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159 lines
5.7 KiB
Markdown
159 lines
5.7 KiB
Markdown
<!--[metadata]>
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+++
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aliases = ["/engine/articles/ambassador_pattern_linking/"]
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title = "Link via an ambassador container"
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description = "Using the Ambassador pattern to abstract (network) services"
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keywords = ["Examples, Usage, links, docker, documentation, examples, names, name, container naming"]
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[menu.main]
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parent = "engine_admin"
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weight = 15
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+++
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<![end-metadata]-->
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# Link via an ambassador container
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Rather than hardcoding network links between a service consumer and
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provider, Docker encourages service portability, for example instead of:
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(consumer) --> (redis)
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Requiring you to restart the `consumer` to attach it to a different
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`redis` service, you can add ambassadors:
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(consumer) --> (redis-ambassador) --> (redis)
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Or
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(consumer) --> (redis-ambassador) ---network---> (redis-ambassador) --> (redis)
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When you need to rewire your consumer to talk to a different Redis
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server, you can just restart the `redis-ambassador` container that the
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consumer is connected to.
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This pattern also allows you to transparently move the Redis server to a
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different docker host from the consumer.
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Using the `svendowideit/ambassador` container, the link wiring is
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controlled entirely from the `docker run` parameters.
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## Two host example
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Start actual Redis server on one Docker host
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big-server $ docker run -d --name redis crosbymichael/redis
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Then add an ambassador linked to the Redis server, mapping a port to the
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outside world
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big-server $ docker run -d --link redis:redis --name redis_ambassador -p 6379:6379 svendowideit/ambassador
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On the other host, you can set up another ambassador setting environment
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variables for each remote port we want to proxy to the `big-server`
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client-server $ docker run -d --name redis_ambassador --expose 6379 -e REDIS_PORT_6379_TCP=tcp://192.168.1.52:6379 svendowideit/ambassador
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Then on the `client-server` host, you can use a Redis client container
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to talk to the remote Redis server, just by linking to the local Redis
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ambassador.
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client-server $ docker run -i -t --rm --link redis_ambassador:redis relateiq/redis-cli
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redis 172.17.0.160:6379> ping
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PONG
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## How it works
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The following example shows what the `svendowideit/ambassador` container
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does automatically (with a tiny amount of `sed`)
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On the Docker host (192.168.1.52) that Redis will run on:
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# start actual redis server
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$ docker run -d --name redis crosbymichael/redis
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# get a redis-cli image for connection testing
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$ docker pull relateiq/redis-cli
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# test the redis server by talking to it directly
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$ docker run -t -i --rm --link redis:redis relateiq/redis-cli
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redis 172.17.0.136:6379> ping
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PONG
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^D
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# add redis ambassador
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$ docker run -t -i --link redis:redis --name redis_ambassador -p 6379:6379 alpine:3.2 sh
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In the `redis_ambassador` container, you can see the linked Redis
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containers `env`:
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/ # env
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REDIS_PORT=tcp://172.17.0.136:6379
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REDIS_PORT_6379_TCP_ADDR=172.17.0.136
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REDIS_NAME=/redis_ambassador/redis
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HOSTNAME=19d7adf4705e
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SHLVL=1
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HOME=/root
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REDIS_PORT_6379_TCP_PORT=6379
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REDIS_PORT_6379_TCP_PROTO=tcp
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REDIS_PORT_6379_TCP=tcp://172.17.0.136:6379
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TERM=xterm
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PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
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PWD=/
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/ # exit
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This environment is used by the ambassador `socat` script to expose Redis
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to the world (via the `-p 6379:6379` port mapping):
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$ docker rm redis_ambassador
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$ CMD="apk update && apk add socat && sh"
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$ docker run -t -i --link redis:redis --name redis_ambassador -p 6379:6379 alpine:3.2 sh -c "$CMD"
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[...]
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/ # socat -t 100000000 TCP4-LISTEN:6379,fork,reuseaddr TCP4:172.17.0.136:6379
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Now ping the Redis server via the ambassador:
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Now go to a different server:
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$ CMD="apk update && apk add socat && sh"
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$ docker run -t -i --expose 6379 --name redis_ambassador alpine:3.2 sh -c "$CMD"
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[...]
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/ # socat -t 100000000 TCP4-LISTEN:6379,fork,reuseaddr TCP4:192.168.1.52:6379
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And get the `redis-cli` image so we can talk over the ambassador bridge.
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$ docker pull relateiq/redis-cli
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$ docker run -i -t --rm --link redis_ambassador:redis relateiq/redis-cli
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redis 172.17.0.160:6379> ping
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PONG
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## The svendowideit/ambassador Dockerfile
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The `svendowideit/ambassador` image is based on the `alpine:3.2` image with
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`socat` installed. When you start the container, it uses a small `sed`
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script to parse out the (possibly multiple) link environment variables
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to set up the port forwarding. On the remote host, you need to set the
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variable using the `-e` command line option.
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--expose 1234 -e REDIS_PORT_1234_TCP=tcp://192.168.1.52:6379
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Will forward the local `1234` port to the remote IP and port, in this
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case `192.168.1.52:6379`.
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#
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# do
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# docker build -t svendowideit/ambassador .
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# then to run it (on the host that has the real backend on it)
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# docker run -t -i -link redis:redis -name redis_ambassador -p 6379:6379 svendowideit/ambassador
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# on the remote host, you can set up another ambassador
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# docker run -t -i -name redis_ambassador -expose 6379 -e REDIS_PORT_6379_TCP=tcp://192.168.1.52:6379 svendowideit/ambassador sh
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# you can read more about this process at https://docs.docker.com/articles/ambassador_pattern_linking/
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# use alpine because its a minimal image with a package manager.
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# prettymuch all that is needed is a container that has a functioning env and socat (or equivalent)
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FROM alpine:3.2
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MAINTAINER SvenDowideit@home.org.au
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RUN apk update && \
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apk add socat && \
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rm -r /var/cache/
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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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