Maestro guest functions

MaestroNG passes into your container’s environment a lot of information that you might need for your application to run: port numbers that you have defined in your environment configuration file, addresses and ports of your dependencies, etc. All these environment variables are defined and documented in How Maestro orchestrates and service auto-configuration.

In order to make groking this environment information easier, MaestroNG provides a Python module of “guest functions” that allow you to write simple container init scripts in Python that act as the glue between this information and your application’s configuration before starting the application itself.

A common use case, for example, is to get your ZooKeeper connection string from this environment – the rest you usually find in ZooKeeper itself via service discovery.

Basic usage

To make use of the Maestro guest utils functions, you’ll need to have the Maestro package installed inside your container. You can easily achieve this by adding the following to your Dockerfile:

ENV DEBIAN_FRONTEND noninteractive
RUN apt-get update \
  && apt-get -y install python python-pip \
  && apt-get clean
RUN pip install maestro-ng

Then, from your Python script, simply do:

from maestro.guestutils import *

And you’re ready to go! Feel free to change the import * to the list of specific functions you actually need in your script.


Here’s a summary of the functions available at your disposal that will make your life much easier. All the examples given here are based on the [Zookeeper+Kafka example environment](examples/zookeeper+kafka.yaml), and assumed to be executed from within the kafka-2 container.


Returns the name of the environment as defined in the description file. Could be useful to namespace information inside ZooKeeper for example.

>> get_environment_name()


Returns the name of the service the container is a member of.

>> get_service_name()


Returns the name of the container instance.

>> get_container_name()


Returns the IP address or hostname of the host of the container. Useful if your application needs to advertise itself to some service discovery system with its publicly reachable address. This would be in our example.

>> get_container_host_address()


Returns the IP address assigned to the container itself by Docker (its private IP address). This is normally the IP address Docker assigned to the eth0 interface inside the container and is usually in the subnet.

>> get_container_internal_address()
# Might be different depending on the number of running containers on
# that host.

get_port(name, default=None)

Returns the exposed internal port number of a given named port for the current container. This is the port number your application uses _inside_ the container. This is useful to automatically configure the port your application should use directly from what you have specified in your environment file.

If no default is provided and the port name does not exist, the function will throw a MaestroEnvironmentError exception.

>> get_port('broker')

>> get_port('unknown', 42)

>> get_port('unknown')
# MaestroEnvironmentError gets raised

get_node_list(service, ports=[], minimum=1)

This function is one of the most useful of the set. It takes in a service name and an optional list of port names and returns the list of IP addresses/hostname of the containers of that service. For each port specified, in order, it will append :<port number> to each host with the external port number.

The minimum parameter allows for specifying a minimum number of hosts to return, under which the function will throw a MaestroEnvironmentError exception. This helps enforce the presence of at least N hosts of that service you depend on in the environment.

Back to our example, you can return the list of ZooKeeper endpoints with their client ports by calling:

>> get_node_list('zookeeper', ports=['client'])
['', '', '']

>> get_node_list('zookeeper', ports=['client', 'peer'])
['', '', '']

Note that Maestro provides information about all your declared dependencies in your environment, but also the information about all the instances of your service itself, so you can easily get a node list of your peers:

>> get_node_list(get_service_name(), ports=['broker'])
['', '', '']

get_specific_host(service, container)

Returns the hostname or IP address of a specific container from a given service.

>> get_specific_host('zookeeper', 'zk-node-2')

get_specific_port(service, container, port, default=None)

Returns the external port number of a specific named port of a given container. This is the externally reachable, routed port number for that particular target.

>> get_specific_port('zookeeper', 'zk-node-2', 'client')

get_specific_exposed_port(service, container, port, default=None)

Returns the exposed internal port number of a specific named port of a given container. This is rarely needed (but is used internally by get_port()).

>> get_specific_exposed_port('zookeeper', 'zk-node-2', 'client')