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Why not curl?

Something like this:

while ! curl http://$POSTGRES_PORT_5432_TCP_ADDR:$POSTGRES_PORT_5432_TCP_PORT/ 2>&1 | grep '52'
do
sleep 1
done

It works for me.

Problem with your solution tiziano is that curl is not installed by default and i wanted to avoid installing additional stuff. Anyway i did what bereal said. Here is the script if anyone would need it.

import socket
import time
import os


port = int(os.environ["DB_PORT"]) # 5432


s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
while True:
try:
s.connect(('myproject-db', port))
s.close()
break
except socket.error as ex:
time.sleep(0.1)

If you have psql you could simply add the following code to your .sh file:

RETRIES=5


until psql -h $PG_HOST -U $PG_USER -d $PG_DATABASE -c "select 1" > /dev/null 2>&1 || [ $RETRIES -eq 0 ]; do
echo "Waiting for postgres server, $((RETRIES--)) remaining attempts..."
sleep 1
done

This will successfully wait for Postgres to start. (Specifically line 6). Just replace npm start with whatever command you'd like to happen after Postgres has started.

services:
practice_docker:
image: dockerhubusername/practice_docker
ports:
- 80:3000
command: bash -c 'while !</dev/tcp/db/5432; do sleep 1; done; npm start'
depends_on:
- db
environment:
- DATABASE_URL=postgres://postgres:password@db:5432/practicedocker
- PORT=3000
db:
image: postgres
environment:
- POSTGRES_USER=postgres
- POSTGRES_PASSWORD=password
- POSTGRES_DB=practicedocker

The simplest solution is a short bash script:

while ! nc -z HOST PORT; do sleep 1; done;
./run-smth-else;

wait-for-it small wrapper scripts which you can include in your application’s image to poll a given host and port until it’s accepting TCP connections.

can be cloned in Dockerfile by below command

RUN git clone https://github.com/vishnubob/wait-for-it.git

docker-compose.yml

version: "2"
services:
web:
build: .
ports:
- "80:8000"
depends_on:
- "db"
command: ["./wait-for-it/wait-for-it.sh", "db:5432", "--", "npm",  "start"]
db:
image: postgres

I've spent some hours investigating this problem and I got a solution. Docker depends_on just consider service startup to run another service. Than it happens because as soon as db is started, service-app tries to connect to ur db, but it's not ready to receive connections. So you can check db health status in app service to wait for connection. Here is my solution, it solved my problem. :) Important: I'm using docker-compose version 2.1.

version: '2.1'


services:
my-app:
build: .
command: su -c "python manage.py runserver 0.0.0.0:8000"
ports:
- "8000:8000"
depends_on:
db:
condition: service_healthy
links:
- db
volumes:
- .:/app_directory


db:
image: postgres:10.5
ports:
- "5432:5432"
volumes:
- database:/var/lib/postgresql/data
healthcheck:
test: ["CMD-SHELL", "pg_isready -U postgres"]
interval: 5s
timeout: 5s
retries: 5


volumes:
database:

In this case it's not necessary to create a .sh file. I hope it helps you guys ;) cya

If the backend application itself has a PostgreSQL client, you can use the pg_isready command in an until loop. For example, suppose we have the following project directory structure,

.
├── backend
│   └── Dockerfile
└── docker-compose.yml

with a docker-compose.yml

version: "3"
services:
postgres:
image: postgres
backend:
build: ./backend

and a backend/Dockerfile

FROM alpine
RUN apk update && apk add postgresql-client
CMD until pg_isready --username=postgres --host=postgres; do sleep 1; done \
&& psql --username=postgres --host=postgres --list

where the 'actual' command is just a psql --list for illustration. Then running docker-compose build and docker-compose up will give you the following output:

Note how the result of the psql --list command only appears after pg_isready logs postgres:5432 - accepting connections as desired.

By contrast, I have found that the nc -z approach does not work consistently. For example, if I replace the backend/Dockerfile with

FROM alpine
RUN apk update && apk add postgresql-client
CMD until nc -z postgres 5432; do echo "Waiting for Postgres..." && sleep 1; done \
&& psql --username=postgres --host=postgres --list

then docker-compose build followed by docker-compose up gives me the following result:

That is, the psql command throws a FATAL error that the database system is starting up.

In short, using an until pg_isready loop (as also recommended here) is the preferable approach IMO.

In your Dockerfile add wait and change your start command to use it:

ADD https://github.com/ufoscout/docker-compose-wait/releases/download/2.7.3/wait /wait
RUN chmod +x /wait


CMD /wait && npm start

Then, in your docker-compose.yml add a WAIT_HOSTS environment variable for your api service:

services:
api:
depends_on:
- postgres
environment:
- WAIT_HOSTS: postgres:5432


postgres:
image: postgres
ports:
- "5432:5432"

This has the advantage that it supports waiting for multiple services:

environment:
- WAIT_HOSTS: postgres:5432, mysql:3306, mongo:27017

For more details, please read their documentation.

#!/bin/sh


POSTGRES_VERSION=9.6.11
CONTAINER_NAME=my-postgres-container


# start the postgres container
docker run --rm \
--name $CONTAINER_NAME \
-e POSTGRES_PASSWORD=docker \
-d \
-p 5432:5432 \
postgres:$POSTGRES_VERSION


# wait until postgres is ready to accept connections
until docker run \
--rm \
--link $CONTAINER_NAME:pg \
postgres:$POSTGRES_VERSION pg_isready \
-U postgres \
-h pg; do sleep 1; done

Inspired by @tiziano answer and the lack of nc or pg_isready, it seems that in a recent docker python image (python:3.9 here) that curl is installed by default and I have the following check running in my entrypoint.sh:

postgres_ready() {
$(which curl) http://$DBHOST:$DBPORT/ 2>&1 | grep '52'
}


until postgres_ready; do
>&2 echo 'Waiting for PostgreSQL to become available...'
sleep 1
done
>&2 echo 'PostgreSQL is available.'

I have managed to solve my issue by adding health check to docker-compose definition.

  db:
image: postgres:latest
ports:
- 5432:5432
healthcheck:
test: "pg_isready --username=postgres && psql --username=postgres --list"
timeout: 10s
retries: 20

then in the dependent service you can check the health status:

  my-service:
image: myApp:latest
depends_on:
kafka:
condition: service_started
db:
condition: service_healthy

source: https://docs.docker.com/compose/compose-file/compose-file-v2/#healthcheck

Sleeping until pg_isready returns true unfortunately is not always reliable. If your postgres container has at least one initdb script specified, postgres restarts after it is started during it's bootstrap procedure, and so it might not be ready yet even though pg_isready already returned true.

What you can do instead, is to wait until docker logs for that instance return a PostgreSQL init process complete; ready for start up. string, and only then proceed with the pg_isready check.

Example:

start_postgres() {
docker-compose up -d --no-recreate postgres
}


wait_for_postgres() {
until docker-compose logs | grep -q "PostgreSQL init process complete; ready for start up." \
&& docker-compose exec -T postgres sh -c "PGPASSWORD=\$POSTGRES_PASSWORD PGUSER=\$POSTGRES_USER pg_isready --dbname=\$POSTGRES_DB" > /dev/null 2>&1; do
printf "\rWaiting for postgres container to be available ... "
sleep 1
done
printf "\rWaiting for postgres container to be available ... done\n"
}


start_postgres
wait_for_postgres

An example for Nodejs and Postgres api.

#!/bin/bash
#entrypoint.dev.sh
echo "Waiting for postgres to get up and running..."
while ! nc -z postgres_container 5432; do
# where the postgres_container is the hos, in my case, it is a Docker container.
# You can use localhost for example in case your database is running locally.
echo "waiting for postgress listening..."
sleep 0.1
done
echo "PostgreSQL started"


yarn db:migrate


yarn dev

# Dockerfile
FROM node:12.16.2-alpine


ENV NODE_ENV="development"


RUN mkdir -p /app


WORKDIR /app


COPY ./package.json ./yarn.lock ./


RUN yarn install


COPY . .


CMD ["/bin/sh", "./entrypoint.dev.sh"]

There are couple of solutions as other answers mentioned.

But don't make it complicated, just let it fail-fast combined with restart: on-failure. Your service will open connection to the db and may fail at the first time. Just let it fail. Docker will restart your service until it green. Keep your service simple and business-focused.

version: '3.7'


services:


postgresdb:
hostname: postgresdb
image: postgres:12.2
ports:
- "5432:5432"
environment:
- POSTGRES_USER=user
- POSTGRES_PASSWORD=secret
- POSTGRES_DB=Ceo


migrate:
image: hanh/migration
links:
- postgresdb
environment:
- DATA_SOURCE=postgres://user:secret@postgresdb:5432/Ceo
command: migrate sql --yes
restart: on-failure # will restart until it's success

Check out restart policies.

You can use the manage.py command "check" to check if the database is available (and wait 2 seconds if not, and check again). For instance, if you do this in your command.sh file before running the migration, Django has a valid DB connection while running the migration command:

...
echo "Waiting for db.."
python manage.py check --database default > /dev/null 2> /dev/null
until [ $? -eq 0 ];
do
sleep 2
python manage.py check --database default > /dev/null 2> /dev/null
done
echo "Connected."
# Migrate the last database changes
python manage.py migrate
...

PS: I'm not a shell expert, please suggest improvements.

If you want to run it with a single line command. You can just connect to the container and check if postgres is running

docker exec -it $DB_NAME bash -c "\
until psql -h $HOST -U $USER -d $DB_NAME-c 'select 1'>/dev/null 2>&1;\
do\
echo 'Waiting for postgres server....';\
sleep 1;\
done;\
exit;\
"
echo "DB Connected !!"

None of other solution worked, except for the following:

version : '3.8'
services :
postgres :
image : postgres:latest
environment :
- POSTGRES_DB=mydbname
- POSTGRES_USER=myusername
- POSTGRES_PASSWORD=mypassword
healthcheck :
test: [ "CMD", "pg_isready", "-q", "-d", "mydbname", "-U", "myusername" ]
interval : 5s
timeout : 5s
retries : 5
otherservice:
image: otherserviceimage
depends_on :
postgres:
condition: service_healthy

Thanks to this thread: https://github.com/peter-evans/docker-compose-healthcheck/issues/16

Trying with a lot of methods, Dockerfile, docker compose yaml, bash script. Only last of method help me: with makefile.

docker-compose up --build -d postgres
sleep 2
docker-compose up --build -d app