bulk-cdk: complete README (#43391)

Co-authored-by: Evan Tahler <[email protected]>

airbyte-cdk/bulk/README.md

@@ -1,15 +1,107 @@
 # Bulk CDK
 
 The Bulk CDK is the "new java CDK" that's currently incubating.
-It's written in Kotlin and consists of a _core_ and a bunch of _toolkits_:
-- The _core_ consists of the Micronaut entry point and other objects which are expected in
-  connectors built using this CDK.
-- The _toolkits_ consist of optional modules which contain objects which are common across
-  multiple (but by no means all) connectors.
-
-While the CDK is incubating, its published version numbers are 0.X where X is monotonically
-increasing based on the maximum version value found on the maven repository that the jars are
-published to: https://airbyte.mycloudrepo.io/public/repositories/airbyte-public-jars/io/airbyte/bulk-cdk/
-
-Jar publication happens via a github workflow triggered by pushes to the master branch, i.e. after
-merging a pull request.
+As the name suggests, its purpose is to help develop connectors which extract or load data in bulk.
+The Bulk CDK is written in Kotlin and uses the Micronaut framework for dependency injection.
+
+## Structure
+
+The Bulk CDK consists of a _core_ and a bunch of _toolkits_.
+
+### Core
+
+The _core_ consists of the Micronaut entry point and other objects which are expected in
+connectors built using this CDK.
+
+The core is broken down into multiple gradle projects; for example the core functionality for
+building sources is in `extract`.
+
+Following up on that example, the expectation for a source connector is that it will use all the
+interfaces and implementations in `extract` unless it has a very good reason not to.
+There is plenty of value in having all source connectors behave predictably.
+
+### Toolkits
+
+The _toolkits_ consist of optional modules which contain objects which are common across
+multiple (but by no means all) connectors.
+
+For example, there's an `extract-jdbc` toolkit to help build source connectors which extract data
+using the JDBC API.
+The expectation for a toolkit is that it provides naive implementations of core interfaces.
+These implementations will be thoroughly tested inside the CDK to serve as a baseline of
+functionality; however the connector may (and in fact often should!) replace parts of these.
+
+Following up on the example of `extract-jdbc`, a source connector needs to implement SQL query
+generation interfaces and, for schema discovery, may prefer to query system tables directly
+instead of relying on the generic JDBC metadata methods.
+
+## Dependencies
+
+The Bulk CDK gradle build relies heavily on so-called [BOM dependencies](https://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html#bill-of-materials-bom-poms).
+This pattern is strongly encouraged to keep transitive version conflicts to a minimum.  This is beneficial for many reasons, including reproducible builds and a good security posture. 
+
+Consider for example the whole Jackson ecosystem.
+Using a BOM allows us to add specific Jackson dependencies without having to figure out which
+version number to use.
+This has some pleasant ripple-effects:
+
+- When the need comes to bump the version, there's only one version number to bump and that's in
+  the BOM import.
+  Consequently, the declared version has a much higher chance of being the effective version
+  picked by gradle during dependency resolution.
+
+- The BOM import is re-exported by the `bulk-cdk-core-base` artifact meaning that the rest of the
+  CDK as well as connectors don't need to worry about Jackson version numbers either.
+
+It gets better when multiple BOMs are involved.
+Consider for example Micronaut and Jackson: Micronaut also depends on Jackson.
+This can (and will!) cause dependency version conflicts; these are much easier to resolve by
+reconciling just two BOM versions.
+
+While BOMs are undoubtedly useful, let's still try to keep external dependencies to a minimum
+outside of tests.
+Less dependencies, less problems.
+
+## Developing
+
+Perhaps the most striking difference with the legacy java CDK from a connector DX perspective is
+that there are no facilities equivalent to `useLocalCdk = true`.
+
+This is deliberate and the intention here is to force the testing of CDK functionality to remain
+in the CDK.
+Recall that this is too often not the case in the legacy java CDK because it's simply not possible
+to do so there.
+
+The Bulk CDK is different.
+Dependency injection makes it possible to mock concrete implementation behavior realistically
+enough that Bulk CDK tests have entire fake connectors defined inside of them.
+
+There's no reason now not to first make changes to the CDK and publish those, and only then make
+downstream changes to a connector.
+
+If there's truly a need to develop both simultaneously, then the way to go may be to:
+1. do experimental development in the connector, keeping the CDK- and the connector-specific code
+   separate;
+2. once the CDK-specific code is reasonably mature, hoist it into the Bulk CDK and test it there;
+3. finally, publish those changes and have the connector depend on the latest Bulk CDK version.
+
+## Publishing
+
+While the CDK is incubating, its published version numbers are 0.X where X is the _build number_.
+This build number is monotonically increasing and is based on the maximum version value found on
+the [maven repository that the jars are published to](https://airbyte.mycloudrepo.io/public/repositories/airbyte-public-jars/io/airbyte/bulk-cdk/).
+
+Artifact publication happens via a [github workflow](../../.github/workflows/publish-bulk-cdk.yml)
+which gets triggered by any push to the master branch, i.e. after merging a pull request.
+
+From a contributor's perspective, this means that there's no need to worry about versions or
+changelogs.
+From a client's perspective, just always use the latest version.
+
+Once the incubation period winds down and the CDK stabilizes, we can start thinking about contracts,
+semantic versioning, and so forth; but not until then.
+
+## Licensing
+
+The license for the Bulk CDK is Elastic License 2.0, as specified by the LICENSE file in the root
+of this git repository.