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moby--moby/docs/articles/ambassador_pattern_linking.md

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<!--[metadata]>
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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"]
[menu.main]
parent = "smn_administrate"
weight = 6
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<![end-metadata]-->
# Link via an ambassador container
## Introduction
Rather than hardcoding network links between a service consumer and
provider, Docker encourages service portability, for example instead of:
(consumer) --> (redis)
Requiring you to restart the `consumer` to attach it to a different
`redis` service, you can add ambassadors:
(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
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
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`
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.
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:
# 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 `env`:
$ 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):
$ 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
Now ping the Redis server via the ambassador:
Now go to a different server:
$ 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.
$ 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