CircleCI is a cloud-based continuous integration server that supports Ruby on Rails, Sinatra, Node, Python, PHP, Java and Clojure. It's a hosted solution designed to reduce as much overhead as possible from the testing process.
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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 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 Can test many code pushes concurrently
You can push multiple batches of code concurrently.
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 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 Support for Queues
Support for RabbitMQ, Beanstalk and Resque through Redis.
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 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 Great customer support
Pro Intelligent notifications
CircleCI can notify via email, Hipchat, Campfire and more. And it does so only when necessary.
Pro Easy configuration with YAML
In most cases CircleCI automatically get settings from your code. When it fails, edit circle.yml.
Pro Supports Docker
CircleCI can continuously deliver Docker images to hosts that support Docker containers.
Pro SSH support
Users can access the Virtual Machine via SSH and run commands.
Con Only supports GitHub
CircleCI has support only for projects hosted on GitHub so teams that use BitBucket or any other alternative to GitHub are forced to rely on another CI tool or use third-party solutions to be able to integrate CircleCI with BitBucket. One of those solutions may be Cloudpipes.