How Docker and Ansible fit together to implement Continuous Delivery/Continuous Deployment

Question

I'm new to the configuration management and deployment tools. I have to implement a Continuous Delivery/Continuous Deployment tool for one of the most interesting projects I've ever put my hands on.

First of all, individually, I'm comfortable with AWS , I know what Ansible is, the logic behind it and its purpose. I do not have same level of understanding of Docker but I got the idea. I went through a lot of Internet resources, but I can't get the the big picture.

What I've been struggling is how they fit together. Using Ansible , I can manage my Infrastructure as Code; building EC2 instances, installing packages... I can even deploy a full application by pulling its code, modify config files and start web server. Docker is, itself, a tool that packages an application and ensures that it can be run wherever you deploy it.

My problems are:

How does Docker (or Ansible and Docker) extend the Continuous Integration process!?

Suppose we have a source code repository, the team members finish working on a feature and they push their work. Jenkins detects this, runs all the acceptance/unit/integration test suites and if they all passed, it declares it as a stable build. How Docker fits here? I mean when the team pushes their work, does Jenkins have to pull the Docker file source coded within the app, build the image of the application, start the container and run all the tests against it or it runs the tests the classic way and if all is good then it builds the Docker image from the Docker file and saves it in a private place? Should Jenkins tag the final image using xyz for example!?

Docker containers configuration :

Suppose we have an image built by Jenkins stored somewhere, how to handle deploying the same image into different environments, and even, different configurations parameters ( Vhosts config, DB hosts, Queues URLs, S3 endpoints, etc...) What is the most flexible way to deal with this issue without breaking Docker principles? Are these configurations backed in the image when it gets build or when the container based on it is started, if so how are they injected?

Ansible and Docker :

Ansible provides a Docker module to manage Docker containers. Assuming I solved the problems mentioned above, when I want to deploy a new version xtz of my app, I tell Ansible to pull that image from where it was stored on, start the app container, so how to inject the configuration settings!? Does Ansible have to log in the Docker image, before it's running ( this sounds insane to me ) and use its Jinja2 templates the same way with a classic host!? If not, how is this handled?!

Excuse me if it was a long question or if I misspelled something, but this is my thinking out loud. I'm blocked for the past two weeks and I can't figure out the correct workflow. I want this to be a reference for future readers.

Please, it would very helpful to read your experiences and solutions because this looks like a common workflow.

Answer 1

I would like to answer in parts

How does Docker (or Ansible and Docker) extend the Continuous Integration process!?

Since docker images same everywhere, you use your docker images as if they are production images. Therefore, when somebody committed a code, you build your docker image. You run tests against it. When all tests pass, you tag that image accordingly. Since docker is fast, this is a feasible workflow. Also docker changes are incremental; therefore, your images will have minimal impact on storage. Also when your tests fail, you may also choose to save that image too. In this way, developer will pull that image and investigate easily why your tests failed. Developer may choose to run tests in their machine too since docker images in jenkins and their machine are not different.

What this brings that all developers will have same environment, same version of all software since you decide which one will be used in docker images. I have come across to bugs that are due to differences between developer machines. For example in the same operating system, unicode settings may affect your code. But in docker images all developers will test against same settings, same version software.

Docker containers configuration :

If you are using a private repository, and you should use one, then configuration changes will not affect hard disk space much. Therefore except security configurations, such as db passwords, you can apply configuration changes to docker images( Baking the Configuration into the Container ). Then you can use ansible to apply not-stored configurations to deployed images before/after startup using environment variables or Docker Volumes.

https://dantehranian.wordpress.com/2015/03/25/how-should-i-get-application-configuration-into-my-docker-containers/

Does Ansible have to log in the Docker image, before it's running ( this sounds insane to me ) and use its Jinja2 templates the same way with a classic host!? If not, how is this handled?!

No, ansible will not log in the Docker image, but ansible with Jinja2 templates can be used to change dockerfile. You can change dockerfile with templates and can inject your configuration to different files. Tag your files accordingly and you have configured images to spin up.

Answer 2

Regarding your question about handling multiple environment configurations using the same Docker image, I have been planning on using a Service Discovery tool like Consul as a centralized config/property management tool. So, when you start your container up, you set an ENV var that tells it what application it is (appID), and what environment config it should use (ex: MyApplication:Dev) and it will pull its config from Consul at startup. I still have to investigate the security around Consul (as if we are storing DB connection credentials in there for example, how do we restrict who can query/update those values). I don't want to just use this for containers, but all apps in general. Another cool capability is to change the config value in Consul and have a hook back into your app to apply the changes immediately (maybe like a REST endpoint on your app to push changes down to and dynamically apply it). Of course your app has to be written to support this!

You might be interested in checking out Martin Fowler's blog articles on immutable infrastructure and on Phoenix servers .

Answer 3

Although not a complete solution, I have suggestions for two of your issues. Although they might not be perfect, these are the practices we are using in our workflow, and prove themselves so far.

1. Defining different environments - supposing you've written a different Ansible role for each environment you launch, we define an environment variable setting the environment we wish the container to belong to. We then download the suitable configuration file from an S3 bucket using the env variable set before into the container (which should be possible if you supply AWS creds or give your server an IAM role) and inject these parameters into the code when building it.

2. Ansible doesn't need to log into the docker app, but the solution is a bit tricky. I've tried two ways of tackling this problem, and both aren't ideal. The first one is to download the configuration file as part of the docker image command line, and build the app on container startup. While this solution works - it breaches the Docker philosophy and makes the image highly prone to build errors. Another solution is pushing several images to your docker hub repo, and then pulling the appropriate image according to the environment at hand.

In a broader stroke, I've tried launching our app completely with Ansible and it was hell, many configuration steps are tricky and get trickier when you try to implement them as a playbook. When I switched to maintaining the severs alone with Ansible, and deploying the app itself with Docker things got a lot easier.