Is Ansible a replacement for a CI tool like Hudson/Jenkins?

Ansible is just a "glorified SSH loop". CI is not only the software running, but the whole process of how success and failure is handled, who gets notification, and how the change is merged into the target version control.

If we only focus on the software, CI is a reactive scheduler triggered by code changes, and triggering typical build-validate-release-deploy sequence of "steps".

So in respect of software, Ansible without additional "sugaring" is just a toolkit to run things, which can be those very steps, but it is not CI. The Ansible (without tower) totally lacks this reactive nature.

If you want to marry Ansible with CI, you can.

Ansible tower is a very Ansible oriented scheduler, but if you need CI software, I think you not necessarily need it. Any CI app capable of running shell script would be capable to launch Ansible playbooks.

Yet unlike Ansible tower - CI tools know to display test reports of all test frameworks, trigger notifications, etc.

Ansible tower can make sense in a complex environment with lots of groups touching Ansible code... The truth is I haven't seen a single real reason to pay for it. But if a manager liked the web interface nothing can stand "but others use it" logic.

I suspect the concept of Ansible tower was in response to puppet enterprise.

:)

First, Jenkins and Hudson are basically the same project. I'll refer to it as Jenkins below. See How to choose between Hudson and Jenkins?, Hudson vs Jenkins in 2012, and What is the most notable difference between Jenkins and Hudson from an user perpective? for more.

Second, Ansible isn't meant to be a continuous integration engine. It (generally) doesn't poll git repos and run builds that fail in a sane way.

When can I simply use Jenkins?

If your machine environment and deployment process is very straightforward (such as Heroku or iron that is configured outside of your team), Jenkins may be enough. You can write a custom script that does a deploy as the final build step (or a chained step).

When can I simply use Ansible?

If you only need to "deploy" without needing to build/test, Ansible might be enough. For instance, you can run a deploy from the commandline or using Ansible Tower. This is great for small projects, static sites, etc.

How do they work together?

A good combination is to use Jenkins to build, test, and save artifacts. Add a step to call Ansible or Ansible Tower to handle the actual deployment process. That allows Ansible to handle machine configuration and lets Jenkins handle the CI process.

What are the alternatives to Jenkins?

I strongly recommend Thoughtworks Go (not to be confused with Go the language) instead of Jenkins. Others include CruiseControl, TravisCI, and Integrity.