Docs: Document creating new extension APIs (apache#11425)

* Docs: Document creating new extension APIs

* fix

* Add clarification about extension APIs. Thanks @ozankabak

* Apply suggestions from code review

Co-authored-by: Mehmet Ozan Kabak <[email protected]>

* Add a paragraph on datafusion-contrib

* prettier

---------

Co-authored-by: Mehmet Ozan Kabak <[email protected]>

datafusion/core/src/lib.rs

@@ -174,7 +174,7 @@
 //!
 //! DataFusion is designed to be highly extensible, so you can
 //! start with a working, full featured engine, and then
-//! specialize any behavior for their usecase. For example,
+//! specialize any behavior for your usecase. For example,
 //! some projects may add custom [`ExecutionPlan`] operators, or create their own
 //! query language that directly creates [`LogicalPlan`] rather than using the
 //! built in SQL planner, [`SqlToRel`].

docs/source/contributor-guide/architecture.md

@@ -25,3 +25,77 @@ possible. You can find the most up to date version in the [source code].
 
 [crates.io documentation]: https://docs.rs/datafusion/latest/datafusion/index.html#architecture
 [source code]: https://github.com/apache/datafusion/blob/main/datafusion/core/src/lib.rs
+
+## Forks vs Extension APIs
+
+DataFusion is a fast moving project, which results in frequent internal changes.
+This benefits DataFusion by allowing it to evolve and respond quickly to
+requests, but also means that maintaining a fork with major modifications
+sometimes requires non trivial work.
+
+The public API (what is accessible if you use the DataFusion releases from
+crates.io) is typically much more stable (though it does change from release to
+release as well).
+
+Thus, rather than forks, we recommend using one of the many extension APIs (such
+as `TableProvider`, `OptimizerRule`, or `ExecutionPlan`) to customize
+DataFusion. If you can not do what you want with the existing APIs, we would
+welcome you working with us to add new APIs to enable your use case, as
+described in the next section.
+
+## `datafusion-contrib`
+
+While DataFusions comes with enough features "out of the box" to quickly start
+with a working system, it can't include everything useful feature (e.g.
+`TableProvider`s for all data formats). The [`datafusion-contrib`] project
+contains a collection of community maintained extensions that are not part of
+the core DataFusion project, and not under Apache Software Foundation governance
+but may be useful to others in the community. If you are interested adding a
+feature to DataFusion, a new extension in `datafusion-contrib` is likely a good
+place to start. Please [contact] us via github issue, slack, or Discord and
+we'll gladly set up a new repository for your extension.
+
+[`datafusion-contrib`]: https://github.com/datafusion-contrib
+[contact]: ../contributor-guide/communication.md
+
+## Creating new Extension APIs
+
+DataFusion aims to be a general-purpose query engine, and thus the core crates
+contain features that are useful for a wide range of use cases. Use case specific
+functionality (such as very specific time series or stream processing features)
+are typically implemented using the extension APIs.
+
+If have a use case that is not covered by the existing APIs, we would love to
+work with you to design a new general purpose API. There are often others who are
+interested in similar extensions and the act of defining the API often improves
+the code overall for everyone.
+
+Extension APIs that provide "safe" default behaviors are more likely to be
+suitable for inclusion in DataFusion, while APIs that require major changes to
+built-in operators are less likely. For example, it might make less sense
+to add an API to support a stream processing feature if that would result in
+slower performance for built-in operators. It may still make sense to add
+extension APIs for such features, but leave implementation of such operators in
+downstream projects.
+
+The process to create a new extension API is typically:
+
+- Look for an existing issue describing what you want to do, and file one if it
+  doesn't yet exist.
+- Discuss what the API would look like. Feel free to ask contributors (via `@`
+  mentions) for feedback (you can find such people by looking at the most
+  recently changed PRs and issues)
+- Prototype the new API, typically by adding an example (in
+  `datafusion-examples` or refactoring existing code) to show how it would work
+- Create a PR with the new API, and work with the community to get it merged
+
+Some benefits of using an example based approach are
+
+- Any future API changes will also keep your example going ensuring no
+  regression in functionality
+- There will be a blue print of any needed changes to your code if the APIs do change
+  (just look at what changed in your example)
+
+An example of this process was [creating a SQL Extension Planning API].
+
+[creating a sql extension planning api]: https://github.com/apache/datafusion/issues/11207