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.

Latest features

Extending functionality of kubernetes-entrypoint by adding an ability to specify dependencies in different namespaces. The new format for writing dependencies is namespace:name, with the exception of pod dependencies which us json. To ensure backward compatibility if the namespace: is omitted, it behaves just like in previous versions so it assumes that dependecies are running at the same namespace as kubernetes-entrypoint. This feature is not implemented for container, config and socket dependency because in such cases the different namespace is irrelevant.

For instance: DEPENDENCY_SERVICE=mysql:mariadb,keystone-api

The new entrypoint will resolve mariadb in mysql namespace and keystone-api in the same namespace as entrypoint was deployed in.

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 or set of jobs with matching name and/or labels succeeded at least once. In order to use labels DEPENDENCY_JOBS_JSON must be used, but DEPENDENCY_JOBS is supported as well for backward compatibility. Examples:

DEPENDENCY_JOBS_JSON='[{"namespace": "foo", "name": "nova-init"}, {"labels": {"initializes": "neutron"}}]' 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

Pod

Checks if at least one pod matching the specified labels is already running, by default anywhere in the cluster, or use "requireSameNode": true to require a a pod on the same node. As seen below the syntax uses JSON to allow for label support. 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_POD_JSON='[{"namespace": "foo", "labels": {"k1": "v1", "k2": "v2"}}, {"labels": {"k1": "v1", "k2": "v2"}, "requireSameNode": true}]'

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.