:title: Link via an Ambassador Container
:description: Using the Ambassador pattern to abstract (network) services
:keywords: Examples, Usage, links, docker, documentation, examples, names, name, container naming

.. _ambassador_pattern_linking:

Link via an Ambassador Container
================================

Rather than hardcoding network links between a service consumer and provider, Docker
encourages service portability.

eg, instead of

.. code-block:: bash

	(consumer) --> (redis)

requiring you to restart the ``consumer`` to attach it to a different ``redis`` service, 
you can add ambassadors

.. code-block:: bash

	(consumer) --> (redis-ambassador) --> (redis)

	or

	(consumer) --> (redis-ambassador) ---network---> (redis-ambassador) --> (redis)

When you need to rewire your consumer to talk to a different redis server, you 
can just restart the ``redis-ambassador`` container that the consumer is connected to.

This pattern also allows you to transparently move the redis server to a different
docker host from the consumer.

Using the ``svendowideit/ambassador`` container, the link wiring is controlled entirely 
from the ``docker run`` parameters.

Two host Example
----------------

Start actual redis server on one Docker host

.. code-block:: bash

	big-server $ docker run -d -name redis crosbymichael/redis

Then add an ambassador linked to the redis server, mapping a port to the outside world

.. code-block:: bash

	big-server $ docker run -d -link redis:redis -name redis_ambassador -p 6379:6379 svendowideit/ambassador

On the other host, you can set up another ambassador setting environment variables for each remote port we want to proxy to the ``big-server``

.. code-block:: bash

	client-server $ docker run -d -name redis_ambassador -expose 6379 -e REDIS_PORT_6379_TCP=tcp://192.168.1.52:6379 svendowideit/ambassador

Then on the ``client-server`` host, you can use a redis client container to talk 
to the remote redis server, just by linking to the local redis ambassador.

.. code-block:: bash

	client-server $ docker run -i -t -rm -link redis_ambassador:redis relateiq/redis-cli
	redis 172.17.0.160:6379> ping
	PONG



How it works
------------

The following example shows what the ``svendowideit/ambassador`` container does 
automatically (with a tiny amount of ``sed``)

On the docker host (192.168.1.52) that redis will run on:

.. code-block:: bash

	# start actual redis server
	$ docker run -d -name redis crosbymichael/redis

	# get a redis-cli container for connection testing	
	$ docker pull relateiq/redis-cli

	# test the redis server by talking to it directly
	$ docker run -t -i -rm -link redis:redis relateiq/redis-cli
	redis 172.17.0.136:6379> ping
	PONG
	^D
	
	# add redis ambassador
	$ docker run -t -i -link redis:redis -name redis_ambassador -p 6379:6379 busybox sh
	
in the redis_ambassador container, you can see the linked redis containers's env

.. code-block:: bash

	$ env
	REDIS_PORT=tcp://172.17.0.136:6379
	REDIS_PORT_6379_TCP_ADDR=172.17.0.136
	REDIS_NAME=/redis_ambassador/redis
	HOSTNAME=19d7adf4705e
	REDIS_PORT_6379_TCP_PORT=6379
	HOME=/
	REDIS_PORT_6379_TCP_PROTO=tcp
	container=lxc
	REDIS_PORT_6379_TCP=tcp://172.17.0.136:6379
	TERM=xterm
	PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
	PWD=/
	
	
This environment is used by the ambassador socat script to expose redis to the world 
(via the -p 6379:6379 port mapping)

.. code-block:: bash

	$ docker rm redis_ambassador
	$ sudo ./contrib/mkimage-unittest.sh
	$ docker run -t -i -link redis:redis -name redis_ambassador -p 6379:6379 docker-ut sh
	
	$ socat TCP4-LISTEN:6379,fork,reuseaddr TCP4:172.17.0.136:6379
	
then ping the redis server via the ambassador

.. code-block::bash

	$ docker run -i -t -rm -link redis_ambassador:redis relateiq/redis-cli
	redis 172.17.0.160:6379> ping
	PONG

Now goto a different server

.. code-block:: bash

	$ sudo ./contrib/mkimage-unittest.sh
	$ docker run -t -i  -expose 6379 -name redis_ambassador docker-ut sh
	
	$ socat TCP4-LISTEN:6379,fork,reuseaddr TCP4:192.168.1.52:6379

and get the redis-cli image so we can talk over the ambassador bridge

.. code-block:: bash

	$ docker pull relateiq/redis-cli
	$ docker run -i -t -rm -link redis_ambassador:redis relateiq/redis-cli
	redis 172.17.0.160:6379> ping
	PONG

The svendowideit/ambassador Dockerfile
--------------------------------------

The ``svendowideit/ambassador`` image is a small busybox image with ``socat`` built in.
When you start the container, it uses a small ``sed`` script to parse out the (possibly multiple)
link environment variables to set up the port forwarding. On the remote host, you need to set the 
variable using the ``-e`` command line option.

``-expose 1234 -e REDIS_PORT_1234_TCP=tcp://192.168.1.52:6379`` will forward the 
local ``1234`` port to the remote IP and port - in this case ``192.168.1.52:6379``.


::

	#
	#
	# first you need to build the docker-ut image 
	# using ./contrib/mkimage-unittest.sh
	# then 
	#   docker build -t SvenDowideit/ambassador .
	#   docker tag SvenDowideit/ambassador ambassador
	# then to run it (on the host that has the real backend on it)
	#   docker run -t -i -link redis:redis -name redis_ambassador -p 6379:6379 ambassador
	# on the remote host, you can set up another ambassador
	#    docker run -t -i -name redis_ambassador -expose 6379 sh

	FROM	docker-ut
	MAINTAINER	SvenDowideit@home.org.au


	CMD	env | grep _TCP= | sed 's/.*_PORT_\([0-9]*\)_TCP=tcp:\/\/\(.*\):\(.*\)/socat TCP4-LISTEN:\1,fork,reuseaddr TCP4:\2:\3 \&/'  | sh && top

