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docker-compose-wait

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A small command-line utility to wait for other docker images to be started while using docker-compose (or Kubernetes or docker stack or whatever).

It permits waiting for:

Usage

This utility should be used in the docker build process and launched before your application starts.

For example, your application "MySuperApp" uses MongoDB, Postgres and MySql (wow!) and you want to be sure that, when it starts, all other systems are available, then simply customize your dockerfile this way:

## Use whatever base image
FROM alpine

## Add the wait script to the image
COPY --from=ghcr.io/ufoscout/docker-compose-wait:latest /wait /wait

## Otherwise you can directly download the executable from github releases. E.g.:
#  ADD https://github.com/ufoscout/docker-compose-wait/releases/download/2.11.0/wait /wait
#  RUN chmod +x /wait

## Add your application to the docker image
ADD MySuperApp.sh /MySuperApp.sh

## Launch the wait tool and then your application
CMD /wait && /MySuperApp.sh

Done! the image is ready.

Now let's modify the docker-compose.yml file:

version: "3"

services:
  mongo:
    image: mongo:3.4
    hostname: mongo
    ports:
      - "27017:27017"

  postgres:
    image: "postgres:9.4"
    hostname: postgres
    ports:
      - "5432:5432"

  mysql:
    image: "mysql:5.7"
    hostname: mysql
    ports:
      - "3306:3306"

  mySuperApp:
    image: "mySuperApp:latest"
    hostname: mySuperApp
    environment:
      WAIT_HOSTS: postgres:5432, mysql:3306, mongo:27017

When docker-compose is started (or Kubernetes or docker stack or whatever), your application will be started only when all the pairs host:port in the WAIT_HOSTS variable are available. The WAIT_HOSTS environment variable is not mandatory, if not declared, the script executes without waiting.

If you want to use the script directly in docker-compose.yml instead of the Dockerfile, please note that the command: configuration option is limited to a single command so you should wrap in a sh call. For example:

command: sh -c "/wait && /MySuperApp.sh"

This is discussed further here and here.

Usage in images that do not have a shell

When using distroless or building images FROM scratch, it is common to not have sh available. In this case, it is necessary to specify the command for wait to run explicitly. The invoked command will be invoked with any arguments configured for it and will completely replace the wait process in your container via a syscall to exec. Because there is no shell to expand arguments in this case, wait must be the ENTRYPOINT for the container and has to be specified in the exec form. Note that because there is no shell to perform expansion, arguments like * must be interpreted by the program that receives them.

FROM golang

COPY myApp /app
WORKDIR /app
RUN go build -o /myApp -ldflags '-s -w -extldflags -static' ./...

## ----------------

FROM scratch

COPY --from=ghcr.io/ufoscout/docker-compose-wait:latest /wait /wait

COPY --from=0 /myApp /myApp
ENV WAIT_COMMAND="/myApp arg1 argN..."
ENTRYPOINT ["/wait"]

Additional configuration options

The behaviour of the wait utility can be configured with the following environment variables:

Supported architectures

From release 2.11.0, the following executables are available for download:

All executables are built with MUSL for maximum portability.

To use any of these executables, simply replace the executable name in the download link: https://github.com/ufoscout/docker-compose-wait/releases/download/{{VERSION}}/{{executable_name}}

Docker images

Official docker images based on scratch can be found here: https://github.com/users/ufoscout/packages/container/package/docker-compose-wait

Using on other systems

The simplest way of getting the wait executable is to download it from

https://github.com/ufoscout/docker-compose-wait/releases/download/{{VERSION}}/wait

or to use one of the pre-built docker images.

If you need it for an architecture for which a pre-built file is not available, you should clone this repository and build it for your target.

As it has no external dependencies, an being written in the mighty rust programming language, the build process is just a simple cargo build --release (well... of course you need to install the rust compiler before...)

For everything involving cross-compilation, you should take a look at Cross.

For example, to build for a raspberry pi, everything you have to do is:

  1. Install the latest stable rust toolchain using rustup
  2. Correctly configure Docker on your machine
  3. Open a terminal and type:
cargo install cross
cross build --target=armv7-unknown-linux-musleabihf --release

Use your shiny new executable on your raspberry device!

Notes

This utility was explicitly written to be used with docker-compose; however, it can be used everywhere since it has no dependencies on docker.