An entrypoint for containers

entrypoing is a Python library that provides an ability to retrieve configuration settings at a start point from etcd. By preserving an ability to use local settings which are present in a container or attached through a volume.

The following snippet of the code shows the common usage pattern of the library:

import entrypoint
from entrypoint import catalog
from entrypoint import config

CONF = config.CONF

...

entrypoint.process_entrypoint(
    "ceagle", default_config_path="/etc/ceagle/config.json")

...

elasticsearch = catalog.get_endpoint("elasticsearch")

print(endpoint.protocol) #-> None or "http"
print(endpoint.address) #-> "127.0.0.1"
print(endpoint.port) #-> 8234
print(endpoint.url()) #-> "http://127.0.0.1:8234/"

The main assumption is that before to use CONF the process_entrypoint function have to be called to populate it. Right now there are only two required variables, such as service_name and default_config_path.

Be default the entrypoint expect that configuration is present locally but this behaviour can be changed through the envrinment variable ENVIRONMENT. There are two possible values for this variable, such as etcd and local.

Fetch configuration from etcd

The first value etcd means that configuration is present in etcd and will be fetched from it. In this cahse etcd hosts have to be specified in the env variable ETCD_HOSTS, they will be used to try to connect to them and fetch configuration.

entrypoint expects in etcd the following key patter for configuration of services:

/<service_name>/config

Before to run application the configuration file for a service have to set in etcd, it could be done by the next command (for the ceagle service):

etcdctl set /ceagle/config < config.json

Only JSON-serialized configuration is supported.

To run a container from an image that uses entrypoint, the following command can be used:

cat > ceagle-env-vars <<EOF
ENVIRONMENT=etcd
ETCD_HOSTS="172.17.0.4:2379 172.17.0.5:2379"
EOF
docker run --env-vars=ceagle-env-vars --name=ceagle ceagle:latest

Read configuration locally

The second value local means that the configuration will be read from a local filesystem by a path which is specified in the variable <SERVICE_NAME>_CONFIG_FILE, e.g. CEAGLE_CONFIG_FILE in case of service_name="ceagle". If the *_CONFIG_FILE variable is not speicified, then the default filename will be used, which is specified by the default_config_path variable in a code of an application.

Nothing specially have to be specified to run a container with the configuration which is shipped by an image.

To use non-default configuration path, e.g. through mounting a host directory, an container can be run in the following way:

cat > ceagle-env-vars <<EOF
CEAGLE_CONFIG_FILE=/etc/ms/ceagle/config.json
EOF
docker run --env-vars=ceagle-env-vars --name=ceagle \
    -v /local/ceagle:/etc/ms/ceagle
    ceagle:latest

A List of Supported Parameters

The full list of supported arguments and environment variables are listed here:

Argument	Environment variable	Default value (type)
--environment	ENVIRONMENT	local (etcd/local)
--etcd-hosts	ETCD_HOSTS	- (a space-delimited list of str:int)
--etcd-attempts	ETCD_ATTEMPTS	10 (int)
--etcd-attempts-delay	ETCD_ATTEMPTS_DELAY	6 (int)
--config-file	<SERVICE_NAME>_CONFIG_FILE	<default_config_path> (str)

Endpoints

A service have to has a definition of its endpoints in etcd in the following key:

/<service_name>/endpoints

The only supported format for endpoints is JSON with the next structure:

{
    "private": {
        "protocol": "http",
        "address": "127.0.0.1",
        "port": 8000,
    }
}

Where the private type of endpoints have to be used for the cross service communication. The protocol is an optional parameter and in case of using the Endpoint.url method will be replaced by "http".

TODO

What we want:

• Add support to handle dependencies between services throught.
• Reporting status of services.