docs: rfc for db-console bazel build

Described proposal and changes required for
`pkg/ui` UI projects to establish Bazel build
workflow and integration with other bazel
rules.

Release note: None

docs/RFCS/20210423_db-cosole-bazel-build.md

@@ -0,0 +1,92 @@
+- Feature Name:
+- Status: draft
+- Start Date: 2021-04-23
+- Authors: Andrii Vorobiov
+- RFC PR: [#65924](https://github.com/cockroachdb/cockroach/pull/65924)
+- Cockroach Issue:
+
+# Summary
+
+Migration from Make builds to Bazel build tool for the entire project is one of OKRAs which is aimed to
+deliver the product in a timely and sustainable manner. This proposal outlines a subset of changes required to perform 
+migration to Bazel build for DB Console (and UI related packages).
+
+# Motivation
+
+Migration should not affect the current build process with `Make` so it can be gradual (without
+breaking changes).
+The migration process can be broken down into the following parts:
+- establish Bazel build for Db Console (with support for both OSS and CCL licensing);
+- establish Bazel build and publishing artifacts to NPM registry for `cluster-ui` package;
+- build `crdb-protobufjs-client` package for both CCL and OSS licences.
+
+Bazel builds (comparing to Make build) allow defining granular steps which can be run independently for particular
+package instead of the entire project, which in turn reduces the build time and a chain of unnecessary rebuilds for
+unchanged parts of code.
+
+Github `Bazel` project already includes following tasks related to migration to Bazel for Db Console   
+- [build: Bazelize UI](https://github.com/cockroachdb/cockroach/issues/56068)
+- [build: Get built UI to bindata (bazel)](https://github.com/cockroachdb/cockroach/issues/56069)
+- [build,bazel: Implement JS protobuf support for Bazel](https://github.com/cockroachdb/cockroach/issues/59329)
+- [build: Get bazel building both ccl and oss builds](https://github.com/cockroachdb/cockroach/issues/56071)
+
+# Technical design
+
+Current UI project has following dependencies
+
+![DbConsole project dependencies](images/db-console-bazel-build.png?raw=true)
+
+where:
+- `*.proto` files are set of protobuf files that are required to build JS protobuf client code
+- `ccl/oss` specifies build condition, whether the project is built for CCL or OSS licence
+- `ccl protos/oss protos` - autogenerated JS protobuf client code
+- `db-console` is main `ui` project
+- `cluster-ui` is a nested `ui/cluster-ui` JS project
+- `ui/(ccl|oss)/bidndata.go` - autogenerated Go sources that embed JS bundled files
+
+#### Dependency graph of bazel rules:
+
+![Bazel rules dependencies](images/db-console-bazel-build-dependency-graph.svg)
+
+Bazel build pipeline for UI can be broken down in following steps:
+- Build protobuf js client code [db-console: bazel build for protobufjs clients #64065](https://github.com/cockroachdb/cockroach/pull/64065)
+- `cluster-ui` standalone build (react + webpack project) 
+- `db-console` standalone build (react + webpack project)
+- embed JS assets into go libraries (`pkg/ui/distoss/distoss.go` and `pkg/ui/distccl/distccl.go`)
+
+### Generate protobuf JS client code
+Protobuf CCL and OSS packages are considered as an independent bazel builds. The installation of `protobufjs` package
+requires additional steps to install required dependencies for protobufjs-cli tool. This problem persists in Makefile
+as well and will be fixed with major release of protobufjs ver 7. Installation of dependencies and binding protobuf cli
+is done according to example provided here: https://github.com/bazelbuild/rules_nodejs/tree/stable/examples/protobufjs.
+
+### Build `db-console` and `cluster-ui` packages
+`db-console` and `cluster-ui` are typescript packages which have their own webpack configuration to build production
+bundle, run dev server and watch for changes.
+To build these projects, additional project changes required:
+- change relative paths for input and output files to be absolute paths in webpack configs because bazel resolves
+  relative paths from workspace root dir (which is root of entire `cockroach` project)
+- optional dependencies that are required only for web development and was defined as a separate NPM package (with its
+own package.json file in `pkg/ui/opts`) is removed and dependencies are moved in `pkg/ui/workspaces/db-console/package.json`
+under `optionalDependencies` section. Along with this, installation of NPM packages is performed with `--ignore-optional`
+flag to prevent installation of optional during production builds (ie `yarn install --ignore-optional`).
+
+### Refactor project structure to use Yarn workspaces
+Existing project structure had multiple `node_modules` directories for different purposes. With Bazel, every 
+`node_modules` is treated as separate external workspaces and with such configuration it restricts to use dependencies 
+from single external workspace.
+
+To overcome this limitation, project structure is refactored to use Yarn workspaces, it allows to hoist all common
+dependencies to the top level in a single `node_modules` which is considered as a single external workspace for bazel.
+- `rules_nodejs` doesn't support Yarn workspaces (https://github.com/bazelbuild/rules_nodejs/issues/266) but for now the
+only limitation is that project specific dependencies are included in main workspaces rather than in external workspace.
+- all hoisted dependencies can be accessed from single external workspace (`@npm`) and project specific dependencies are
+bounded with `filegroup`'s.
+
+### Embeding JS assets into go libraries
+`go:embed` directive (available starting from ver 1.6) is used instead of `go-bindata` for embedding JS assets into 
+go libraries. It simplifies build process for both Make and Bazel builds because it eliminates running external
+commands to generate go sources with embedded data.
+
+# Unresolved questions
+- TBD