TechDocs: Document basic and recommended architecture

docs/features/techdocs/architecture.md

@@ -1,44 +1,153 @@
 ---
 id: architecture
-title: Architecture
-description: Documentation on Architecture
+title: TechDocs Architecture
+description: Documentation on TechDocs Architecture
 ---
 
 ## Basic (out-of-the-box)
 
 When you deploy Backstage (with TechDocs enabled by default), you get a basic
-out-of-the box experience. This is how the architecture of it looks.
-
-Note: See below for our recommended production architecture which takes care of
-stability, scalability and speed.
+out-of-the box experience.
 
 ![TechDocs Architecture diagram](../../assets/techdocs/architecture-basic.drawio.svg)
 
-We store the timestamp in memory.
+> Note: See below for our recommended deployment architecture which takes care
+> of stability, scalability and speed.
 
-Publishing to external storage is also possible. But status: none of the cloud
-storage solutions are supported yet.
+When you open a TechDocs site in Backstage, the
+[TechDocs Reader](./concepts.md#techdocs-reader) makes a request to
+`techdocs-backend` with the entity ID and the path of the current page you are
+looking at. In response, it receives the static files (HTML, CSS, JSON, etc.) to
+render on the page in TechDocs/Backstage.
 
-Speed is a bottleneck if doc repositories are very large (>100MBs).
+The static files consist of HTML, CSS and Images generated by MkDocs. We remove
+all the Javascript before adding them to Backstage for security reasons. And
+there are some additional techdocs metadata JSON files that TechDocs needs to
+render a site.
+
+The TechDocs Reader then applies a list of "Transformers" (see
+[Concepts](./concepts.md)) which modify the generated static HTML files for a
+number of use cases e.g. Remove certain headers, filter out some HTML tags, etc.
+
+Currently, we use the Backstage server's (or techdocs-backend's) local file
+system to store the generated files. Publishing to an external storage system
+(AWS S3, GCS, etc.) is also possible, but has not been implemented yet.
+
+A word about `UrlReader` vs Git preparer - Right now, we have two ways to fetch
+files from its source repository for docs site generation. 1. By using Git
+and 2. By directly using Source control (GitHub, Azure, etc.) APIs. This work is
+heavily in progress. Please reach out to us on Discord in the #docs-like-code
+channel to talk about it.
 
 ## Recommended deployment
 
-This is how we recommend deploying TechDocs for potential production use.
+This is how we recommend deploying TechDocs in production environment.
 
 ![TechDocs Architecture diagram](../../assets/techdocs/architecture-recommended.drawio.svg)
 
-TechDocs Backend: Responsible for access control and sending files over to
-TechDocs.
+The key difference in the recommended deployment approach is where the docs are
+built.
 
-Tokens (if possible): Read and Write tokens.
+We assume each entity lives in a repository somewhere (GitHub, GitLab, etc.). We
+recommend using a CI/CD pipeline with the repository that has a dedicated
+step/job to build docs for TechDocs. The generated static files are then stored
+in a cloud storage solution of your choice.
+[Track progress here](https://github.com/backstage/backstage/issues/3096).
 
-## FAQ
+Similar to how it is done in the Basic setup, the TechDocs Reader requests
+`techdocs-backend` plugin for the docs site. `techdocs-backend` then requests
+your configured storage solution for the necessary files and returns them to
+TechDocs Reader.
 
-Q: Why don't use Backstage server to serve static files? A: Not scalable.
-[Investigate more]
+We will provide instructions, scripts and/or templates (e.g. GitHub actions) to
+build docs in your CI/CD system.
+[Track progress here.](https://github.com/backstage/backstage/issues/3400) You
+will be able to use `techdocs-cli` to build docs and publish the generated docs
+site files to your cloud storage system.
+
+Note about caching: We have noticed internally that some storage providers can
+be quite slow, which is why we are recommending a cache that sits between the
+TechDocs Reader and the Storage.
+
+_Feel free to suggest better ideas to us in #docs-like-code channel in Discord
+or via a GitHub issue._
+
+### Security consideration
+
+Our biggest security concern is managing the access to the docs in the cloud
+storage. We also want to have only one security solution for all different types
+of storage (GCS, AWS, custom SFTP server, etc.) Restricting access to the
+storage and only allowing `techdocs-backend` to fetch files is a good way to
+achieve this.
+
+This would also allow us to use the access control management Backstage when
+that is ready.
+[Track progress here.](https://github.com/backstage/backstage/issues/3218)
+
+In theory, you can directly enable TechDocs Reader to read from your storage.
+But, you will have to think about how to do it without the docs being public and
+how access to user groups is managed.
+
+For cloud storage access tokens, `techdocs-backend` only needs a token with Read
+permissions. But in your CI/CD system, there needs to be a token with Write
+permissions to publish the generated docs site files.
+
+## FAQs
+
+**Q: Why do you have separate "basic" and "recommended" deployment approaches?**
+
+A: The basic or out-of-the-box setup is what you get when you create a new app
+or do a git clone of the Backstage repository. We want the first experience to
+_just work magically_ so that you can have your first experience with TechDocs
+which is smooth. However, if you decide to deploy Backstage/TechDocs for
+production use, the basic setup would work but there are going to be downsides
+as you scale with the number of documentation sites and sizes of them. So you
+would want to make sure the deployment is as stable as possible. Hence there is
+a recommended approach. There can be even more deployment approaches to TechDocs
+and we welcome such "Alternative" ideas from the community.
+
+**Q: Why don't you recommend techdocs-backend local filesystem to serve static
+files?**
+
+A: It would make scaling a Backstage instance harder. Think about the case where
+we have distributed Backstage deployments. Using a separate file storage system
+for TechDocs makes it easier to do some operations like delete a docs site and
+wipe its contents.
+
+**Q: Why aren't docs built on the fly i.e. when users visits a page, generate
+docs site in real-time?**
+
+A: Generating the content from Markdown on the fly is not optimal (although that
+is how the basic out-of-the-box setup is implemented). Storage solutions act as
+a cache for the generated static content. TechDocs is also currently built on
+MkDocs which does not allow us to build docs per-page, so we would have to build
+all docs for a entity on every request.
 
 # Future work
 
-Instead of storing frontend assets in cloud storage, it can only store texts and
-images. And all of frontend rendering is handled by Backstage and TechDocs
-reader.
+_Ideas here are far fetched and not in the project's milestone for near future
+(~6 months)._
+
+We currently depend on MkDocs to parse doc sites written in Markdown. And we
+store the generated static assets and re-use it later to render in Backstage. A
+better (futuristic) approach will be to directly parse whatever type of source
+files you have in your docs repository and directly render in Backstage in
+real-time.
+
+# Features status
+
+Status of all the features mentioned above.
+
+**In place ✅**
+
+- Basic setup with techdocs-backend file server as storage.
+
+**Work in progress 🚧**
+
+- Basic setup with cloud storage solution.
+
+**Not implemented yet ❌**
+
+- `techdocs-cli` is able to generate docs in CI/CD environment.
+- `techdocs-cli` is able to publish docs site to any storage.
+- `techdocs-backend` integration with Backstage access control management.