GitLab Modular Monolith
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Summary
The main GitLab Rails
project has been implemented as a large monolithic application, using
Ruby on Rails framework. It has over 2.2 million
lines of Ruby code and hundreds of engineers contributing to it every day.
The application has been growing in complexity for more than a decade. The
monolithic architecture has served us well during this time, making it possible
to keep high development velocity and great engineering productivity.
Even though we strive for having an approachable open-core architecture
we need to strengthen the boundaries between domains to retain velocity and
increase development predictability.
As we grow as an engineering organization, we want to explore a slightly
different, but related, architectural paradigm:
a modular monolith design,
while still using a monolithic architecture
with satellite services.
This should allow us to increase engineering efficiency, reduce the cognitive
load, and eventually decouple internal components to the extend that will allow
us to deploy and run them separately if needed.
Motivation
Working with a large and tightly coupled monolithic application is challenging:
Engineering:
- Onboarding engineers takes time. It takes a while before engineers feel
productive due to the size of the context and the amount of coupling.
- We need to use
CODEOWNERS file feature for several domains but
these rules are complex.
- It is difficult for engineers to build a mental map of the application due to its size.
Even apparently isolated changes can have far-reaching repercussions
on other parts of the monolith.
- Attrition/retention of engineering talent. It is fatiguing and demoralizing for
engineers to constantly deal with the obstacles to productivity.
Architecture:
- There is little structure inside the monolith. We have attempted to enforce
the creation of some modules
but have no company-wide strategy on what the functional parts of the
monolith should be, and how code should be organized.
- There is no isolation between existing modules. Ruby does not provide
out-of-the-box tools to effectively enforce boundaries. Everything lives
under the same memory space.
- We rarely build abstractions that can boost our efficiency.
- Moving stable parts of the application into separate services is impossible
due to high coupling.
- We are unable to deploy changes to specific domains separately and isolate
failures that are happening inside them.
Productivity:
- High median-time-to-production for complex changes.
- It can be overwhelming for the wider-community members to contribute.
- Reducing testing times requires diligent and persistent efforts.
Goals
- Increase the development velocity and predicability through separation of concerns.
- Improve code quality by reducing coupling and introducing useful abstractions.
- Build abstractions required to deploy and run GitLab components separately.
How do we get there?
While we do recognize that modularization is a significant technical endeavor,
we believe that the main challenge is organizational, rather than technical. We
not only need to design separation in a way that modules are decoupled in a
pragmatic way which works well on GitLab.com but also on self-managed
instances, but we need to align modularization with the way in which we want to
work at GitLab.
There are many aspects and details required to make modularization of our
monolith successful. We will work on the aspects listed below, refine them, and
add more important details as we move forward towards the goal:
- Deliver modularization proof-of-concepts that will deliver key insights.
- Align modularization plans to the product structure by defining bounded contexts.
- Separate domains into modules that will reflect product structure.
- Start a training program for team members on how to work with decoupled domains (TODO)
- Build tools that will make it easier to build decoupled domains through inversion of control (TODO)
- Introduce hexagonal architecture within the monolith
- Introduce clean architecture with one-way-dependencies and host application (TODO)
- Build abstractions that will make it possible to run and deploy domains separately (TODO)
Status
In progress.
- A working group Bounded Contexts
was concluded in April 2024 which defined a list of bounded contexts to be enforced for GitLab Rails domain and
infrastructure layer.
Decisions
- ADR-001: Modularize application domain? Start with modularizing
the application domain and infrastructure code.
- ADR-002: Define bounded context around feature categories as a SSoT in the code.
- ADR-003: Assign stewards to all modules and libraries.
Glossary
modules are Ruby modules and can be used to nest code hierarchically.namespaces are unique hierarchies of Ruby constants. For example, Ci:: but also Ci::JobArtifacts:: or Ci::Pipeline::Chain::.packages are Packwerk packages to group together related functionalities. These packages can be big or small depending on the design and architecture. Inside a package all constants (classes and modules) have the same namespace. For example:
- In a package
ci, all the classes would be nested under Ci:: namespace. There can be also nested namespaces like Ci::PipelineProcessing::.
- In a package
ci-pipeline_creation all classes are nested under Ci::PipelineCreation, like Ci::PipelineCreation::Chain::Command.
- In a package
ci a class named MergeRequests::UpdateHeadPipelineService would not be allowed because it would not match the package’s namespace.
- This can be enforced easily with Packwerk’s based RuboCop Cops.
bounded context is a top-level Packwerk package that represents a macro aspect of the domain. For example: Ci::, MergeRequests::, Packages::, etc.
- A bounded context is represented by a single Ruby module/namespace. For example,
Ci:: and not Ci::JobArtifacts::.
- A bounded context can be made of 1 or multiple Packwerk packages. Nested packages would be recommended if the domain is quite complex and we want to enforce privacy among all the implementation details. For example:
Ci::PipelineProcessing:: and Ci::PipelineCreation:: could be separate packages of the same bounded context and expose their public API while keeping implementation details private.
- A new bounded context like
RemoteDevelopment:: can be represented a single package while large and complex bounded contexts like Ci:: would need to be organized into smaller/nested packages.
References
List of references
The general steps of refactoring existing code to modularization could be:
Use the same namespace for all classes and modules related to the same bounded context.
Why? Without even a rough understanding of the domains at play in the codebase it is difficult to draw a plan. Having well namespaced code that everyone else can follow is also the pre-requisite for modularization. If a domain is already well namespaced and no similar or related namespaces exist, we can move directly to the next step.
Historical context Until May 2024 the GitLab codebase didn’t have a clear domain structure. We have forced the creation of some modules as a first step but we didn’t have a well defined strategy for doing it consistently.
The majority of the code was not properly namespaced and organized:
Ruby namespaces used didn’t always represent the SSoT. We had overlapping concepts spread across multiple namespaces. For example: Abuse:: and Spam:: or Security::Orchestration:: and Security::SecurityOrchestration.
Background This design document supersedes the previous Composable GitLab Codebase where we explored the idea of separating the codebase into technical runtime profiles: for example, run the monolith solely as a Sidekiq node. With a modular monolith and the use of an Hexagonal Architecture, we can achieve both separation of domains as well as separation of application adapters, which may include the usage of engines and/or different runtime profiles.
Summary TL;DR: Change the Rails monolith from a big ball of mud state to a modular monolith that uses an Hexagonal architecture (or ports and adapters architecture).
Context Before we modularize a codebase we first needed to define how we are going to divide it.
Decision We start by focusing on the application domain (backend business logic) leaving the application adapters (Web controllers and views, REST/GraphQL endpoints) outside the scope of the modularization initially.
The reasons for this are:
Code in application adapters may not always align with a specific domain. For example: a project settings endpoint or a merge request page contain references to many domains.
Context With the focus primarily on the application domain we needed to define how to modularize it.
Decision The application domain is divided into bounded contexts which define the top-level modules of GitLab application. The term bounded context is widely used in Domain-Driven Design.
Defining bounded contexts means to organize the code around product structure rather than organizational structure.
From the research in Proposal: split GitLab monolith into components it seems that following product categories, as a guideline, would be much better than translating organization structure into folder structure (for example, app/modules/verify/pipeline-execution/.
Context How do we assign stewardship to domain and platform modules? We have a large amount of shared code that does not have explicit stewards who can provide a vision and direction on that part of code.
Decision We use the term stewards instead of owners to be more in line with GitLab principle of everyone can contribute. Stewards are care takers of the code. They know how a specific functionality is designed and why.
Modularization of our monolith is a complex project. There will be many unknowns. One thing that can help us mitigate the risks and deliver key insights are Proof-of-Concepts that we could deliver early on, to better understand what will need to be done.
Inter-module communicaton A PoC that we plan to deliver is a PoC of inter-module communication. We do recognize the need to separate modules, but still allow them to communicate together using a well defined interface.
Related design docs Composable codebase design doc Related Issues Split GitLab monolith into components Make it simple to build and use “Decoupled Services” Use nested structure to organize CI classes Create new models / classes within a module / namespace Make teams to be maintainers of their code Add backend guide for Dependency Injection Internal Slack Channels #modular_monolith #architecture Reference Implementations / Guides Gusto / RubyAtScale:
RubyAtScale toolchain for modularization Gusto’s engineering blog Gradual modularization (successor to CBRA) Component-Based Rails Applications (“deprecated”) Shopify:
