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Pros
Pro Docker integration
Shippable is built using Docker, a popular open source Linux container. It was originally built using it's own container but when that started to become too complex, they switched to using Docker. Since the beginning Shippable was different from other CI tools because while Shippable uses a container (Docker), traditionally CI tools have used virtual machines to manage their workloads.
Pro Built in devops state
Understanding the state of your application development steps for e.g. whats running in prod, whats the last stable release that has been test approved, what versions and commit SHAs make up a particular release are all critical pieces of information that streamlines your devops workflow. Shippable is the only provider that has this built in.
Pro Builds are faster
The fact that Shippable runs inside of Docker means that it keeps a persistent state and every build will not have to revert to initial state where it needs to install every dependency from the ground up. Classic CI tools that run on virtual machines need to reset their environment every time and every time install the gems, packages and services needed.
Cons
Con Docker security measures may be a hindrance
Shippable runs inside Docker containers. Docker has some specific security measures which may or may not become a hindrance in using Shippable. It may be harder for users who are not very comfortable with a Linux container environment and that can create some security problems. Even for more advanced users, it's still something more that they have to address while using Shippable.