Concept
The four kinds of open
Open source, open formats, open interfaces, and open governance are separate claims, and conflating them is how lock-in survives.
Open is the most overloaded word in this subject. It does four different jobs, and a platform can satisfy one while failing the rest. Keeping them separate is the most useful habit available when evaluating anything in this space.
Open source
The implementation is licensed so anyone can use, modify, and redistribute it, including competitors. This is a claim about code.
It does not tell you whether the data format is documented, whether a second implementation exists, or whether you could leave.
Open format
The file or table layout is specified in public, in enough detail to implement from. This is a claim about data.
The strongest signal is not the specification’s existence but the number of independent implementations. Until a second one exists, nobody has tested whether the document was sufficient. An open format with one implementation is a published internal format.
Open interface
The protocol between components is documented and implemented by more than one party, so a client written against one implementation works against another. This is a claim about the seams.
For day-to-day freedom this often matters more than open source. A proprietary component behind a widely implemented interface is frequently easier to replace than an open source one that nothing else speaks.
Open governance
The specification changes in public, under a process no single commercial interest controls, with versioning and a visible record of decisions. This is a claim about the future.
It is the one people check last and the one that determines whether the other three stay true. A format governed by one vendor can be extended in ways only that vendor implements, which produces lock-in inside a compliant wrapper.
Applying it
Run the four questions against each layer separately rather than against the platform as a whole.
The common shape of a partly open system is open files, an open table format, and a proprietary catalog. Every byte is in a documented format, and the only thing that can tell you which bytes constitute the current table refuses to talk to anyone else. Your data is portable in principle and captured in practice.
That is not an accusation against any particular product. It is a description of where to look, because the layer that gets left closed is usually the one that controls access rather than the one that holds bytes.
Primary sources
Documentation and specifications move faster than any summary. Treat these as the authority and this page as orientation.
- Open Source and the Data Lakehouse Why the openness of each layer has to be assessed separately.