Rather than hardcoding network links between a service consumer and provider, Docker encourages service portability.
eg, instead of 2 containers talking directly to each other:
requiring you to restart the consumer to attach it to a different redis service, you can add ambassador containers:
(consumer) --> (redis-ambassador) --> (redis)
or
(consumer) --> (redis-ambassador) ---network---> (redis-ambassador) --> (redis)
When you need to rewire your consumer to talk to a different resdis 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 dockerrun 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’s 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
then ping the redis server via the ambassador
Now goto 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 1234port 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
(this is pull request https://github.com/dotcloud/docker/pull/3038 so will eventually find its way into the Docker documentation)
