使用docker-compose时如何从golang应用连接到Postgres？

Question

My docker-compose file

version: "2" 
services:   db:
   restart: always
   image: postgres:latest
   ports:
     - "5435:5432"
   environment:
      POSTGRES_PASSWORD: password
      POSTGRES_USER: user
      POSTGRES_DB: db   adminer:    
   web:
      image: golang:1.7
      working_dir: /go/src/app
      command: go run bot.go
      ports:
        - "3000:3000"
      volumes:
        - ./bot:/go/src/app
      links:
        - db
      environment:
      PORT: 3000
      CONNECTION_STRING_DEV: postgres://user:password@db/db

and my bot.go, where I try connect

db, err = sql.Open("postgres", "user=user password=password host=db dbname=db port=5432 sslmode=verify-full ")

When I bring up my containers, I see errors:

panic: dial tcp 5.61.14.99:5432: getsockopt: connection refused

I changed the port on 5432 and tried connect like this:

db, err = sql.Open("postgres", "postgres://user:password@db/db")

but I get the same errors

What's wrong with my docker-compose setup?

Answer 1

Your docker-compose looks a little messy but that's probably from copy and pasting. It's likely that postgres is not yet up and running when Go tries to connect. To test if that's the problem, first:

docker-compose up -d db

Then wait until postgres is ready by checking:

docker-compose logs -f db

and look out for a log line like:

db_1   | LOG:  database system is ready to accept connections

When that line appears, quit the log command (Ctrl+C) and run your bot:

docker-compose up web

If it is now working, that was indeed your problem.

Solution: Wait until postgres is ready. Easy ways to achieve this are:

• sleep for an amount of time (e.g. 1 min) before running web
• sleep inside web before connecting
• when connecting fails, sleep for 5 seconds and retry indefinitely

The disadvantage of these are that you don't know when postgres is ready, so you could wait too long or not long enough. A better solution is to run your bot only after a successful connection to postgres has been made.

Example from https://docs.docker.com/compose/startup-order/:

#!/bin/bash
# wait-for-postgres.sh

set -e

host="$1"
shift
cmd="$@"

until psql -h "$host" -U "postgres" -c '\l'; do
  >&2 echo "Postgres is unavailable - sleeping"
  sleep 1
done

>&2 echo "Postgres is up - executing command"
exec $cmd

Add this script as wait-for-postgres.sh and in you docker-compose.yml change the command for web like so:

command: ["./wait-for-postgres.sh", "db", "go", "run", "bot.go"]