Packages

Rails is a monorepo of 12 independently-published Ruby gems plus a top-level meta-gem (rails) that depends on them. Each component lives in its own root-level directory with the same shape: lib/, test/, bin/test, a *.gemspec, a CHANGELOG.md, and a README.

This section documents each component as a "package" — its purpose, layout, key abstractions, and where to start when modifying it. The components are loosely coupled: in principle, a change in one shouldn't break another unless an explicit dependency is in play.

Dependency layering

graph TD
    AS[Active Support] --> AM[Active Model]
    AS --> AR[Active Record]
    AS --> AP[Action Pack]
    AS --> AV[Action View]
    AS --> AJ[Active Job]
    AM --> AR
    AP --> AV
    AP --> AMail[Action Mailer]
    AR --> AStor[Active Storage]
    AJ --> AStor
    AStor --> AT[Action Text]
    AR --> AMbox[Action Mailbox]
    AS --> AC[Action Cable]
    AS --> RT[Railties]
    AP --> RT
    AV --> RT

Components

In rough order from the bottom of the dependency stack to the top:

Page	Component	Lines (lib/)	One-line purpose
active-support	activesupport/	32,407	Ruby core extensions and shared infrastructure
active-model	activemodel/	9,017	Validations, callbacks, dirty tracking for plain Ruby objects
active-record	activerecord/	69,692	ORM and database abstraction
action-pack	actionpack/	28,574	Routing + Action Controller + Action Dispatch
action-view	actionview/	19,401	Templates, ERB, and view helpers
action-mailer	actionmailer/	2,755	Outbound email
action-mailbox	actionmailbox/	728	Inbound email
active-job	activejob/	4,743	Background job abstraction
action-cable	actioncable/	4,198	WebSocket integration
active-storage	activestorage/	3,904	File uploads to disk and cloud
action-text	actiontext/	2,580	Rich text content (Trix-backed)
railties	railties/	15,862	The rails CLI, generators, and application boot

For the high-level relationships and the request lifecycle, see overview/architecture. For Rails-internal vocabulary, see overview/glossary.