Add architecture explanation to application structure overview

[brainwane]

Let's update our application structure overview with a writeup like Zulip's architecture summary or a curated list of links to conference talks, blog posts, etc. that would get us 30% of the way towards a history and application overview like this MediaWiki overview. We'd mention frameworks and components we use, like:

• Pyramid
• Alembic
• Postgres

and engineering approaches we recommend people know about as they learn Warehouse.

Reasoning: Developers who are new to a codebase need is to know the design rationale of confusing bits -- why it was made this way, what decisions are embedded in particular choices, whether particular components are the result of a feature request, a quick fix after an outage, an experiment, etc. (This is based on research summarized in Making Software.)

Discussed a bit on the pypa-dev mailing list.

[brainwane]

@lgh2 just chatted with Ernest and got some notes that I'll be turning into a PR. Here are those notes for reference -- they are very rough because I requested very quick notes, so that's my fault, not hers:

The Warehouse codebase

Warehouse uses the Pyramid web framework, the SQLAlchemy ORM, and Postgres for its database. Warehouse's front end uses Jinja2 templates.

The application exists within two Docker containers, one of which contains static files for the website, and the other which contains the Python web application code running in a virtual environment and the database. In the development environment, Docker Compose manages running the containers and the connections between them.

The top-level directory of the Warehouse repo contains a number of files. Among them are the license file, contributing.rst and readme. The requirements.txt file is for the Warehouse virtual environment. The Dockerfile creates the Docker containers that Warehouse runs in, and the docker-compose yml file configures docker compose. Test configuration is in setup.cfg. Heroku uses runtime.txt. The makefile contains commands to spin up Docker compose and the Docker containers. There are also some files associated with Warehouse's front end.

# add files

Since Warehouse was built on top of a pre-existing database, some of the code in the ORM may not look like code from SQLAlchemy’s documentation in order to make it fit the existing tables. There are some places where joins are done using logic instead of a foreign key.

Warehouse also uses Pyramid’s hybrid URL traversal and dispatch. Using factory classes, URLs are pre-populated before the view is requested.

bin/ - high-level scripts for Docker
dev/ - assets for dev env
tests/ - tests
warehouse/ - code in modules
    legacy/ - most of the implementation
    forklift/ - APIs for upload
    accounts/ - user accounts
    admin/ - administrator-specific
    cache/ - Warehouse - more goes out than goes in - cache as much as possible
    classifiers/ - frame classifiers
    cli/ - entry scripts
    i18n/ - internationalization
    locales/ - internationalization
    manage/ - DB
    migrations/ - DB
    packaging/ - models
                - rate limiting to prevent abuse
                - RSS feeds
                - site maps
    utils/

• sql - some code not in docs because relations already existed

• some use logic and not foreign key and are joined on names (this may change)

• factory methods prepopulate url before view requested

Pyramid hybrid URL Traversal and Dispatch:

https://docs.pylonsproject.org/projects/pyramid/en/latest/narr/hybrid.html

Pyramid: https://docs.pylonsproject.org/projects/pyramid/en/latest/index.html
SQLAlchemy: https://docs.sqlalchemy.org/en/latest/
Postgres: https://www.postgresql.org/docs/

Docker: https://docs.docker.com/

Docker Compose: https://docs.docker.com/compose/overview/

[brainwane]

It would be great if this documentation also explained what files/directories/libraries Warehouse uses to produce its various APIs.

[di]

@brainwane Could you outline what was missing from #2937 that would fully resolve this issue?

[brainwane]

Thanks for asking @di. I'd like the Warehouse developer documentation to include:

• PyPI's general load and performance expectations
  • how big the database is
  • our Service Level Agreement-type expectations of the service; "pre-production" on Warehouse has always generally meant "we're not monitoring it, we make no promises on uptime, some things are missing and objects may move around", but "beta" and "production" mean higher reliability promises
  • (nice to have) distinction between app and infrastructure?
• any other components or related repositories developers should know about, and (if it's not obvious) why/how Warehouse uses them (alembic, celery, readme-renderer, Elastic Search, pypi-theme)
• usage assumptions/concepts, such as:
  • we focus on architecture and features for the PyPI and Test PyPI, and people who want to run their own package indexes usually use this, bandersnatch, or devpi
  • most people browsing are not logged in; there are probably <10 users who'd access the admin UI, tens of thousands of project owners/maintainers, etc.
  • projects are mutable but releases are not (and relationship to file name/hash reuse)
  • projects, releases, packages, authors, owners, maintainers (De-duplicate "Owner" and "Maintainer" roles  #2745, Author vs Maintainer in the metadata vs Owner vs Maintainer in PyPI #2059)
  • Warehouse has never been deployed with its own database, and has always shared a database with production; the very first thing warehouse did was take over ownership of migrations to the database (it was easier to do that than to try to create a mirror database, but part of it was purposeful, since if it was affecting production from day 1, then there could be no mistake where you did something sloppy the first time around (e.g. don't check permissions or something) with the idea that you'd come back and fix it before launch, which you might forget) -- explanation from Donald
• any special security concerns Warehouse has above and beyond your usual web app
• (nice to have) likely future architectural changes (e.g., user groups)
• (nice to have) pointer to curated list of presentations

[brainwane]

In today's Warehouse developers' meeting we decided to pare down our near-future milestones on our development roadmap so they really only contain the essential bugfixes and features we need to launch, replace legacy PyPI, and shut down the old site. So I'm moving this issue into a milestone further in the future.