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This was discussed and agreed in #246. It will probably break a few outstanding PRs as well as a few external links but it's for the better good long term.
224 lines
5.3 KiB
Markdown
224 lines
5.3 KiB
Markdown
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class: title
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# Connecting containers with links
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---
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## Objectives
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Links were the "legacy" way of connecting containers (before the implementation of the CNM).
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They are still useful in some scenarios.
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---
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## How *links* work
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* Links are created *between two containers*
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* Links are created *from the client to the server*
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* Links associate an arbitrary name to an existing container
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* Links exist *only in the context of the client*
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---
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class: extra-details
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## The plan
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* We will create the `redis` container first.
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* Then, we will create the `www` container, *with a link to the previous container.*
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* We don't need to use a custom network for this to work.
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---
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class: extra-details
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## Create the `redis` container
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Let's launch a container from the `redis` image.
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```bash
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$ docker run -d --name datastore redis
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<yourContainerID>
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```
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Let's check the container is running:
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```bash
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$ docker ps -l
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CONTAINER ID IMAGE COMMAND ... PORTS NAMES
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9efd72a4f320 redis:latest redis-server ... 6379/tcp datastore
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```
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* Our container is launched and running an instance of Redis.
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* We used the `--name` flag to reference our container easily later.
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* We could have used *any name we wanted.*
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---
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class: extra-details
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## Create the `www` container
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If we create the web container without any extra option, it will not be able to connect to redis.
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```bash
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$ docker run -dP jpetazzo/trainingwheels
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```
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Check the port number with `docker ps`, and connect to it.
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We get the same red error page as before.
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---
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class: extra-details
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## How our app connects to Redis
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Remember, in the code, we connect to the name `redis`:
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```python
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redis = redis.Redis("redis")
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```
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* This means "try to connect to 'redis'".
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* Not 192.168.123.234.
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* Not redis.prod.mycompany.net.
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*Obviously* it doesn't work.
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---
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class: extra-details
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## Creating a linked container
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Docker allows to specify *links*.
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Links indicate an intent: "this container will connect to this other container."
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Here is how to create our first link:
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```bash
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$ docker run -ti --link datastore:redis alpine sh
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```
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In this container, we can communicate with `datastore` using
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the `redis` DNS alias.
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---
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class: extra-details
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## DNS
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Docker has created a DNS entry for the container, resolving to its internal IP address.
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```bash
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$ docker run -it --link datastore:redis alpine ping redis
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PING redis (172.17.0.29): 56 data bytes
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64 bytes from 172.17.0.29: icmp_seq=0 ttl=64 time=0.164 ms
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64 bytes from 172.17.0.29: icmp_seq=1 ttl=64 time=0.122 ms
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64 bytes from 172.17.0.29: icmp_seq=2 ttl=64 time=0.086 ms
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^C--- redis ping statistics ---
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3 packets transmitted, 3 packets received, 0% packet loss
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round-trip min/avg/max/stddev = 0.086/0.124/0.164/0.032 ms
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```
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* The `--link` flag connects one container to another.
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* We specify the name of the container to link to, `datastore`, and an
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alias for the link, `redis`, in the format `name:alias`.
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---
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class: extra-details
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## Starting our application
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Now that we've poked around a bit let's start the application itself in
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a fresh container:
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```bash
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$ docker run -d -P --link datastore:redis jpetazzo/trainingwheels
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```
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Now let's check the port number associated to the container.
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```bash
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$ docker ps -l
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```
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---
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class: extra-details
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## Confirming that our application works properly
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Finally, let's browse to our application and confirm it's working.
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```bash
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http://<yourHostIP>:<port>
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```
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---
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## Links and environment variables
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In addition to the DNS information, Docker will automatically set environment variables in our container, giving extra details about the linked container.
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```bash
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$ docker run --link datastore:redis alpine env
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PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
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HOSTNAME=0738e57b771e
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REDIS_PORT=tcp://172.17.0.120:6379
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REDIS_PORT_6379_TCP=tcp://172.17.0.120:6379
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REDIS_PORT_6379_TCP_ADDR=172.17.0.120
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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_NAME=/dreamy_wilson/redis
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REDIS_ENV_REDIS_VERSION=2.8.13
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REDIS_ENV_REDIS_DOWNLOAD_URL=http://download.redis.io/releases/redis-2.8.13.tar.gz
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REDIS_ENV_REDIS_DOWNLOAD_SHA1=a72925a35849eb2d38a1ea076a3db82072d4ee43
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HOME=/
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RUBY_MAJOR=2.1
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RUBY_VERSION=2.1.2
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```
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* Each variables is prefixed with the link alias: `redis`.
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* Includes connection information PLUS any environment variables set in
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the `datastore` container via `ENV` instructions.
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---
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## Differences between network aliases and links
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* With network aliases, you can start containers in *any order.*
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* With links, you have to start the server (in our example: Redis) first.
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* With network aliases, you cannot change the name of the server once it is running. If you want to add a name, you have to create a new container.
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* With links, you can give new names to an existing container.
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* Network aliases require the use of a custom network.
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* Links can be used on the default bridge network.
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* Network aliases work across multi-host networking.
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* Links (as of Engine 1.11) only work with local containers (but this might be changed in the future).
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* Network aliases don't populate environment variables.
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* Links give access to the environment of the target container.
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---
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class: extra-details
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## Section summary
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We've learned how to:
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* Create links between containers.
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* Use names and links to communicate across containers.
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