# Kubernetes Entrypoint

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Kubernetes-entrypoint enables complex deployments on top of Kubernetes.

## Overview

Kubernetes-entrypoint is meant to be used as a container entrypoint, which means it has to bundled in the container. 
Before launching the desired application, the entrypoint verifies and waits for all specified dependencies to be met.

The Kubernetes-entrypoint queries directly the Kubernetes API and each container is self-aware of its dependencies and their states. 
Therefore, no centralized orchestration layer is required to manage deployments and scenarios such as failure recovery or pod migration become easy.

## Usage

Kubernetes-entrypoint reads the dependencies out of environment variables passed into a container.
There is only one required environment variable "COMMAND" which specifies a command (arguments delimited by whitespace) which has to be executed when all dependencies are resolved:

`COMMAND="sleep inf"`

Kubernetes-entrypoint introduces a wide variety of dependencies which can be used to better orchestrate once deployment.

## Supported types of dependencies

All dependencies are passed as environement variables in format of `DEPENDENCY_<NAME>` delimited by colon. For dependencies to be effective please use [readiness probes](http://kubernetes.io/docs/user-guide/production-pods/#liveness-and-readiness-probes-aka-health-checks) for all containers.

### Service
Checks whether given kubernetes service has at least one endpoint.  
Example:

`DEPENDENCY_SERVICE=mariadb,keystone-api`

### Container
Within a pod composed of multiple containers, it waits for the containers specified by their names to start.
This dependency requires a `POD_NAME` environement variable which can be easily passed through the [downward api](http://kubernetes.io/docs/user-guide/downward-api/).
Example:

`DEPENDENCY_CONTAINER=nova-libvirt,virtlogd`

### Daemonset
Checks if a specified daemonset is already running on the same host, this dependency requires a `POD_NAME`
env which can be easily passed through the [downward api](http://kubernetes.io/docs/user-guide/downward-api/).
Example:

`DEPENDENCY_DAEMONSET=openvswitch-agent`

### Job
Checks if a given job succeded at least once.
Example:

`DEPENDENCY_JOBS=nova-init,neutron-init`

### Config
This dependency performs a container level templating of configuration files. It can template an ip address `{{ .IP }}` and hostname `{{ .HOSTNAME }}`. 
Templated config has to be stored in an arbitrary directory `/configmaps/<name_of_file>/<name_of_file>`.
This dependency requires `INTERFACE_NAME` environment variable to know which interface to use for obtain ip address. 
Example:

`DEPENDENCY_CONFIG=/etc/nova/nova.conf`

The Kubernetes-entrypoint will look for the configuration file `/configmaps/nova.conf/nova.conf`, template 
`{{ .IP }} and {{ .HOSTNAME }}` tags and save the file as `/etc/nova/nova.conf`.

### Socket
Checks whether a given file exists and container has rights to read it.
Example:

`DEPENDENCY_SOCKET=/var/run/openvswitch/ovs.socket`

## Examples

[Stackanetes](http://github.com/stackanetes/stackanetes) uses kubernetes-entrypoint to manage dependencies when deploying OpenStack on Kubernetes.