# Three things I love about CircleCI

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I recently had to beef up the continuous deployment of Jupyter Book, and used it as an opportunity to learn a bit more about CircleCI’s features. It turns out, they’re pretty cool! Here are a few of the things that I learned this time around.

For those who aren’t familiar with CircleCI, it is a service that runs Continuous Integration and Continuous Deployment (CI/CD) workflows for projects. This basically means that they manage many kinds of infrastructure that can launch jobs that run test suites, deploy applications, and test on many different environments.

Here are some cool things that I now have a much better appreciation for:

## Re-run a job with SSH access

Often when tests don’t pass or a build otherwise fails, it’s really helpful to be able to get into the machine itself and just start poking around. It turns out that CircleCI makes this really easy! If a build has failed, then you can use the drop-down menu next to the “Restart Job” button to select “Restart Job with SSH”. The next time the job fails (which it probably will, since you’ve just restarted a job that already failed once), CircleCI will print the IP address and SSH command to connect to that machine remotely.

## Persisting files between jobs with Workspaces

Everything CircleCI does is based around containers - each job has a Docker image environment specified (and CircleCI curates a large list of containers for testing). One challenge this introduces is that it can be more complex to use jobs that have multiple languages or tools installed. You can always manually configure this, but I’ve found that another easy solution is to split your task across multiple jobs, and persist some of the files between them with CircleCI workspaces.

To set up a CircleCI workspace, you first need two jobs, then

jobs:
build_files:
docker:
# We use a Python image to test our files and run the test suite
- image: circleci/python:3.6-stretch
steps:
# Some steps to build files we need in another job
# Assume it places the built files into a folder called _build/
- run: build_stuff_with_python

# Persist the specified paths (see https://circleci.com/docs/2.0/workflows/#using-workspaces-to-share-data-among-jobs)
- persist_to_workspace:
# The root of the workspace, here just the CWD
root: .
# The sub-paths of the workspace to persist
paths:
- _build/

deploy_files:
docker:
# We'll use a Ruby image to deploy our files
- image: circleci/ruby:2.6
steps:
# Connect the files from the last job to this job
- attach_workspace:
# Must be absolute path or relative path from working_directory
at: /tmp/workspace

# Our final deployment steps
- run: deploy_files_with_ruby


Finally, we’ll set up a CircleCI workflow that runs the deployment job only after the build job as finished, since the deploy job depends on files that are created by the build job.

workflows:
version: 2
default:
jobs:
- build_files
#
- deploy_files:
requires:
- build_files


In this way, we’ve split our task (build a bunch of files, then deploy them online) into two different jobs. One that builds files with a Python container, and another that deploys them with a Ruby container.

## Re-use code snippets with Commands

Finally, many times you’d like to re-use the same set of snippets across multiple points of your CircleCI jobs. In 2.1, CircleCI added a new feature called Commands that does this fairly simply. Commands are kind of like functions in that they wrap up a collection of steps that can be re-used and parameterized. This means you can define a “template” of a collection of steps, then fill-in missing fields in that template in order to modify its behavior.

For example, here’s a Command template to build a site with Jekyll:

commands:
build_site:
description: "Build the site with Jekyll"
parameters:
# We'll define one parameter that lets us pass build arguments to Jekyll build
build_args:
type: string
default: ""
steps:
- run:
name: Build the website
# Note the << parameters.param >> syntax that lets you define your own inputs
command: bundle exec jekyll build << parameters.build_args >>


Now, we can re-use this command throughout our build steps. Here are two jobs that use this command in different ways:

jobs:
# Build the site to store artifacts
build_with_params:
steps:
# Build the site's HTML w/ the base_url for CircleCI artifacts
- build_site:
build_args: --baseurl /0/html/
# Build the site to store artifacts
build_without_params:
steps:
# Build the site's HTML w/ defaults for Jekyll
- build_site:
build_args: ""


Each of these jobs uses the command specified above in build_site, but they use the build_args parameter to modify its behavior each time.

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