The most fundamental principle, historically reaching back to the Four Freedoms of the Free Software Foundation, is Open Source. Any software developed under the guidance of the Four Opens must be released under an open source license. It means that anyone should be able to study a program, modify it, and redistribute either the original or the modified version so that others may benefit from their work. But the Four Opens is much more than just access to the source code, as you will see in the following chapters.
As discussed earlier, free and open access to the source code is just just one factor, but it doesn't ensure anything about the quality, usability and scalability of that code. Open Source also entails the possibility to openly collaborate on any enhancement or new development direction to the software, and it should not be limited in features or performance to enable a company's business model. Open Core is a collaboration model where some of the functionality is open source, while there are features that are withheld by a company controlling the open source project, allowing to sell a proprietary "Enterprise Edition".
While the Open Core model is compatible with open source and might seem like a good model at first, it has a lot of shortcomings that prevents you from building a healthy and balanced community around it. It cannot engage participants from other vendor companies long term as not all functionality is available to add to the community version of the software. This can lead to copy the existing code base and develop it further under a different name and maybe license, this is what is called forking. This behavior creates fragmentation in the community that can result in tension and unhealthy environments.
Users of the open source edition of an Open Core software may fall into similar traps, as they can be forced to purchase the Enterprise Edition to access extra functionality that is only available there. This can result in lock-in, despite of them originally choosing an open source solution, which then didn't give them the freedom it was supposed to. This can lead them to the choice of looking for a different community or solution overall.
The Open Core model always fails when a contributor eventually tries to add a feature that is not supposed to be made available in the open source version according to the company in control, who owns the productized version. This circumstance leads to confusion, frustration and tension which undermines the concept of open collaboration on a level playing field, which the Four Opens are meant to ensure.
In addition, these enterprise features often fall in the groups of security or scalability, which are desirable by all users.
The Open Source Principle in Practice
Open Source begins with the choice of license a community applies to its project. In most cases, the communities supported by the Open Infrastructure Foundation use v2.0 of the Apache License1. It is:
This license meets the requirements of being able to modify and redistribute the work. It includes a number of provisions that also protect end-users by granting copyright and patent licenses to all, while limiting liability to the original copyright holder. This patent protection is one of the distinguishing features in comparison to other open source licenses, like the MIT License.
In practice, regardless if you are an individual or corporate contributor, you need to understand the consequences of contributing code to an Apache Licensed project, particularly the granting of copyright and patent licenses to all users of the software. To acknowledge this, many projects require that all contributors sign a "Contributor License Agreement" (CLA)5 or "Developer Certificate of Origin" (DCO).
Typically, a CLA is a stronger statement, attesting that you, as a contributor, have a right to submit work to the project and that they are granting copyright and patent licenses in accordance with the Apache License along with your submissions.
A DCO, on the other hand, is a bit lighter weight and is more of a statement (rather than a license) that you, as developer, are indeed authorized to submit changes to the project and understand that your contributions will be used in accordance with the license.
A CLA, being a stronger document, is also often considered to be a barrier to entry. A DCO, in contrast, lowers the barrier to entry by removing the requirement to consent to a legal document6.
Apache 2.0 is very liberal in allowing companies to modify and use the code in any way they want, and doesn't place requirements to release changes (although it doesn't prohibit them from doing so). This, along with the patent protections of the license, is one of the reasons why it is so popular. It has also been used in practice since 2004, and is fairly well understood amongst the corporate and open source legal communities.
This liberal view on modification does have some downsides, though. It becomes very easy for companies to withhold enhancements that would be beneficial to the wider community, or to make changes to their version of the software that breaks interoperability. While the license doesn't address these directly, there are guard-rails that a project can put in place to mitigate these risks.
Trademark programs based on interoperability or conformance testing are one such tool. The Open Infrastructure Foundation uses such a program. In order to qualify for the trademark, a product can not modify API code (thus adding a stronger modification restriction than provided by the Apache License), and it must successfully demonstrate compatibility by passing a suite of interoperability tests, run against the public API of the product. In this way, the scope of modifications is limited.
Furthermore, in case of fast-evolving infrastructure software, it's worth noting that keeping local changes private is not a great long-term strategy. Maintaining a delta between code running in production and fast-evolving upstream open source code is very costly, and gets more difficult as time passes. Technical debt adds up quickly to a point where paying it back is impossible. Engaging upstream to propose your local improvements and finally getting most of them in the open source project itself is the only sustainable way forward over the long run.