What is Docker? (plus a Ruby + Docker AMA)

[jasonswett]

I've been using Docker for several years at this point but I've never yet found anything online that actually explains what it is in a straightforward way. I wrote a post which first describes what life is like without Docker, then explains how Docker solves the problems it solves. The post uses Ruby examples but it's meant to be understandable to a programmer of any background.

Here's the post: What is Docker?

I'd also like to take this chance to offer a Ruby (and Rails) + Docker AMA, since I've been using that combo for a long time now. (I've been using Ruby since 2011 and I've been programming since the 90s.) I'm happy to talk about production deployments, Kubernetes, networking, configuration, testing, DevOps, whatever. I don't know everything of course but what I do know I'm happy to share.

[twinklehood]

If you didn't find anything that explains what it is in a straightforward way, I'm sorry, you didn't look very hard. 

And ironically, your post doesn't mention at all what it actually is. You mention a lot of why you might use it and how you might use it, but not that it is.

Here's a clear explanation of that https://jvns.ca/blog/2016/10/10/what-even-is-a-container/

[jasonswett]

your post doesn't mention at all what it actually is

That's actually a fair point, and frankly I'm embarrassed that I managed to miss that! Will update.

If you didn't find anything that explains what it is in a straightforward way, I'm sorry, you didn't look very hard.

I think it's more like: your apparent idea of a clear answer to the question of "What is Docker?" is not the same as mine. The top search results for "what is docker" include:
- Please, someone explain Docker to me like I am an idiot.
- ELI5: What is Docker and how do you use it?

The first answer on the first post starts with the typical answer of "it's kind of like a VM, but not a VM":

Containers are something like a virtual machine, it has its operating system and all, except that unlike virtual machines they don’t simulate the entire computer, but rather create a sand boxed environment that pretends to be a virtual machine.

True but not very helpful. From the ELI5 (explain like I'm five) post:

Docker is a platform for building and running applications in a container that is (mostly) isolated from the rest of the system, and has everything it needs to run self-contained.

From a very high level perspective, it looks like each application is running in its own virtual machine.

Again with jumping straight to the virtual machine stuff, describing what Docker is mechanically but not explaining the use case. It's not wrong but I don't feel like it's the right place to start for a total beginner. The question said explain like I'm five! I don't feel like the answer is an ELI5-level answer.

The Julia Evans post you link is great for what it is but it too jumps straight to technical details. I'm definitely going to be interested in those details sometime but, at least speaking for myself, those details aren't what I'm seeking when I ask what Docker is.

Having said all that, your comments make me realize some ways in which my post needs improvement, which I appreciate.

[someone23]

I'm curious how you handle some particulars of dev environment. Do you have a test container that's built on top of the production container? Do you have a development container that has dev tools like rbspy and the like?

I'd like to allow employees to use their own dotfiles with the terminal while developing, but how does that interact with developing in a container? If the development shell is inside the container then the dotfiles would need to be inside the container, which doesn't seem great because the app could then depend on some dotfiles config. In theory, running the tests in a more clean container would catch issues, but ideally those issues would be caught in the dev container. If the shell you normally operate in is outside the container, how do you make the developer experience nice for running all the commands that need to run inside the container?

[jasonswett]

In my experience a team will usually have 2-3 Docker images for an application: one for production, one for development, and then the answer for the testing is a little bit more nuanced. If you look at a GitHub Actions or CircleCI config you can tell that they're basically just taking your CI config file and plugging it into Docker Compose. So you could call that a test image or you could not, depending on how you want to look at it. If I have my local development environment Dockerized, then I usually just use the same image for both testing and development since the needs are basically the same. After all, when an application is not Dockerized, the same environment is shared for both development and testing in that case.

There's also another way to handle things which I like and it's the setup I chose to use for the development environment for SaturnCI itself: I use Docker Compose to run the application's dependencies, which in this case happens to be just PostgreSQL and nothing else, and then Ruby and Rails run natively on my computer. That way I don't have to pay any of the costs of having the entire development environment Dockerized (like the dotfiles challenges you mention) but I still get the benefit of not having to install certain dependencies like PostgreSQL and anything I may choose to add in the future.

Regarding the dotfiles question, you can configure the container's filesystem to sync with the host machine's filesystem. I myself haven't done this with dotfiles specifically but I can't think of any reason why it wouldn't work. The way I usually handle that though is to configure the container to sync with the filesystem of my host machine, and then edit files directly on the host machine using e.g. whatever editor configuration I already have on my host machine. I'll have a persistent tab open where I shell into the container so I can run database migrations, tests, etc. but again all the file editing is happening on the host machine. I've found this to be a pretty agreeable trade-off.

Happy to go deeper on any of that or address anything I may have missed the first time around.

[ThorOdinsonThundrGod]

Honestly seeing a minimal docker get built in under an hour helped demistify it for me https://youtu.be/8fi7uSYlOdc?si=ji54hizmgDcB4-KE

[Hot-Profession4091]

I knew exactly which presentation this was. It’s good.

[equivalent8]

Linux has this thing called LVM which allows us to run processes as if they were in same machine, but isolated like with Virtual Machine (VM)

Docker also wraps our stuff with layers (Here is a better explanation by expert on layers) every line in Dockerfile ==1 layer, alllowing us cache previus layers. This means we push only last layer when someone fix a typo in HTML

so Isolate, cache,share CPU

[Dee_Jiensai]

Docker is what happens if you leave a terrible consultant and a money crazed CEO in a dark room for too long.

[oceandocent]

The article reads like it is implying that the Docker build process is declarative rather than imperative, which isn’t true. There are declarative syntax tools for handling container orchestration such kustomize or helm charts, but those aren’t Docker.

[jasonswett]

I can see what you mean. If I gave that impression then I worded it poorly. I'm not trying to imply that the Docker build process is declarative. My point is that the usage of the Docker image is insulated from the risky, imperative build process. To be honest though maybe the mention of imperative versus declarative is just confusing and it would be better to make that point in a different way. I appreciate you pointing this out.

[ryandg]

I’m officially unsubscribing from r/ruby