)。

Generally in MongoDB, you would not use an auto-increment pattern for
the _id field, or any field, because it does not scale for databases
with large numbers of documents. Typically the default value ObjectId
is more ideal for the _id.

不过好消息是，_id不一定非得是ObjectId类型，它可以是任何类型。而且，在mongo里面，有一个findAndModify命令是原子性的。我们可以使用它实现auto increment ID。
在golang的mgo库里，Apply命令可以实现findAndModify功能

假设我们要把test表的_id字段设置为自增的，我们需要先创建一个counter表，这个表里面保存一个seq字段，通过在seq上使用Apply，获取新doc的seq值作为test表中下一个id的值。

package main

import (
    "fmt"
    "gopkg.in/mgo.v2"
    "gopkg.in/mgo.v2/bson"
    "log"
    "math/rand"
    "sync"
)

func init() {
    log.SetFlags(log.Lshortfile | log.LstdFlags)
}
func main() {
    session, err := mgo.Dial("usr:pwd@127.0.0.1:27017/dbname")
    if err != nil {
        log.Fatal("无法打开MongoDB！")
        return
    }
    defer session.Close()

    clt := session.DB("dbname").C("test")
    getNextSeq := func() func() int {
        counter := session.DB("dbname").C("counter")
        cid := "counterid"
        return func() int {
            change := mgo.Change{
                Update:    bson.M{"$inc": bson.M{"seq": 1}},
                Upsert:    true,
                ReturnNew: true,
            }
            doc := struct{ Seq int }{}
            if _, err := counter.Find(bson.M{"_id": cid}).Apply(change, &doc); err != nil {
                panic(fmt.Errorf("get counter failed:", err.Error()))
            }
            log.Println("seq:", doc)
            return doc.Seq
        }
    }()

    wg := sync.WaitGroup{}
    // 创建10个 go routine模拟多线程环境
    for i := 0; i < 10; i++ {
        wg.Add(1)
        go func(i int) {
            _row := map[string]interface{}{
                "_id":       getNextSeq(),
                "username":  fmt.Sprintf("name%2d", i),
                "telephone": fmt.Sprintf("tel%4d", rand.Int63n(1000))}
            if err := clt.Insert(_row); err != nil {
                log.Printf("insert %v failed: %v
", _row, err)
            } else {
                log.Printf("insert: %v success!
", _row)
            }
            wg.Done()
        }(i)
    }
    wg.Wait()
}

2016/04/28 17:05:28 main.go:37: seq: {201}
2016/04/28 17:05:28 main.go:54: insert: map[_id:201 username:name 9 telephone:tel 410] success!
2016/04/28 17:05:28 main.go:37: seq: {204}
2016/04/28 17:05:28 main.go:37: seq: {203}
2016/04/28 17:05:28 main.go:37: seq: {202}
2016/04/28 17:05:28 main.go:37: seq: {206}
2016/04/28 17:05:28 main.go:37: seq: {205}
2016/04/28 17:05:28 main.go:54: insert: map[_id:202 username:name 5 telephone:tel  51] success!
2016/04/28 17:05:28 main.go:54: insert: map[_id:204 username:name 2 telephone:tel 551] success!
2016/04/28 17:05:28 main.go:54: insert: map[telephone:tel 821 _id:203 username:name 1] success!
2016/04/28 17:05:28 main.go:37: seq: {207}
2016/04/28 17:05:28 main.go:54: insert: map[_id:205 username:name 6 telephone:tel 320] success!
2016/04/28 17:05:28 main.go:54: insert: map[_id:206 username:name 4 telephone:tel 937] success!
2016/04/28 17:05:28 main.go:54: insert: map[_id:207 username:name 3 telephone:tel 758] success!
2016/04/28 17:05:28 main.go:37: seq: {208}
2016/04/28 17:05:28 main.go:54: insert: map[username:name 7 telephone:tel 148 _id:208] success!
2016/04/28 17:05:28 main.go:37: seq: {209}
2016/04/28 17:05:28 main.go:54: insert: map[_id:209 username:name 0 telephone:tel 216] success!
2016/04/28 17:05:28 main.go:37: seq: {210}
2016/04/28 17:05:28 main.go:54: insert: map[_id:210 username:name 8 telephone:tel 449] success!

db.test.find({_id:{$gt:200}})
{ "_id" : 201, "username" : "name 9", "telephone" : "tel 410" }
{ "_id" : 202, "username" : "name 5", "telephone" : "tel  51" }
{ "_id" : 203, "username" : "name 1", "telephone" : "tel 821" }
{ "_id" : 204, "username" : "name 2", "telephone" : "tel 551" }
{ "_id" : 205, "username" : "name 6", "telephone" : "tel 320" }
{ "_id" : 206, "username" : "name 4", "telephone" : "tel 937" }
{ "_id" : 207, "username" : "name 3", "telephone" : "tel 758" }
{ "_id" : 208, "username" : "name 7", "telephone" : "tel 148" }
{ "_id" : 209, "username" : "name 0", "telephone" : "tel 216" }
{ "_id" : 210, "username" : "name 8", "telephone" : "tel 449" }