4.2 KiB
Kubernetes Entrypoint
============
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 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.
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.
The POD_NAME variable is mandatory and is used to resolve dependencies.
Example:
DEPENDENCY_DAEMONSET=openvswitch-agent
Simple example how to use downward API to get POD_NAME can be found here.
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
Image
Build process for image is trigged after each commit.
Can be found here, and pulled by executing:
docker pull quay.io/stackanetes/kubernetes-entrypoint:v0.1.0
Examples
Stackanetes uses kubernetes-entrypoint to manage dependencies when deploying OpenStack on Kubernetes.