I'm trying to set up my basic Go app to make a connection to a Postgres database. I have both being spun up as services using docker-compose. When I run docker-compose up, I end up getting a timeout error when the Go app tries to connect to Postgres. Something is wrong or missing that is preventing my Go app from being able to make a connection to Postgres.

docker-compose.yml

services:
  api:
    build: .
    ports:
      - 8080:8080
    environment:
      - GOPATH=/go # set up GOPATH in container to reference modules
      - DB_USERNAME=${DB_USERNAME}  # this is `postgres`
      - DB_PASSWORD=${DB_PASSWORD}  # this is an empty string
    volumes:
      - $GOPATH/pkg/mod:/go/pkg/mod

  db:
    image: postgres:11.3
    ports:
      - 5432:5432
    volumes:
      - ../db/postgres/data:/var/lib/postgresql/data

main.go

import (
    "database/sql"
    "log"

    "github.com/lib/pq"
)

func main() {
    dbConn, err := sql.Open("postgres", "sslmode=disable host=db port=5432 user=postgres dbname=postgres connect_timeout=30")
    if err != nil {
        log.Fatalf("error defining connection to database: %v", err)
    }
    defer func() { _ = dbConn.Close() }()

    // This forces the connection to be created
    err = dbConn.Ping()
    if err != nil {
        log.Fatalf("error opening connection to database: %v", err)
    }
    log.Println("Never get here because we timeout...")
}

I expect to make the connection and get to the end of main.go. Instead, I get the following error: error opening connection to database: dial tcp <container-ip>:5432: i/o timeout

I've tried starting the Postgres container first (docker-compose up db), to make sure it was ready, then spinning up my Go app (docker-compose up api). Same error.

I've logged into the Postgres container and connected to Postgres by using the connection string, manually: psql "sslmode=disable host=localhost port=5432 user=postgres dbname=postgres connect_timeout=30" (notice that only the host field changed from db to localhost, as compared to the connection string used in the main.go code, above). This works, so the connection string is fine.

When logged in to the Postgres container, I've verified that there is a database named postgres that I use in the dbname field in the connection string:

postgres=# \l
                                 List of databases
   Name    |  Owner   | Encoding |  Collate   |   Ctype    |   Access privileges   
-----------+----------+----------+------------+------------+-----------------------
 postgres  | postgres | UTF8     | en_US.utf8 | en_US.utf8 | 
 template0 | postgres | UTF8     | en_US.utf8 | en_US.utf8 | =c/postgres          +
           |          |          |            |            | postgres=CTc/postgres
 template1 | postgres | UTF8     | en_US.utf8 | en_US.utf8 | =c/postgres          +
           |          |          |            |            | postgres=CTc/postgres
(3 rows)

I've also tried making my own database and using that in my connection string.

In the main.go code, above, I've also tried switching the sql.Open line out for the alternative approach:

c, err := pq.NewConnector("sslmode=disable host=db port=5432 user=postgres dbname=postgres connect_timeout=30")
dbConn = sql.OpenDB(c)

If I run my app with go run main.go (not running in a container) and make sure to switch the host to localhost in the Postgres connection string, it works fine. So, it's something to do with the communication between my app's container and the Postgres container.

0 Answers