# 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 alpine:3.2 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 SHLVL=1 HOME=/root REDIS_PORT_6379_TCP_PORT=6379 REDIS_PORT_6379_TCP_PROTO=tcp 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=/ / # exit 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 $ CMD="apk update && apk add socat && sh" $ docker run -t -i --link redis:redis --name redis_ambassador -p 6379:6379 alpine:3.2 sh -c "$CMD" [...] / # socat -t 100000000 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: $ CMD="apk update && apk add socat && sh" $ docker run -t -i --expose 6379 --name redis_ambassador alpine:3.2 sh -c "$CMD" [...] / # socat -t 100000000 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 based on the `alpine:3.2` image with `socat` installed. 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`. # # do # docker build -t svendowideit/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 svendowideit/ambassador # on the remote host, you can set up another ambassador # docker run -t -i -name redis_ambassador -expose 6379 -e REDIS_PORT_6379_TCP=tcp://192.168.1.52:6379 svendowideit/ambassador sh # you can read more about this process at https://docs.docker.com/articles/ambassador_pattern_linking/ # use alpine because its a minimal image with a package manager. # prettymuch all that is needed is a container that has a functioning env and socat (or equivalent) FROM alpine:3.2 MAINTAINER SvenDowideit@home.org.au RUN apk update && \ apk add socat && \ rm -r /var/cache/ CMD env | grep _TCP= | sed 's/.*_PORT_\([0-9]*\)_TCP=tcp:\/\/\(.*\):\(.*\)/socat -t 100000000 TCP4-LISTEN:\1,fork,reuseaddr TCP4:\2:\3 \& wait/' | sh