Link via an ambassador container
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 \&/' && echo wait) | sh
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