如何在Docker容器之间建立连接

Question

I am having connection issues when I try to connect my goLang GORM service to a Docker Postgress container. I believe the problem is my golang code at the bottom at the connection string.

docker-compose up
Recreating postgress_postgre_1 ... done
Attaching to postgres
postgres   | 2018-12-11 21:08:48.283 UTC [1] LOG:  listening on IPv4 address "0.0.0.0", port 5432
postgres   | 2018-12-11 21:08:48.283 UTC [1] LOG:  listening on IPv6 address "::", port 5432
postgres   | 2018-12-11 21:08:48.291 UTC [1] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
postgres   | 2018-12-11 21:08:48.316 UTC [20] LOG:  database system was shut down at 2018-12-11 21:08:44 UTC
postgres   | 2018-12-11 21:08:48.328 UTC [1] LOG:  database system is ready to accept connections

=== when I run the golang I get... panic: failed to connect database

=============== docker-compose.yml

version: '3.6'
services:
  postgre:
    image: postgres:11.1-alpine
    ports:
      - '5432:5432'
    network_mode: bridge
    container_name: postgres

    environment:
      POSTGRES_USER: 'user'
      POSTGRES_PASSWORD: 'password'
      POSTGRESS_DB: 'db_amex01'
    volumes:
      - ./init:/docker-entrypoint-initdb.d/

==== main.go

package main

import (
    "fmt"
    "log"
    "net/http"

    "github.com/gorilla/mux"
    "github.com/jinzhu/gorm"

    // _ "github.com/jinzhu/gorm/dialects/sqlite"
    _ "github.com/jinzhu/gorm/dialects/postgres"
)

// _ "github.com/jinzhu/gorm/blob/master/dialects/postgres"

type ToDo struct {
    gorm.Model
    ID          int    `json:"id"`
    TASK_STRING string `json:"task_string"`
    DONE        bool   `json:"done"`
}

var db *gorm.DB
var err error

func main() {
    const (
        host     = "localhost"
        port     = 5432
        user     = "postgres"
        password = "password"
        dbname   = "db_amex01"
    )


// this is the problem I believe I am having...

    db, err := gorm.Open("postgres", "host='postgres' port=5432 user=user dbname='db_amex01' password='password'")



    defer db.Close()

    if err != nil {
        panic("failed to connect database")
    }

    defer db.Close()

Answer 1

There's a typo in your compose file, you name the service postgre but connect to postgres. Docker uses the service name for the DNS alias on a shared network, so this will break your connection attempt. To fix, just rename your service:

version: '3.6'
services:
  postgres:
    image: postgres:11.1-alpine
    ports:
      - '5432:5432'
    environment:
      POSTGRES_USER: 'user'
      POSTGRES_PASSWORD: 'password'
      POSTGRESS_DB: 'db_amex01'
    volumes:
      - ./init:/docker-entrypoint-initdb.d/

Answer 2

It appears as though you're discarding the error returned from gorm.Open() . To know exactly why gorm failed to open the connection, you'd need to output the error.

Instead of:

panic("failed to connect database")

Use:

panic("failed to connect database: " + err)

At a glance, it could be that your host is misconfigured

host='postgres' port=5432 user=user dbname='db_amex01' password='password'

Should be:

host=localhost port=5432 user=user dbname=db_amex01 password=password

But it's hard to tell without the error from gorm.Open()

One other thing to note, too; don't call db.Close() before first checking the error. It's possible that in the case of an error, db may be nil , causing calls to any of db 's methods to panic. You're also calling db.Close() twice in the code snippet you posted. Don't do this. Golang's documentation on io.Closer :

Closer is the interface that wraps the basic Close method.

The behavior of Close after the first call is undefined. Specific implementations may document their own behavior.

Edit (12/12/2018):

When I ran your code locally, the error I got from gorm was regarding SSL, because you're running your postgres server via docker-compose without any SSL configuration. You can add the sslmode=disable flag to your connection string to fix this issue.

Also there's a typo in your docker-compose.yml : POSTGRESS_DB should be POSTGRES_DB .

Here's a full working example that I got running locally:

docker-compose.yml :

version: '3.6'
services:
  postgres:
    image: postgres:11.1-alpine
    ports:
      - '5432:5432'
    environment:
      POSTGRES_USER: 'test_user'
      POSTGRES_PASSWORD: 'test_password'
      POSTGRES_DB: 'test_database'
    volumes:
      - ./init:/docker-entrypoint-initdb.d/

main.go :

package main

import (
    "fmt"
    "log"

    "github.com/jinzhu/gorm"
    _ "github.com/jinzhu/gorm/dialects/postgres"
)

const (
    host     = "localhost"
    port     = "5432"
    user     = "test_user"
    password = "test_password"
    dbname   = "test_database"
)

func main() {
    url := fmt.Sprintf("postgres://%s:%s@%s:%s/%s?sslmode=disable",
        user,
        password,
        host,
        port,
        dbname,
    )

    db, err := gorm.Open("postgres", url)
    if err != nil {
        log.Fatalf("error connecting to database: %v", err)
    }
    defer db.Close()

    if err := db.DB().Ping(); err != nil {
        log.Fatalf("error pinging database: %v", err)
    }

    fmt.Println("Success!")
}