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Hanoverd - The docker handover daemon

Hanoverd ("Hanover-Dee") is responsible for managing seamless transitions from one application version to another with Docker containers.

It's the docker replace command you always wanted.

Status: beta. Things may change. We may need your feedback to make it work for you.

What is hanoverd good for?

Let's say you have your web application running in docker, and you want to upgrade it seamlessly.

You want to move fast and break things with continuous deployment, making updates to your app, but you don't want your web application to go down while it updates; or if it fails to restart.

Hanoverd solves the problem by leaving the first working version running until a second one has started and correctly accepted a request.

Hanoverd is ideal for stateless web applications running in docker where you can easily have multiple instances running side-by-side.

Hanoverd can pull or build images from a variety of sources, and also listen to webhooks from github and elsewhere using an outbound TCP connection, rather than having to expose a port for listening to such events. This is done via hookbot.

Installation

Hanoverd currently requires the ability to run iptables. This can be achieved with setcap to avoid using root. A suitably capable iptables command must be on the $PATH. This can be achieved with this command, for example:

cp $(which iptables) .
sudo setcap 'cap_net_admin,cap_net_raw=+ep' iptables
PATH=.:$PATH hanoverd

(or you can use some directory other than .).

You should bear in mind that the ability to run iptables is the ability to do almost arbitrary things to the network. However, hanoverd can run as a separate user from the container, so it does not share this privilege.

User experience

• You run one hanoverd per application you wish to run in Docker.
• You run hanoverd in a directory containing a Dockerfile.
• Starting hanoverd causes that Dockerfile to be built and the application is started.
• Sending hanoverd a signal results in hanoverd rebuilding the container and transitioning to the new version.
• When the new version successfully accepts a connection, the new version becomes live.
• If the new version fails to accept any connection, the old version will chug along happily until a new request comes in to start a new application version.

Building and running

First build it:

$ go get -v
$ go build -v

Then launch it (in a directory with a Dockerfile!):

$ ./hanoverd

Supported command line options are the same sort of things as docker. However not all are implemented - please submit an issue if there is one you need which is missing.

• --env, -e for environment, e.g. --env HOME to pass $HOME through or --env HOME=/home/foo
• --publish, -p for specifying port mappings
• --volume, -v for volumes

Other things:

• --status-uri (defaults to /), specify a status URL to send HTTP pings to to determine initial health
• --hookbot, specify a hookbot websocket URL to listen on

Environment variables which the docker client (and boot2docker) use can be set first.

DOCKER_CERT_PATH
DOCKER_HOST
DOCKER_TLS_VERIFY

Additionally, Hanoverd injects internal environment variables that can be used by the container:

HANOVERD_IMAGE
HANOVERD_IMAGE_REPO
HANOVERD_IMAGE_TAGDIGEST

Method

Hanoverd has a few phases. It commences this when it first starts or when it is signalled.

• Obtain image (can be done a few ways)
• Start container with the new image
• Rapidly poll container with requests until it gives a 200 response
• Redirect traffic to the new container using iptables rules
• Stop the old container

If a signal comes in to start a new container and the previous one has not had new traffic directed to it yet, image obtaining is cancelled or the container is stopped and deleted.

Obtaining an image

Images can be obtained via building them or pulling them from a registry. This part is extensible, so coding something to import a tar from S3 would also be doable, for example.

Hookbot

Hookbot is a service that allows applications such as hanoverd to listen for webhooks by using an outbound websocket.

Hanoverd can make an outbound connection to a hookbot server in order to respond to webhook events without needing to listen for them. Hanoverd supports a few different URL types, for example to build and run a github project on a specific branch whenever the branch changes:

hanoverd --hookbot wss://[email protected]/sub/github.com/repo/scraperwiki/project/branch/master

Or to pull from a docker registry:

hanoverd --hookbot wss://[email protected]/sub/docker-pull/localhost.localdomain:5000/pdftables.com/tag/master

Instead of using the --hookbot parameter one can also use the HOOKBOT_URL environment variable.

Plans (not a promise, may never happen)

Right now restarts are not-quite-zero-downtime. We haven't seen a broken connection because of it, but if you have long running requests it could happen. This could be reasonably easily fixed by giving the old container some grace time.