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CircleCI is cloud-based and 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.
Read CircleCI vs. GitLab: Choosing the Right CI/CD Tool.
SpecsUpdate
Pros
Pro Supports 8 languages and 16 databases
Support for Ruby, Python, Node, Java, PHP, RoR, DJ, JavaScript. It also detects settings for Ruby, Python, Node.js, Java and Clojure.
It als has support for: MySQL, MongoDB, PostgreSQL, Cassandra, Riak, Redis, SQLite, Solr, CouchDB, ElasticSearch, Neo4j, Couchbase, Lucene, Sphinx, ThriftDB, Memcache.
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 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.
Cons
Con Only supports GitHub and Bitbucket
CircleCI has support only for projects hosted on GitHub or Bitbucket. Teams that use other alternatives are forced to rely on another CI tool or use third-party solutions to be able to integrate CircleCI.