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# Architecture based on systemd, docker, haproxy, and some bash scripts
Our architecture revolves around a
[set of systemd unit files](https://github.com/indiehosters/indiehosters/tree/master/unit-files). They come in various types:
## Server-wide processes
The haproxy.* and postfix.* unit files correspond to two server wide processes. They run Docker containers from images in the
[server-wide/ folder of our dockerfiles repo](https://github.com/indiehosters/dockerfiles/tree/master/server-wide).
The haproxy-confd service unit starts configuration service for haproxy. It monitors `etcdctl ls /services` to see if any new backends were created, and updates the haproxy configuration files. The files lives in `/data/runtime/haproxy/` on the host sytem. It is required by the haproxy.* unit. That means that when you run `systemctl start haproxy`, and then run `docker ps` or `systemctl list-units`, you will see that systemd not only started the haproxy container, but also the haproxy-confd container.
There is currently no similar service for updating `/data/runtime/postfix/`, so you will have to update the configuration files in that folder manually, and then run `systemctl restart postfix`.
Etcd is configured in the `cloud-config` file. The `scripts/setup.sh` takes care of enabling and starting the haproxy and postfix service, and the haproxy-confd to listen for changes in the backends configuration in [etcd](https://github.com/coreos/etcd). New backends are automatically added to the haproxy configuration as soon as their private docker IP address is written into etcd.
## HAProxy backends: static, static-git, wordpress
A per user process is a haproxy backend for a specific domain name. For now, we have three applications available: static, static-git and wordpress. But we are working on adding more. Please, if you want, you can create more :)
You will notice there are also some other units in the `unit-files/` folder of this repository, like the gitpuller and mysql ones.
Whenever you start a wordpress unit, it requires a mysql service.
Whenever you start a static-git unit, it wants a static-git-puller unit.
In all three cases, an -importer unit and a -discovery unit are required.
This works through a
[`Requires=` directive](https://github.com/indiehosters/indiehosters/blob/0.1.0/unit-files/nginx@.service#L6-L7) which systemd interprets, so that if you start one service, its dependencies are also started (you can see that in `systemctl list-units`).
## Discovery
The `-discovery` units check find out the local IP address of the backend, checks if it is up by doing a `curl`, and if so, writes the IP address into etcd. The `haproxy-confd` service notices this, and update the haproxy config.
The `-import` units check if data exists, and if it doesn't, tries to import from the `/data/import` folder, and create initial data state, for instance by doing a git clone, untarring the content of a vanilla wp-content for wordpress installation.
Note that some initialization is also done by the Docker images themselves - for instance the wordpress image runs a [shell script](https://github.com/pierreozoux/tutum-docker-wordpress-nosql/blob/master/run-wordpress.sh) at container startup, that creates the initial mysql database if it didn't exist yet.
We are working on merging everything inside the docker image.
The `-gitpuller` unit is scheduled to run every 10 minutes by the .timer file. When it runs, it does a git pull to update the website content at one of the haproxy backends from the git repository mentioned in the GITURL file.
There is one important script you can run at your server. You can also run the commands they contain manually, then you just use them as a cheatsheet of how to [set up a new server](https://github.com/indiehosters/indiehosters/tree/master/scripts/setup.sh).
There are also deploy scripts which do the same from a jump box, so you can orchestrate multiple servers from one central vantage points. They are in the
[deploy/](https://github.com/indiehosters/indiehosters/tree/master/deploy)
folder of this repo, and they are the scripts referred to in the 'how to deploy a server' document. They basically run the scripts from the scripts/ folder over ssh.