Docker Zootopia Networking

I want to talk a little bit about Docker Compose, networking, and services calling each other. This is a super regular scenario that ‘ve already talked/posted/written, but I keep these post mostly for myself? if that makes sense.

So, from the beginning;

A docker-compose file, there is service-a (the main service) and service-b. I need service-a to ping to service-b.
And service-a has an environment variable to define the URL for service-b.

The happy case works right out of the box; service-a can reach http://service-b:1080

why? cause there is a default Docker network configuration (I pretend that this is already written in my compose file)

networks:
  default:
    driver: bridge

Which means:

• Each service you define in the compose joins the network tagged default (its named like {folder-name}_default ).
• Each service is automatically assigned automatically an IP address (like 172.x.x.x).
• key thing: service-a can reach service-b by name cause this network already has a DNS resolution built-in

(I made up these IPs):

# DNS resolution example inside the Docker network
service-a -> 172.18.0.1:8080
service-b -> 172.18.0.2:1080

Quick note!

Inside the Docker network they can call each other using these names… but from your machine you can’t use these names.

It’s like in Zootopia.

In Zootopia, a bunny can call another bunny "cute," but when other animals do it, it's a little… well, you know what I mean.

This is the same, you computer can’t use the name alias referring to service-a, it needs to use localhost and the forwarded port, that’s why we do this forwarding port thing like 4080:1080

---

Anyway, this isn’t what I want to focus 👯,

because my particular use case has an additional key requirement:
service-a can not use http://service-b:1080 (there was an explicit code guard)  and NEEDED an actual ip-like address.

So, yay, easy right? just create an alias:

service-b:
  image: ...
  networks:
    default:
      ports:
        - 4080:1080
+     aliases:
+       - service-b.local

So instead of

- http://service-b:1080
+ http://service-b.local:1080

Except this needs an extra step.

---

Remember that DNS mapping we have by default? we neet to create a subnet and assign the IPs to each service manually….

…maybe there’s a more ‘DevOps-ish’ way to do this? but I find statically assigning the IPs pretty straightforward, im opened to alternatives:

services:
  service-a:
    image: ...
+   networks:
+     default:
+       ipv4_address: 172.21.0.10
    environment:
      SERVICE_B_URL: http://service-b:1080

  service-b:
    image: ...
    networks:
      default:
+       ipv4_address: 172.21.0.20
        aliases:
          - service-b.local
    ports:
      - 4080:1080

+ networks:
+   default:
+     ipam:
+       driver: default
+       config:
+         - subnet: 172.21.0.0/24

IPAM means “Ey Docker, Im to assign manually the IPs”; The subnet part: 172.21.0.0/24 is creating a range of IPs between:

172.21.0.1 , 172.21.0.2 , 172.21.0.3 … 172.21.0.254

And I assign each service… honestly, somewhat random (.0.10, 0.20).

---

Just one thing left, the DNS; we need to tell service-a to map the name and the IP:

  service-a:
    image: ...
    networks:
      default:
        ipv4_address: 172.21.0.10
+   extra_hosts:
+     - "service-b.local:172.21.0.20"
    environment:
      SERVICE_B_URL: http://service-b:1080

And that’s it. Happy ending. service-a can now reach service-b using the name service-b.local, and everything works inside the Docker network.