Error 1040: Too many connections

[codeisallaround]

I open the database connection once, and reuse it for all spawned goroutines (see code below).
Why do I get the "too many connection" error, as I opened only one connection?

(I read #112 and #111 , but I think I have a different issue.)

ApacheBench:

ab -k -c 3000 -n 3000 -k http://localhost:8080/test/entry

Code:

package main

import (
    "database/sql"
    "fmt"
    _ "github.com/go-sql-driver/mysql" // tested it with master and v. 1.2
    "log"
    "net/http"
    "time"
)

func main() {
    db, err := sql.Open("mysql", "root:@/test") 
    if err != nil {
        log.Fatal(err)
    }
    //db.SetMaxIdleConns(10000)
    defer db.Close()

    err = db.Ping()
    if err != nil {
        log.Fatal("DB not ready")
    }

    http.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
        wwwHandler(w, req, db)
    })

    // accepts new connections in an infinite loop and 
    // spawns new goroutines to handle them:
    http.ListenAndServe(":8080", nil)
}

func wwwHandler(w http.ResponseWriter, r *http.Request, db *sql.DB) {
    fmt.Fprintf(w, "%s: Hi there '%s'!", time.Now().Format(time.RFC3339), r.URL.Path[1:])
    gopher(db)
}

func gopher(db *sql.DB) {
    var sStmt string = "INSERT INTO data (name, acc_id, value) VALUES (?, 2, ?);"

    stmt, err := db.Prepare(sStmt)
    if err != nil {
        log.Fatal(err)
    }

    res, err := stmt.Exec(time.Now(), 1)
    if err != nil || res == nil {
        log.Fatal(err)
    }

    stmt.Close()
}

[julienschmidt]

First, sql.DB is not a connection, it is a handle for a database maintaining a pool of connections.
You never explicitly opened a connection. The database/sql package internally opens as much connections as are required.

Secondly, this is not how prepared statements are meant to be used. Prepare the statement once (in the main) and then only execute it. If you need a new statement every time, just use db.Exec instead. Preparing new statements can cause some trouble with the connection-scheduling.

Thirdly, this error is completely fine (from the driver and database/sql perspective). You did not set a maximum number of connections that can be used, so there is no guarantee that in a peek not more connections are used than the server can handle. You should set a limit corresponding to your server settings with http://golang.org/pkg/database/sql/#DB.SetMaxOpenConns