mirror of
https://github.com/jpetazzo/container.training.git
synced 2026-05-20 15:52:55 +00:00
018282f392
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
|
|
class: title
|
|
|
|
# Connecting containers with links
|
|
|
|
|
|
|
|
---
|
|
|
|
## Objectives
|
|
|
|
Links were the "legacy" way of connecting containers (before the implementation of the CNM).
|
|
|
|
They are still useful in some scenarios.
|
|
|
|
---
|
|
|
|
## How *links* work
|
|
|
|
* Links are created *between two containers*
|
|
* Links are created *from the client to the server*
|
|
* Links associate an arbitrary name to an existing container
|
|
* Links exist *only in the context of the client*
|
|
|
|
---
|
|
|
|
class: extra-details
|
|
|
|
## The plan
|
|
|
|
* We will create the `redis` container first.
|
|
* Then, we will create the `www` container, *with a link to the previous container.*
|
|
* We don't need to use a custom network for this to work.
|
|
|
|
---
|
|
|
|
class: extra-details
|
|
|
|
## Create the `redis` container
|
|
|
|
Let's launch a container from the `redis` image.
|
|
|
|
```bash
|
|
$ docker run -d --name datastore redis
|
|
<yourContainerID>
|
|
```
|
|
|
|
Let's check the container is running:
|
|
|
|
```bash
|
|
$ docker ps -l
|
|
CONTAINER ID IMAGE COMMAND ... PORTS NAMES
|
|
9efd72a4f320 redis:latest redis-server ... 6379/tcp datastore
|
|
```
|
|
|
|
|
|
* Our container is launched and running an instance of Redis.
|
|
* We used the `--name` flag to reference our container easily later.
|
|
* We could have used *any name we wanted.*
|
|
|
|
---
|
|
|
|
class: extra-details
|
|
|
|
## Create the `www` container
|
|
|
|
If we create the web container without any extra option, it will not be able to connect to redis.
|
|
|
|
```bash
|
|
$ docker run -dP jpetazzo/trainingwheels
|
|
```
|
|
|
|
Check the port number with `docker ps`, and connect to it.
|
|
|
|
We get the same red error page as before.
|
|
|
|
---
|
|
|
|
class: extra-details
|
|
|
|
## How our app connects to Redis
|
|
|
|
Remember, in the code, we connect to the name `redis`:
|
|
|
|
```python
|
|
redis = redis.Redis("redis")
|
|
```
|
|
|
|
* This means "try to connect to 'redis'".
|
|
* Not 192.168.123.234.
|
|
* Not redis.prod.mycompany.net.
|
|
|
|
*Obviously* it doesn't work.
|
|
|
|
---
|
|
|
|
class: extra-details
|
|
|
|
## Creating a linked container
|
|
|
|
Docker allows to specify *links*.
|
|
|
|
Links indicate an intent: "this container will connect to this other container."
|
|
|
|
Here is how to create our first link:
|
|
|
|
```bash
|
|
$ docker run -ti --link datastore:redis alpine sh
|
|
```
|
|
|
|
In this container, we can communicate with `datastore` using
|
|
the `redis` DNS alias.
|
|
|
|
---
|
|
|
|
class: extra-details
|
|
|
|
## DNS
|
|
|
|
Docker has created a DNS entry for the container, resolving to its internal IP address.
|
|
|
|
```bash
|
|
$ docker run -it --link datastore:redis alpine ping redis
|
|
PING redis (172.17.0.29): 56 data bytes
|
|
64 bytes from 172.17.0.29: icmp_seq=0 ttl=64 time=0.164 ms
|
|
64 bytes from 172.17.0.29: icmp_seq=1 ttl=64 time=0.122 ms
|
|
64 bytes from 172.17.0.29: icmp_seq=2 ttl=64 time=0.086 ms
|
|
^C--- redis ping statistics ---
|
|
3 packets transmitted, 3 packets received, 0% packet loss
|
|
round-trip min/avg/max/stddev = 0.086/0.124/0.164/0.032 ms
|
|
```
|
|
|
|
|
|
* The `--link` flag connects one container to another.
|
|
* We specify the name of the container to link to, `datastore`, and an
|
|
alias for the link, `redis`, in the format `name:alias`.
|
|
|
|
---
|
|
|
|
class: extra-details
|
|
|
|
## Starting our application
|
|
|
|
Now that we've poked around a bit let's start the application itself in
|
|
a fresh container:
|
|
|
|
```bash
|
|
$ docker run -d -P --link datastore:redis jpetazzo/trainingwheels
|
|
```
|
|
|
|
Now let's check the port number associated to the container.
|
|
|
|
```bash
|
|
$ docker ps -l
|
|
```
|
|
|
|
---
|
|
|
|
class: extra-details
|
|
|
|
## Confirming that our application works properly
|
|
|
|
Finally, let's browse to our application and confirm it's working.
|
|
|
|
```bash
|
|
http://<yourHostIP>:<port>
|
|
```
|
|
|
|
---
|
|
|
|
## Links and environment variables
|
|
|
|
In addition to the DNS information, Docker will automatically set environment variables in our container, giving extra details about the linked container.
|
|
|
|
```bash
|
|
$ docker run --link datastore:redis alpine env
|
|
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
|
|
HOSTNAME=0738e57b771e
|
|
REDIS_PORT=tcp://172.17.0.120:6379
|
|
REDIS_PORT_6379_TCP=tcp://172.17.0.120:6379
|
|
REDIS_PORT_6379_TCP_ADDR=172.17.0.120
|
|
REDIS_PORT_6379_TCP_PORT=6379
|
|
REDIS_PORT_6379_TCP_PROTO=tcp
|
|
REDIS_NAME=/dreamy_wilson/redis
|
|
REDIS_ENV_REDIS_VERSION=2.8.13
|
|
REDIS_ENV_REDIS_DOWNLOAD_URL=http://download.redis.io/releases/redis-2.8.13.tar.gz
|
|
REDIS_ENV_REDIS_DOWNLOAD_SHA1=a72925a35849eb2d38a1ea076a3db82072d4ee43
|
|
HOME=/
|
|
RUBY_MAJOR=2.1
|
|
RUBY_VERSION=2.1.2
|
|
```
|
|
|
|
|
|
* Each variables is prefixed with the link alias: `redis`.
|
|
* Includes connection information PLUS any environment variables set in
|
|
the `datastore` container via `ENV` instructions.
|
|
|
|
---
|
|
|
|
## Differences between network aliases and links
|
|
|
|
* With network aliases, you can start containers in *any order.*
|
|
* With links, you have to start the server (in our example: Redis) first.
|
|
* 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.
|
|
* With links, you can give new names to an existing container.
|
|
* Network aliases require the use of a custom network.
|
|
* Links can be used on the default bridge network.
|
|
* Network aliases work across multi-host networking.
|
|
* Links (as of Engine 1.11) only work with local containers (but this might be changed in the future).
|
|
* Network aliases don't populate environment variables.
|
|
* Links give access to the environment of the target container.
|
|
|
|
---
|
|
|
|
class: extra-details
|
|
|
|
## Section summary
|
|
|
|
We've learned how to:
|
|
|
|
* Create links between containers.
|
|
* Use names and links to communicate across containers.
|
|
|