插入第一个元素时为什么 collection.InsertOne 的速度较慢？

在使用 Go 的官方 MongoDB 驱动程序时，首次调用 collection.InsertOne 会出现明显延迟，而后续调用则非常迅速。这是因为该驱动程序使用延迟连接，首次调用时需要建立连接。可以通过使用 context.WithTimeout 强制连接来避免这种延迟。

我正在测试 go 的新官方 mongodb 驱动程序，我注意到第一次调用 collection.insertone 总是需要大量时间，而所有后续调用都非常快。为什么？如何避免这种破坏性行为？

package main

import (
    "context"
    "log"
    "time"

    "go.mongodb.org/mongo-driver/mongo"
    "go.mongodb.org/mongo-driver/mongo/options"
)

type trainer struct {
    name string
    age  int
    city string
}

func main() {

    t1 := time.now()

    // set client options
    clientoptions := options.client().applyuri("mongodb://localhost:27017")

    log.println("setting client options took", time.now().sub(t1))
    t1 = time.now()

    // connect to mongodb
    client, err := mongo.connect(context.todo(), clientoptions)
    if err != nil {
        log.fatal(err)
    }

    log.println("connecting took", time.now().sub(t1))
    t1 = time.now()

    // some dummy data to add to the database
    ash := trainer{"ash", 30, "pallet town"}

    // get a handle for your collection
    collection := client.database("test").collection("trainers")
    // insert a single document

    log.println("getting the collection took", time.now().sub(t1))
    t1 = time.now()

    for i := 0; i < 10; i++ {
        _, err := collection.insertone(context.todo(), ash)
        if err != nil {
            log.fatal(err)
        }
        log.println("inserting document took", time.now().sub(t1))
        t1 = time.now()
    }

    err = client.disconnect(context.todo())

}

我预计所有插入操作都需要毫秒或纳秒，而第一个插入操作大约需要 0.6 秒。这是带有计时的日志：

2019/07/31 17:41:39 Setting client options took 0s
2019/07/31 17:41:39 Connecting took 0s
2019/07/31 17:41:39 Getting the collection took 0s
2019/07/31 17:41:40 Inserting document took 606.0339ms
2019/07/31 17:41:40 Inserting document took 0s
2019/07/31 17:41:40 Inserting document took 0s
2019/07/31 17:41:40 Inserting document took 0s
2019/07/31 17:41:40 Inserting document took 0s
2019/07/31 17:41:40 Inserting document took 0s
2019/07/31 17:41:40 Inserting document took 875.2µs
2019/07/31 17:41:40 Inserting document took 0s
2019/07/31 17:41:40 Inserting document took 0s
2019/07/31 17:41:40 Inserting document took 0s

解决方案

只是为了结束评论中的问题：

mongodb 驱动程序使用 lazy connection。具体请参阅 MongoDB docs：

ctx, _ = context.WithTimeout(context.Background(), 2*time.Second)
err = client.Ping(ctx, readpref.Primary())

这将强制连接并消除第一次插入时的插入延迟。

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