connect: cannot assign requested address

Question

I have the following error message from a Go app in Docker:

panic: failed to connect to `host=localhost user=postgres-dev database=dev`: dial error (dial tcp [::1]:5432: connect: cannot assign requested address)

which appear in the environment of the next Dockerfile and docker-compose.yml file:

FROM golang:latest

WORKDIR /WD

COPY go.mod go.sum ./

RUN go mod download

COPY . .

the docker-compose.yml file:

version: '3'
services:
    db:
        image: postgres:latest
        environment:
            POSTGRES_DB: dev
            POSTGRES_USER: postgres-dev
            POSTGRES_PASSWORD: [~secret~]
            ports: 5432:5432
    app:
        build: .
        command: ["./wait-for-it.sh", "db:5432", "--", "go", "run", "main.go"]
        volumes:
            - .:/WD
        ports:
            - "8000:8000"
        depends_on: 
            - db
        links: 
            - db

here the main.go file:

package main

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

    api "github.com/[placeholder]/[placeholder]/api"

    db "github.com/[placeholder]/[placeholder]/db"

    pgx "github.com/jackc/pgx/v4"
)

func main() {
    fmt.Println("Init")
    r := api.InitRoutes()

    conn, err := pgx.Connect(context.Background(), "postgresql://postgres-dev:[~secret~]@localhost:5432/dev")
    if err != nil {
        panic(err) // the error appears from this line.
    }

    dbInstance := &db.DbService{Conn: conn}

    dbInstance.Conn.Ping(context.Background())

    dbInstance.Migrate("/db/db.sql")

    http.ListenAndServe(":8000", r)
}

May this be helpful?

in console logs I found the next lines, which I think related to the problem:

db_1   | 2019-12-07 08:08:59.350 UTC [1] LOG:  starting PostgreSQL 12.1 (Debian 12.1-1.pgdg100+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 8.3.0-6) 8.3.0, 64-bit
db_1   | 2019-12-07 08:08:59.351 UTC [1] LOG:  listening on IPv4 address "0.0.0.0", port 5432
# read the next line:
db_1   | 2019-12-07 08:08:59.351 UTC [1] LOG:  listening on IPv6 address "::", port 5432 

the address of the database is :: with port 5432 and while the program try to connect to ::1 with port 5432, could this be the cause of the problem?

Answer 1

Within a container attatched to a bridge network (the default) localhost (127.0.0.1) is the container itself. So your app container is trying to access the database at port 5432 on itself (not on the host or on the db container). The easiest fix is to change the connection string from:

postgresql://postgres-dev:[~secret~]@localhost:5432/dev

to

postgresql://postgres-dev:[~secret~]@db:5432/dev

Note: I think you have a typo in your docker-compose.yml - ports: 5432:5432 is setting an environmental variable rather than mapping a port (note that this is not actually needed for app to talk to db if they are both on the same bridged network as is the case by default).

Note2: You should not need to use links in this case (this is a legacy feature).