30 January 2016

While I work less and less on down-to-earth development, there are times when I get a chance to hack something. In this post I’ll describe a devtest deployment system I got a chance to be work on.

In a system we are developing we wanted to do a persistent deployment of the whole system to ECS on each branch repository push where each subservice would be accessible as <branch>-<service>.dev.example.com1. This would make it easy for developer to verify integration tests before merging (pull request) to master2, to show their work to other developers and to allow non-developer stakeholders easy access to features as they are being developed.

Testing pipeline

Flow of each push through the CI. Integration tests are run against a deployment on ECS.

There are multiple ways to accomplish this such as using dynamic DNS updates, ELBs and so on, and after some discussions and testing we settled on the following setup.

  1. There is a separate frontend deployment that consists of etcd and nginx servers.

  2. Each branch deployment service registers itself to the frontend by addings its address to etcd registry with its branch and service names.

  3. A watcher process on nginx container notices changes to etcd registry and updates nginx configuration so that the branch-service.dev address gets proxied to the correct docker container.

Request processing in deployment

Request processing with multiple branch deployments. Each service registers its address and port to etcd registry.

Registration is done during service startup and is also pretty simple (REGISTRY_URL, SERVICE, NAME and PORT are passed as Docker environment variables, ip is fetched from EC2 metadata service):

if [ -n "$REGISTRY_URL" -a -n "$PORT" -a -n "$SERVICE" -a -n "$NAME" ]; then
    echo "INFO: Registering to $REGISTRY_URL as $NAME/$SERVICE at $ip:$PORT" >&2
    now=$(date -u +"%Y-%m-%dT%H:%M:%SZ")
    $curl $url/ip -XPUT -d value=$ip >/dev/null || exit 1
    $curl $url/port -XPUT -d value=$PORT >/dev/null || exit 1
    $curl $REGISTRY_URL/v2/keys/deployment/$NAME/created -XPUT -d value=$now >/dev/null || exit 1

Similarly when the service is shut down it’ll issue DELETE on its keys to unregister itself.

The nginx container is based on the standard docker registry nginx container, but it will start a “regenerate” process alongside the actual nginx server. It uses etcdwatch to keep track of registry changes and then finally a separate regenerate.py script takes the registry contents and updates the nginx configuration file3.

if [ -n "$REGISTRY_URL" ]; then
    echo "Registry is at $REGISTRY_URL" >&2
    echo "Starting etcdwatch to generate configuration files" >&2
    etcdwatch -u $REGISTRY_URL -d /deployment -- sh -c './regenerate.py && nginx -s reload && echo `date -u`: Regenerated configuration' &

Now some caveats. THIS IS NOT FOR PRODUCTION USE. We use this only for devtest deployments. This setup is meant to make it easy and straighforward to test and verify things during development, even when working on incomplete and definitely-not-ready-for-merge code changes.

I have also omitted a lot of details. Where does PORT come from? (It needs to be container’s host port, and it needs to be unique per container instance.) How to use frontend also on local (developer machine) deployment? What to use as REGISTRY_URL? How to actually integrate all of this into a CI pipeline? — I’ll leave those as a home exercise either to solve yourself, or to pester me to write about them :-)

Anyway we are pretty happy about the setup. It means that any branch will get full integration test love on an ECS-deployed setup, automatically and without any extra work on developer’s side. It also means that errors and problems are a bit easier to debug, since any URL from any log or test user report will automatically reveal branch and service names.

It is also easy to determine a correct url to pass to other people (developers, stakeholders, internal alpha testers) as it is always branch.dev.example.com. We special-cased the default entry point to drop the service name.

Sprint demo coming up? Merge master to demo, push and it’ll be up in a jiffy.

  1. Not example.com in reality, of course. 

  2. Before you say that a developer should be able to run integration tests locally and tests should always match ECS-based tests thanks to the magic of docker I’ll answer that yes, they can, it’s one command, and no, since there are inherent differences in deployment in local vs. remote deployment the results are not always the same. Although usually there are no problems, and when there are they usually are configuration and not code problems. 

  3. The regenerate script actually just dumps the registry information to a Jinja2 template, writing the result to /etc/nginx/conf.d directory. 

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