docker-compose for Swarm: docker stack

docker compose + swarm = docker stackImagine you configured your new shiny Docker cluster and now ready to fill it with dockerized applications. How exactly are you going to do that? Not by manually typing docker service create for every app, right? Especially when average application that requires cluster will contain more than one service in it.

In standalone Docker we had docker-compose tool, which allowed us to describe all app containers in single docker-compose.yml file and then start it with docker-compose up. Can we use the same for Swarm? Absolutely.

Introducing docker stack

Among the other commands brought by Docker in Swarm mode there is docker stack, which, believe it or not, works almost the same as docker-compose. With little to no modification it even can use compose YAML files. If you’d want to update your existing compose file to be compatible with Stack, you’d need to do the following:

  1. Use at least 3rd version of compose file. E.g. version: '3.1'
  2. Make sure there’re no build (and few others) commands in it.

And that’s it! Now you’re ready to type docker stack deploy -c my-compose-file.yml my-stack-name and send YAML file to the cloud.

Example

In last article we ended up with Docker cluster and two services in it: viz for visualizing cluster state and replicated web service for performance testing. Even though it wasn’t much of a typing to create those, commands themselves were reasonably complicated and involved Swarm features like constraints and replicas. Instead, we can create two docker-compose-like YAML files for viz and web services and make their deployment absolutely trivial.

0. Prerequisites

To follow along you’ll need a configured instance of Docker in Swarm mode and previous article explains how to create one (steps 0 and 1). You don’t even need multi-host cluster: local machine in Swarm mode is perfectly fine.

1. Stack definition for viz

In case you forgot (like I did) how visualization service command looked like, it was the following:

YAML file for it will look pretty much like the one you’d create for docker-compose:

The difference is in latest version number 3.1 and deploy section which was introduced specifically for Swarm. The following command will deploy the stack:

-c viz.yml specifies file name to use and viz is the stack name. Now, when command has finished, we can list deployed stacks with docker stack ls and navigate to port 8080 to see visualization service in action:

viz

2. Stack definition for web

web service was good old nginx container with port 80 being published. To make things a little bit more interesting, let’s put two services into the same file. Just to make sure it works the same way as it did in docker-compose. Let’s call the first service web1, the second – web2 and this is how their YAML looks like:

As for deployment command, it hasn’t changed a bit:

Again, checking the stack status:

web

So far, so good.

3. How to update existing stack

I remember, when I had to update container deployed with docker-compose, I had to do at least three steps: docker-compose stop srv, docker-compose build srv and docker-compose up -d --no-deps srv. It got a little bit simpler with stacks.

Original web service definition from previous article ended up being upscaled to two replicas. For today’s service we can specify number of replicas directly in YAML file and that also will be a good reason to update the whole stack. What’s more, why not replicate both web1 and web2 services? More containers, more colored rectangles at visualization page.

And this is how stack update command would look like:

Yup, this is the same command as for stack initial deployment: deploy does both installs and upgrades.

Updated web service

Summary

There’re good news: docker-compose approach for complex containerized applications deployment is still there. For Docker in Swarm mode it’s called docker stack and it’s not something new to learn. If you know compose, you already know Stack. It uses slightly newer YAML file format with new Swarm specific settings in it and few limitations, but other than that it’s good old compose in disguise.

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