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Pro Very fast parallel testing
Tests can be parallelized across multiple machines reducing test times drastically. They support up to 8-way parallelization. Additionally, CircleCI caches the build environment.
Pro Quick setup
CircleCI excels with its setup process. All that's needed is a GitHub login and CircleCI automatically detects the settings for Ruby, Python, Node.js, Java and Clojure. The setup process is their most widely praised feature.
Pro SSH support
Users can access the Virtual Machine via SSH and run commands.
Pro Simple and intuitive GitHub integration
CircleCI can be connected to any project that is hosted on GitHub by logging in using the GitHub OAuth and adding the desired repository.
Whenever a new commit is pushed to GitHub, CircleCI runs the tests that have been already defined and if none of them fails, the build is deployed to the runtime environment.
Pro Easy configuration with YAML
In most cases CircleCI automatically get settings from your code. When it fails, edit circle.yml.
Pro Clean, intuitive UI
Circle CI's web UI is clean and easy to use.
It gives all the information for a single build in a feed and gives the explanation for each step of the build, what it's doing and what the step is related to. On the top it displays author information and the time and date when the build was started and finished.
This is all done by giving only the most essential information without clogging the screen.
Pro Headless browser support
Alongside latest Chrome, Firefox and Webkit (installed using xvfb), CircleCi supports the use of Selenium, PhantomJS as well as tools like Capybara and Cucumber.
Pro Supports 8 languages and 16 databases
It als has support for: MySQL, MongoDB, PostgreSQL, Cassandra, Riak, Redis, SQLite, Solr, CouchDB, ElasticSearch, Neo4j, Couchbase, Lucene, Sphinx, ThriftDB, Memcache.
Pro Intelligent notifications
CircleCI can notify via email, Hipchat, Campfire and more. And it does so only when necessary.
Pro Can test many code pushes concurrently
You can push multiple batches of code concurrently.
Pro Supports 10 Continuous Deployment solutions
Support for Heroku, AWS, Engine Yard, dotCloud, Fabric, Nodejitsu, AppFog, Capistrano, Rockspace, Joynet.
Integration with Heroku is solid with the ability to automatically deploy or merge branches.
CircleCI is also very flexible with the deployment arrangement allowing SSH key management, deployment freedom including directly to a PaaS, using Capistrano, Fabric, arbitrary bash commands, or by auto-merging to another branch, or packaging code up to S3.
Pro Support for Queues
Support for RabbitMQ, Beanstalk and Resque through Redis.
Pro Supports Docker
CircleCI can continuously deliver Docker images to hosts that support Docker containers.
Con Changes the environment without warning
Con Docker is way outdated on the VM provided
Currently (October 5th 2016), Docker installed on the VM is: 1.9.1-circleci-cp-workaround, build 517b158, and docker-compose is 1.5.2, build 7240ff3. docker-compose in particular is almost too old to be used.
Con Does not cache docker images
The way to fake it is to save the image on disk, in the cache folder (it tars it), and restore it afterwards. But in tests it was slower than not caching.