Circle CI pros & cons, features, benefits of Continuous Integration server

	Circle CI https://circleci.com
Unique feature	Premium Support
Type of product	SaaS, Self-hosted / On Premise
Offers a free plan	Yes Provides a free plan for it's SaaS solution, for up to 1000 build minutes per month, 1 container and 1 concurrent job. On top of this, it has a special Free Tier for open source projects, which extend the number of containers and concurrent jobs available.
Predictable pricing	Yes Provides a calculator, based on the number of concurrent jobs and containers, which can be used to determine pricing before purchase. They also give a guideline based on the number of developers you employ (2-3 containers per full-time developer)
Support / SLA	Yes While it's community is not quite as vibrant as it is for Jenkins, CircleCI even offers support premium support for companies who cannot afford any downtime in their CI/CD pipelines (https://circleci.com/support/premium-support/). For the regular plans, all plans except the free tier offer official email support. The free tier only includes community support (Discuss, StackOverflow, etc.)
Paralellism Every CI servers tends to address this differently (parallel, distributed, build matrix). Some of it is just marketing, and some is just nuance. For this table, parallel means that tasks can be run concurrently on the same machine, distributed means that tasks can be scaled horizontally, on multiple machines How to split tests in parallel in the optimal way with Knapsack Pro	Yes CircleCI can run builds in parallel, each build in a completely isolated environment by using containers.
Distributed builds distributed means that tasks can be scaled horizontally, on multiple machines How to split tests in parallel in the optimal way with Knapsack Pro	Yes The SaaS version distributes builds by default, while the self-hosted version has all the tools built-in for managing the cluster of builder machines.
Containers support / Build environment	Yes CircleCI runs every build in a container, or VM, ensuring an isolated, local scope for each build. The environment is also reset with each build, which can highlight hard-to-track issues related to assumptions about the environment that the project is deployed to.
Analytics / Status overview Analytics and overview referrs to the ability to, at a glance, see what's breaking (be it a certain task, or the build for a specific project)	Yes Available by default in Circle CI: https://circleci.com/docs/2.0/status/#section=jobs. Aditionally, the Insights Dashboard provides a very useful overview on build times, error rates and status for your projects: https://circleci.com/docs/2.0/insights/#section=jobs
Management support How easy is it to manage users / projects / assign roles and permissions and so on	No (partial) Using the Cloud plan for Circle CI requires no dedicated person for maintanence / management of the service. The CircleCI Server option (self-hosted) is also hassle free, as the process of installing and managing CircleCI and it's dependencies is automated. Developers are also given SSH acces to the builds (not the whole environment), while sys-admins can work on the host machine without worry about affecting the builds (which are containerized).
Self-hosted option	Yes
Hosted plans / SaaS	Yes
Build pipelines A continuous delivery pipeline is a description of the process that the software goes through from a new code commit, through testing and other statical analysis steps all the way to the end-users of the product.	Yes Pipelines in CircleCI are defined declaratively using an Yaml config file. CircleCI has special provisions for storing secrets in these files.
Reports Reports are about the abilty to see specific reports (like code coverage or custom ones), but not necesarily tied in into a larger dashboard.	Yes Reports are available
Ecosystem Besides the official documentation and software, is there a large community using this product? Are there any community-driven tools / plugins that you can use?	Yes, partially While CircleCI doesn't natively support plugins, it's core features address all of the core functionality that a CI/CD service needs. For integrating with other platforms or tools, there are integrations such as https://slack.com/apps/A0F7VRE7N-circleci. Certain jobs are available as CircleCI Orbs: https://circleci.com/orbs/
Specific language support: Ruby Some CI servers have built-in support for parsing RSpec or Istanbul output for example and we mention those. Some others make it even easier by detecting Gemfiles or package.json and automate parts of the process for the developer.	Yes For collecting test metadata and coverage, there is built-in support for Ruby (specifically, Cucumber and RSpec). Setting this up takes very little time and is well-documented on CircleCI's docs.
Specific language support: JavaScript	Yes For collecting test metadata and coverage, there is built-in support for Javascript. For code coverage, CircleCI understands Istanbul output (Jest also uses Istanbul for the code coverage reports), while for test metadata, the JUnit output format is natively supported.
Integrations 1st party support for common tools (like Slack notifications, various VCS platforms, etc)	Yes Available as CircleCI Orbs: https://circleci.com/orbs/
API Custom integreation is available, via an API or otherwise, it's mentioned separately as it allows further customization than any of the Ecosystem/Integration options	Yes REST API documentation available at https://circleci.com/docs/api/#api-overview. Creating Orbs is also documented at https://circleci.com/docs/2.0/orb-author/#introduction
Auditing	N/A
Additional notes

Programming Language	Supported test runners	Installation guide	Knapsack Pro Library README / Source
Ruby	RSpec, Cucumber, Minitest, test-unit, Spinach, Turnip	Install	knapsack_pro gem
JavaScript	Cypress.io	Install	@knapsack-pro/cypress
JavaScript	Jest	Install	@knapsack-pro/jest
JavaScript / TypeScript	Any test runner in JavaScript	How to build native integration with Knapsack Pro API to run tests in parallel for any test runner	@knapsack-pro/core
Any programming language	Any test runner	How to build a custom Knapsack Pro API client from scratch in any programming language	-

Circle CI pros & cons features of Continuous Integration server Why choose Circle CI? See product overview

Circle CI

Premium Support

SaaS, Self-hosted / On Premise

Yes

Yes

Yes

Yes

Yes

Yes

Yes

No (partial)

Yes

Yes

Yes

Yes

Yes, partially

Yes

Yes

Yes

Yes

N/A

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Circle CI pros & cons features of Continuous Integration server
Why choose Circle CI? See product overview

How to run parallel tests on Circle CI
to execute 1-hour test suite in 2 minutes?

How much can you save with faster tests
on Circle CI?