Feedback on Docker or how to get system administrators and developers to agree
This application container management system has become standard practice for developers and system engineers in the space of just two years. Docker is now a major player and is widely used in cloud systems architectures.
So what is Docker?
Docker is a microcontainer management tool that uses libcontainer. Developed in Go by Frenchman Solomon Hykes, it became open source in 2013 and was quickly adopted by key accounts. The tool’s flexibility was a game changer.
Before its arrival, creating an application container required a mastery of relatively advanced concepts. LXC had already started to grab the lion’s share, giving “pure virtualization” solutions a run for their money. OpenVZ and Xen also played their roles. But those systems were mostly designed for server solutions and demanded considerable configuration work.
Let’s be clear: Docker is not a replacement for LXC, OpenVZ or Xen. And it’s not a virtualization solution the way that KVM, VirtualBox and VMWare are. Docker has another vision, another method of operation, and does not serve the exact same purposes.
Like OpenVZ, Xen and LXC, Docker uses the principle of rootfs, which is nothing more than a root file system. It uses that tree structure as the root for a remote system (like chroot would do) and offers a network layer and a set-up system. But it also has its share of differences.
First, its images and containers are layered using UnionFS. On the one hand, this saves disk space, but it also makes it possible to quickly build a container without copying an entire root.
The other difference is that Docker avoids the initialization phase of the guest system. In other words, the container’s root is only used as an environment for the targeted application.
Lastly, Docker comes standard with the ability to manage an image versioning registry. By default, the public registry is used. That registry offers a multitude of off-the-shelf images (either official images or images submitted by users in the community), as well as a private space that can be made available for a fee. In theory, Docker is quite similar to Git and its Hub could be compared to a service like GitHub. It also uses common concepts like commits, tags and a remote registry server.
The community has been active around the project, proposing tools for autostarts (fig, now Docker Compose) or for simplifying cloud integration and administration processes (CoreOS), monitoring tools (cAdvisor) and the list goes on.
Today, Docker is flooding the IT world. OpenStack, Amazon, Google, CoreOS and more: they’re all looking into this technology, if they haven’t already integrated it with their infrastructure.
But competition is coming!
Convenient for systems
Docker’s primary appeal is undoubtedly the ease of creating containers to manage microservices. A container is, first and foremost, a way to completely isolate an application. Thanks to its libcontainer library, Docker uses that memory and process isolation through the management of cgroups.
Docker has become very popular in the cloud for the low level of resources that it uses, its volume management and its UnionFS mode, which reduces the disk space needed. In studying how Docker works, you realize that it is very simple to create a scalable and/or high availability system.
The “docker” command is in fact a simple REST client that communicates with the daemon. By default, the service (daemon) creates a unix socket (/var/run/docker.sock) that supplies that API. The “docker” commands merely use the API.
The API can be used to listen to events like when a container is created, started or stopped. When you work with the information provided by this API, you can determine which container is running which service, on which port, etc.
If you don’t want to or are unable to use the structures required by CoreOS, OpenStack, etc., you may be able to create your own architecture and tools relatively easily. The API is fairly simple to access and highly effective.
Here is an example of architecture used in one of our projects:
- Each physical slave server has a discovery service that listens to the Docker socket.
- When a container starts or stops, the service sends the information to the master server.
The master server can then take appropriate action, such as by modifying the nginx configuration and deleting or removing an upstream server.
Another method can be set up by changing Docker’s configuration so it displays the API in TCP mode (making it network accessible). This allowed us to set up this architecture:
In this specific case, a small client connects to all the slave servers and listens to all their events. As for the rest, the principle is the same as above: each event will allow an upstream server to be added or deleted in nginx (for example).
The difference here is that the master must handle all the connections to all the slaves. The previous method is less costly, because the slaves are the ones connecting to the master.
In both cases, the primary server is notified each time a container starts or stops and can modify the primary nginx server, restart containers or prevent an incident.
It’s also certainly possible to set up two master servers for failover management. The solution is adaptable and easy to maintain.
Also convenient for developers
Docker is really suited for systems administrators. But it can also be seriously useful to production, i.e. for developers. One solution that we used for a customer was a combination of Dockerfiles and docker-compose files (for retrieval of the fig project by Docker).
The idea is to define what a developer’s workstation needs to run the project and then to create Dockerfiles (if needed) that will build images in line with the constraints, as well as a docker-compose.yml file linking the containers.
Then, when using a version control server (Git, Mercurial, SVN, etc.), you simply set up the project directory to include those files and the project’s source code and to specify the volumes to be fed to the containers. Next, the team retrieves the project, and the only command needed to start the services is “docker-compose up”.
Let’s take the example of a Drupal project. Two containers can be used:
- A MySQL container.
- A container with Apache + the PHP module.
Drupal’s source codes are placed in “/src” and sent to the Apache container. This is the principle of volume: a local directory or file on the host can be attached to one or more containers in a specific directory. You then do the same for the MySQL storage directory so as not to lose the records saved to the database.
The Dockerfile could for example be:
FROM debian:7 MAINTAINER firstname.lastname@example.org # Installation des paquets RUN apt-get update && \ apt-get install \ apache2 \ php5 \ php5-mysql \ libapache2-mod-php5 # Lancer apache CMD /usr/sbin/apache2ctl -D FOREGROUND
And the docker-compose.yml file:
web: dockerfile: . volumes: - "./src:/var/www/drupal" ports: - "8080:80" links: - "db" db: image: mysql volumes: - "./data:/var/lib/mysql"
The “web” service is linked to “db” (see the “links” directive in the above file), so it is possible to read environment variables that supply MySQL addresses and ports (presented in its Dockerfile).
Here for example, the following variables are accessible in the “web” container:
But resolved container names can also be used, and “web” and “db” are considered to be machine names in two containers.
In other words, the address “mysql://db” is resolved. You then just need to modify the Drupal configuration file to access the database and your work is done.
The project will have the following structure:
”data/” to store MySQL data.
”src/” containing Drupal’s source code.
”dockerfile” to create the Apache/PHP image.
”docker-compose.yml” which describes how to launch the containers.
Developers don’t necessarily have the rights to use port 80, so we map the local port 8080 to the Apache container’s port 80. The developer just needs to use http://127.0.0.1:8080 to see the Drupal instance.
To understand this connection, take a look at Figure 3.
Docker is clearly a technology suited to both development and system administration. Its contributions toward simplification and performance, as well as its adaptability, make it possible to easily set up services in a way that was previously inaccessible. Regardless of your infrastructure’s size, Docker has options for you.
But its strengths can also be put to use for production. Development is becoming standardized and the line between developers and systems is becoming ever finer. It remains to be seen how much evolution the next versions will present and what the competition will offer.