[#]: collector: (lujun9972) [#]: translator: ( ) [#]: reviewer: ( ) [#]: publisher: ( ) [#]: url: ( ) [#]: subject: (Docker Compose: a nice way to set up a dev environment) [#]: via: (https://jvns.ca/blog/2021/01/04/docker-compose-is-nice/) [#]: author: (Julia Evans https://jvns.ca/) Docker Compose: a nice way to set up a dev environment ====== Hello! Here is another post about [computer tools that I’ve appreciated][1]. This one is about Docker Compose! This post is mostly just about how delighted I was that it does what it’s supposed to do and it seems to work and to be pretty straightforward to use. I’m also only talking about using Docker Compose for a dev environment here, not using it in production. I’ve been thinking about this kind of personal dev environment setup more recently because I now do all my computing with a personal cloud budget of like $20/month instead of spending my time at work thinking about how to manage thousands of AWS servers. I’m very happy about this because previous to trying Docker Compose I spent two days getting frustrated with trying to set up a dev environment with other tools and Docker Compose was a lot easier and simpler. And then I told my sister about my docker-compose experiences and she was like “I KNOW, DOCKER COMPOSE IS GREAT RIGHT?!?!” So I thought I’d write a blog post about it, and here we are. ### the problem: setting up a dev environment Right now I’m working on a Ruby on Rails service (the backend for a sort of computer debugging game). On my production server, I have: * a nginx proxy * a Rails server * a Go server (which proxies some SSH connections with [gotty][2]) * a Postgres database Setting up the Rails server locally was pretty straightforward without resorting to containers (I just had to install Postgres and Ruby, fine, no big deal), but then I wanted send `/proxy/*` to the Go server and everything else to the Rails server, so I needed nginx too. And installing nginx on my laptop felt too messy to me. So enter `docker-compose`! ### docker-compose lets you run a bunch of Docker containers Docker Compose basically lets you run a bunch of Docker containers that can communicate with each other. You configure all your containers in one file called `docker-compose.yml`. I’ve pasted my entire `docker-compose.yml` file here for my server because I found it to be really short and straightforward. ``` version: "3.3" services: db: image: postgres volumes: - ./tmp/db:/var/lib/postgresql/data environment: POSTGRES_PASSWORD: password # yes I set the password to 'password' go_server: # todo: use a smaller image at some point, we don't need all of ubuntu to run a static go binary image: ubuntu command: /app/go_proxy/server volumes: - .:/app rails_server: build: docker/rails command: bash -c "rm -f tmp/pids/server.pid && source secrets.sh && bundle exec rails s -p 3000 -b '0.0.0.0'" volumes: - .:/app web: build: docker/nginx ports: - "8777:80" # this exposes port 8777 on my laptop ``` There are two kinds of containers here: for some of them I’m just using an existing image (`image: postgres` and `image: ubuntu`) without modifying it at all. And for some I needed to build a custom container image – `build: docker/rails` says to use `docker/rails/Dockerfile` to build a custom container. I needed to give my Rails server access to some API keys and things, so `source secrets.sh` puts a bunch of secrets in environment variables. Maybe there’s a better way to manage secrets but it’s just me so this seemed fine. ### how to start everything: `docker-compose build` then `docker-compose up` I’ve been starting my containers just by running `docker-compose build` to build the containers, then `docker-compose up` to run everything. You can set `depends_on` in the yaml file to get a little more control over when things start in, but for my set of services the start order doesn’t matter, so I haven’t. ### the networking is easy to use It’s important here that the containers be able to connect to each other. Docker Compose makes that super simple! If I have a Rails server running in my `rails_server` container on port 3000, then I can access that with `http://rails_server:3000`. So simple! Here’s a snippet from my nginx configuration file with how I’m using that in practice (I removed a bunch of `proxy_set_header` lines to make it more clear) ``` location ~ /proxy.* { proxy_pass http://go_server:8080; } location @app { proxy_pass http://rails_server:3000; } ``` Or here’s a snippet from my Rails project’s database configuration, where I use the name of the database container (`db`): ``` development: <<: *default database: myproject_development host: db # <-------- this "magically" resolves to the database container's IP address username: postgres password: password ``` I got a bit curious about how `rails_server` was actually getting resolved to an IP address. It seems like Docker is running a DNS server somewhere on my computer to resolve these names. Here are some DNS queries where we can see that each container has its own IP address: ``` $ dig +short @127.0.0.11 rails_server 172.18.0.2 $ dig +short @127.0.0.11 db 172.18.0.3 $ dig +short @127.0.0.11 web 172.18.0.4 $ dig +short @127.0.0.11 go_server 172.18.0.5 ``` ### who’s running this DNS server? I dug into how this DNS server is set up a very tiny bit. I ran all these commands outside the container, because I didn’t have a lot of networking tools installed in the container. **step 1**: find the PID of my Rails server with `ps aux | grep puma` It’s 1837916. Cool. **step 2**: find a UDP server running in the same network namespace as PID `1837916` I did this by using `nsenter` to run `netstat` in the same network namespace as the `puma` process. (technically I guess you could run `netstat -tupn` to just show UDP servers, but my fingers only know how to type `netstat -tulpn` at this point) ``` $ sudo nsenter -n -t 1837916 netstat -tulpn Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name tcp 0 0 127.0.0.11:32847 0.0.0.0:* LISTEN 1333/dockerd tcp 0 0 0.0.0.0:3000 0.0.0.0:* LISTEN 1837916/puma 4.3.7 udp 0 0 127.0.0.11:59426 0.0.0.0:* 1333/dockerd ``` So there’s a UDP server running on port `59426`, run by `dockerd`! Maybe that’s the DNS server? **step 3**: check that it’s a DNS server We can use `dig` to make a DNS query to it: ``` $ sudo nsenter -n -t 1837916 dig +short @127.0.0.11 59426 rails_server 172.18.0.2 ``` But – when we ran `dig` earlier, we weren’t making a DNS query to port 59426, we were querying port 53! What’s going on? **step 4**: iptables My first guess for “this server seems to be running on port X but I’m accessing it on port Y, what’s going on?” was “iptables”. So I ran iptables-save in the container’s network namespace, and there we go: ``` $ sudo nsenter -n -t 1837916 iptables-save .... redacted a bunch of output .... -A DOCKER_POSTROUTING -s 127.0.0.11/32 -p udp -m udp --sport 59426 -j SNAT --to-source :53 COMMIT ``` There’s an iptables rule that sends traffic on port 53 to 59426. Fun! ### it stores the database files in a temp directory One nice thing about this is: instead of managing a Postgres installation on my laptop, I can just mount the Postgres container’s data directory at `./tmp/db`. I like this because I really do not want to administer a Postgres installation on my laptop (I don’t really know how to configure Postgres), and conceptually I like having my dev database literally be in the same directory as the rest of my code. ### I can access the Rails console with `docker-compose exec rails_server rails console` Managing Ruby versions is always a little tricky and even when I have it working, I always kind of worry I’m going to screw up my Ruby installation and have to spend like ten years fixing it. With this setup, if I need access to the Rails console (a REPL with all my Rails code loaded), I can just run: ``` $ docker-compose exec rails_server rails console Running via Spring preloader in process 597 Loading development environment (Rails 6.0.3.4) irb(main):001:0> ``` Nice! ### small problem: no history in my Rails console I ran into a problem though: I didn’t have any history in my Rails console anymore, because I was restarting the container all the time. I figured out a pretty simple solution to this though: I added a `/root/.irbrc` to my container that changed the IRB history file’s location to be something that would persist between container restarts. It’s just one line: ``` IRB.conf[:HISTORY_FILE] = "/app/tmp/irb_history" ``` ### I still don’t know how well it works in production Right now my production setup for this project is still “I made a digitalocean droplet and edited a lot of files by hand”. I think I’ll try to use docker-compose to run this thing in production. My guess is that it should work fine because this service is probably going to have at most like 2 users at a time and I can easily afford to have 60 seconds of downtime during a deploy if I want, but usually something goes wrong that I haven’t thought of. A few notes from folks on Twitter about docker-compose in production: * `docker-compose up` will only restart the containers that need restarting, which makes restarts faster * there’s a small bash script [wait-for-it][3] that you can use to make a container wait for another service to be available * You can have 2 docker-compose.yaml files: `docker-compose.yaml` for DEV, and `docker-compose-prod.yaml` for prod. I think I’ll use this to expose different nginx ports: 8999 in dev and 80 in prod. * folks seemed to agree that docker-compose is fine in production if you have a small website running on 1 computer * one person suggested that Docker Swarm might be better for a slightly more complicated production setup, but I haven’t tried that (or of course Kubernetes, but the whole point of Docker Compose is that it’s super simple and Kubernetes is certainly not simple :) ) Docker also seems to have a feature to [automatically deploy your docker-compose setup to ECS][4], which sounds cool in theory but I haven’t tried it. ### when doesn’t docker-compose work well? I’ve heard that docker-compose doesn’t work well: * when you have a very large number of microservices (a simple setup is best) * when you’re trying to include data from a very large database (like putting hundreds of gigabytes of data on everyone’s laptop) * on Mac computers, I’ve heard that Docker can be a lot slower than on Linux (presumably because of the extra VM). I don’t have a Mac so I haven’t run into this. ### that’s all! I spent an entire day before this trying to configure a dev environment by using Puppet to provision a Vagrant virtual machine only to realize that VMs are kind of slow to start and that I don’t really like writing Puppet configuration (I know, huge surprise :)). So it was nice to try Docker Compose and find that it was straightforward to get to work! -------------------------------------------------------------------------------- via: https://jvns.ca/blog/2021/01/04/docker-compose-is-nice/ 作者：[Julia Evans][a] 选题：[lujun9972][b] 译者：[译者ID](https://github.com/译者ID) 校对：[校对者ID](https://github.com/校对者ID) 本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出 [a]: https://jvns.ca/ [b]: https://github.com/lujun9972 [1]: https://jvns.ca/#cool-computer-tools---features---ideas [2]: https://github.com/yudai/gotty/ [3]: https://github.com/vishnubob/wait-for-it [4]: https://docs.docker.com/cloud/ecs-integration/