dockerize

Utility to simplify running applications in docker containers.

dockerize is a utility to simplify running applications in docker containers. It allows you to:

generate application configuration files at container startup time from templates and container environment variables
Tail multiple log files to stdout and/or stderr
Wait for other services to be available using TCP, HTTP(S), unix before starting the main process.

The typical use case for dockerize is when you have an application that has one or more configuration files and you would like to control some of the values using environment variables.

For example, a Python application using Sqlalchemy might not be able to use environment variables directly. It may require that the database URL be read from a python settings file with a variable named SQLALCHEMY_DATABASE_URI. dockerize allows you to set an environment variable such as DATABASE_URL and update the python file when the container starts. In addition, it can also delay the starting of the python application until the database container is running and listening on the TCP port.

Another use case is when the application logs to specific files on the filesystem and not stdout or stderr. This makes it difficult to troubleshoot the container using the docker logs command. For example, nginx will log to /var/log/nginx/access.log and /var/log/nginx/error.log by default. While you can sometimes work around this, it's tedious to find a solution for every application. dockerize allows you to specify which logs files should be tailed and where they should be sent.

See A Simple Way To Dockerize Applications

Installation

Download the latest version in your container:

Docker Base Image

The jwilder/dockerize image is a base image based on gcr.io/distroless/static. dockerize is installed in the $PATH and can be used directly.

FROM jwilder/dockerize
...
ENTRYPOINT dockerize ...

Ubuntu Images

ENV DOCKERIZE_VERSION v0.8.0

RUN apt-get update \
    && apt-get install -y wget \
    && wget -O - https://github.com/jwilder/dockerize/releases/download/$DOCKERIZE_VERSION/dockerize-linux-amd64-$DOCKERIZE_VERSION.tar.gz | tar xzf - -C /usr/local/bin \
    && apt-get autoremove -yqq --purge wget && rm -rf /var/lib/apt/lists/*

For Alpine Images:

ENV DOCKERIZE_VERSION v0.8.0

RUN apk update --no-cache \
    && apk add --no-cache wget openssl \
    && wget -O - https://github.com/jwilder/dockerize/releases/download/$DOCKERIZE_VERSION/dockerize-alpine-linux-amd64-$DOCKERIZE_VERSION.tar.gz | tar xzf - -C /usr/local/bin \
    && apk del wget

Usage

dockerize works by wrapping the call to your application using the ENTRYPOINT or CMD directives.

This would generate /etc/nginx/nginx.conf from the template located at /etc/nginx/nginx.tmpl and send /var/log/nginx/access.log to STDOUT and /var/log/nginx/error.log to STDERR after running nginx, only after waiting for the web host to respond on tcp 8000:

CMD dockerize -template /etc/nginx/nginx.tmpl:/etc/nginx/nginx.conf -stdout /var/log/nginx/access.log -stderr /var/log/nginx/error.log -wait tcp://web:8000 nginx

Command-line Options

You can specify multiple templates by passing using -template multiple times:

$ dockerize -template template1.tmpl:file1.cfg -template template2.tmpl:file3

Templates can be generated to STDOUT by not specifying a dest:

$ dockerize -template template1.tmpl

Template may also be a directory. In this case all files within this directory are processed as template and stored with the same name in the destination directory. If the destination directory is omitted, the output is sent to STDOUT. The files in the source directory are processed in sorted order (as returned by ioutil.ReadDir).

$ dockerize -template src_dir:dest_dir

If the destination file already exists, dockerize will overwrite it. The -no-overwrite flag overrides this behaviour.

$ dockerize -no-overwrite -template template1.tmpl:file

You can tail multiple files to STDOUT and STDERR by passing the options multiple times.

$ dockerize -stdout info.log -stdout perf.log

If inotify does not work in your container, you can use -poll to poll for file changes instead.

$ dockerize -stdout info.log -stdout perf.log -poll

If your file uses {{ and }} as part of it's syntax, you can change the template escape characters using the -delims.

$ dockerize -delims "<%:%>"

Http headers can be specified for http/https protocols.

$ dockerize -wait http://web:80 -wait-http-header "Authorization:Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ=="

Waiting for other dependencies

It is common when using tools like Docker Compose to depend on services in other linked containers, however oftentimes relying on links is not enough - whilst the container itself may have started, the service(s) within it may not yet be ready - resulting in shell script hacks to work around race conditions.

Dockerize gives you the ability to wait for services on a specified protocol (file, tcp, tcp4, tcp6, http, https and unix) before starting your application:

$ dockerize -wait tcp://db:5432 -wait http://web:80 -wait file:///tmp/generated-file

Timeout

You can optionally specify how long to wait for the services to become available by using the -timeout # argument (Default: 10 seconds). If the timeout is reached and the service is still not available, the process exits with status code 1.

$ dockerize -wait tcp://db:5432 -wait http://web:80 -timeout 10s

See this issue for a deeper discussion, and why support isn't and won't be available in the Docker ecosystem itself.

Using Templates

Templates use Golang text/template. You can access environment variables within a template with .Env.

{{ .Env.PATH }} is my path

There are a few built-in functions as well:

default $var $default - Returns a default value for one that does not exist. {{ default .Env.VERSION "0.1.2" }}
contains $map $key - Returns true if a string is within another string
exists $path - Determines if a file path exists or not. {{ exists "/etc/default/myapp" }}
split $string $sep - Splits a string into an array using a separator string. Alias for strings.Split. {{ split .Env.PATH ":" }}
replace $string $old $new $count - Replaces all occurrences of a string within another string. Alias for strings.Replace. {{ replace .Env.PATH ":" }}
parseUrl $url - Parses a URL into it's protocol, scheme, host, etc. parts. Alias for url.Parse
atoi $value - Parses a string $value into an int. {{ if (gt (atoi .Env.NUM_THREADS) 1) }}
add $arg1 $arg - Performs integer addition. {{ add (atoi .Env.SHARD_NUM) -1 }}
isTrue $value - Parses a string $value to a boolean value. {{ if isTrue .Env.ENABLED }}
lower $value - Lowercase a string.
upper $value - Uppercase a string.
jsonQuery $json $query - Returns the result of a selection query against a json document.
loop - Create for loops.

jsonQuery

Objects and fields are accessed by name. Array elements are accessed by index in square brackets (e.g. [1]). Nested elements are separated by dots (.).

Examples:

With the following JSON in .Env.SERVICES

{
  "services": [
    {
      "name": "service1",
      "port": 8000,
    },{
      "name": "service2",
      "port": 9000,
    }
  ]
}

the template expression jsonQuery .Env.SERVICES "services.[1].port" returns 9000.

loop

loop allows for creating for loop within a template. It takes 1 to 3 arguments.

# Loop from 0...10
{{ range loop 10 }}
i = {{ . }}
{{ end }}

# Loop from 5...10
{{ range $i := loop 5 10 }}
i = {{ $i }}
{{ end }}

# Loop from 5...10 by 2
{{ range $i := loop 5 10 2 }}
i = {{ $i }}
{{ end }}