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Hello everyone! For RailsConf 2020, my name is Vladimir, and I'm going to talk about what lies between monoliths and microservices.
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Rails applications tend to turn into giant monoliths, and keeping the codebase maintainable usually requires architectural changes.
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Microservices? A distributed monolith is still a monolith. So, what about breaking the code into pieces and rebuilding a monolithic puzzle out of them?
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In Rails, we have the right tool for the job: engines. With engines, you can break your application into components—the same way Rails combines all its parts, which are engines, too.
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However, this path is full of snares and pitfalls. Let me be your guide on this journey and share the story of monolith engine-ification.
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A typical Rails application is often doomed to become a monolithic monster. The question is: how do we prevent this?
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We need to keep our applications maintainable as they grow, and make developers happy to work with them. One may suggest, 'Why not just split it into microservices?' But that hype train is starting to derail because we know that microservices usually end up becoming a distributed monolith, which is just as daunting to manage.
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The issue with both monoliths and distributed monoliths lies in forgetting the core principle of separation. We need to split the application into logically independent units, components that are connected together, almost like a kind of bus.
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The idea is to maintain loose coupling, which means that if one component is broken, it doesn’t affect the others. Refactoring becomes much easier this way, unlike with monoliths, where if there's a bug, fixing it can be slow and painful. We need a system where we can guarantee that a bug won't reappear in the next version.
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Today, we are going to discuss component-based monoliths or modular monoliths. A particular example of such a model is Shopify.
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Shopify has a great article describing their architecture, showcasing components with a gem-like structure inside their application.
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However, the problem with Shopify's approach is that we cannot directly apply it to our applications because their solution hasn't been open-sourced yet. Yet, we can still borrow the idea, and that's what we'll do during this talk.
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Let's also look at another Ruby framework that allows you to use component-based architecture. They call themselves Molly Thirst, which means you can start with a single component but add other components later as your application grows.
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Another example is umbrella projects in Elixir, which actually date back to Perl, but they share the same idea: multiple logical applications working as a single system.
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In Rails, we also have this concept, known as engines, and that's what we're going to discuss today.
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Let me take a quick detour and talk about myself a little. How did I end up here, speaking about component-based architecture in Rails?
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You can find me on GitHub under the name Falcon. I'm working at a company called Evil Martians, which has been off in Ramallah for over ten years. We have bases around the world with agents scattered across various continents.
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You've probably heard of open-source projects from the Ruby world, development tools, and our beautiful blog with a series of technical posts.
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Today, however, I'm not talking about that; I'm sharing the darker side. A large part of my everyday life is working on commercial projects, and about half of all our clients use Rails as their core technology.
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Today, I want to share one particular case study: a story involving a project called Common and Cocaine. This is where we started actively using component-based architecture.
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Let’s take a quick overview of what these services are and why we decided to use this architecture. Common assault started as a long-term Cleveland rental service working in major cities in the U.S. and Canada.
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Before we joined their team, they only had a management application—managing properties, losses, billing staff, and so on.
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Our task was to build a community portal for users, and one of the requirements was that we needed to do this within the same Rails app.
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They didn’t want to go with microservices right away, and that was a wise decision, considering that most of the logic was implemented in this Rails monolith.
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This was when the model started to risk becoming a monster if we didn't handle it correctly.
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So, what exactly is a community portal? It’s a mix of something like Meetup.com, a chat platform, and other smaller features.
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The components are almost independent of each other, even at a logical level, as they have well-defined boundaries.
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That’s why, from day one, we started using namespaces to isolate the lexical and structural levels of all the code related to a particular feature or domain.
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Even though we don’t have namespaces in Ruby, we can use modules to contain all the code. This is a very fast way of creating a pseudo-component-based architecture.
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However, we cannot guarantee that code from one namespace doesn't access the code from a different namespace, which it shouldn’t.
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This was pretty much how we established the MVP, the minimum viable product. It was an initial version, but then the requirements evolved.
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It became clear that we were going to build some spin-off projects and other community portals with very similar functionality.
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We thought we could reuse most of the feature code, but we needed a way to ensure isolation and portability.
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That’s when we began to use engines and project gems. This became the final architecture for the project, which I will explain in more detail later.
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One key point that motivated us to make this decision was the modular model of architecture. This is a strong starting point to learn about component-based architecture in Rails and identify potential initial problems.
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It turned out that there were many more pitfalls, and that is what I would like to address today.
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We’ll dive into the technical details of how to make engines work well in a component-based system.
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As I mentioned, we used engines and also local gems, which we call ‘soul gems’—exclusive to this application, similar to a mono-repo without public publishing.
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We refer to both concepts as engines, so let’s start with an engine. What is a Rails engine?
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Simply put, it’s like any other gem that you can generate by running 'rails plugin new'.
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However, let’s go a bit deeper. We need to understand what an engine is before diving into the details.
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First, an engine is indeed a gem. You can build it and publish it like any other gem.
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It has a gem specification that lists both its required dependencies and the lib folder that contains all the code.
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What differentiates an engine from a typical gem is that it includes some Rails-specific entities.
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It has your app folder with controllers, models, and other standard components.
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The idea is that when you add the engine to a Rails application, its content is accessible through the outer loading mechanism.
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You can just use these constants without having to require them explicitly.
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Engines also come with their own test suites, allowing us to test them in isolation.
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Additionally, they have their own class in the namespace that acts as a glue between the Rails application and the engine’s functionality.
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You can add initializers, configurations, and other elements similar to those in a Rails application.
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Lastly, you can add routes to the engine and use those routes in your main application.
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Have you ever seen engines in real life? Of course, you have! These are some of the most popular engines that are part of Rails applications.
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Rails indeed offers a component-based architecture with many full-featured engines, in addition to Rails itself.
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One popular engine outside the Rails world is Devise. Many of us use Devise or its extensions to manage user authentication.
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Thus, using engines can lead to happiness and help in building component-based applications.
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However, the process of engine-ification in applications is not always straightforward.
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Realizing what an engine is is just the beginning; it hides many complexities and problems that you are likely to face.
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I won’t cover all of these issues today, but I will highlight some of the more interesting ones.
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Let’s begin discussing the architecture of the apple head. Here are a few pictures of what the final result was.
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We had an 'engines' folder with a few engine gems, while another folder consisted only of mount directives.
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Our application.js file contained just these mount directives.
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In addition, our gem files contained definitions pointing to our local engines, utilizing half of the gem specification.
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The application diagram, as a components diagram, could appear as a unified structure, representing our approach.
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The components implementing the business logic share similar naming conventions, often using a verb with a suffix.
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We also employed a core engine, which included basic models like users and generic entities.
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Another layer utilized this core engine as a dependency and included other layers, which also represented the engines.
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We had two utility engines for search proxy and common events, which will be discussed later.
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Finally, we formed one umbrella engine which integrated all the implementations.
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Let’s begin investigating the journey from here. Our first stop is dependency management.
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This is essentially the first problem you will encounter when using engines. There are many questions that arise when you start to extract functionality into isolated gems.
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The first issue occurs when you have non-Rubygems dependencies. Each gem may need to load from GitHub, local sources, or other repositories.
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You cannot specify these dependencies in the gem file, so you end up duplicating definitions in your root gem file.
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This makes it difficult to maintain and synchronize dependencies, and it's easy to overlook some dependencies.
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As a solution, we evolved a gem file structure that enhances functionality with bundler.
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Every engine has a runtime gem file that includes non-Ruby gem dependencies required at runtime and another for development dependencies.
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This way, we can ensure that our dependencies are synchronized cleanly.
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Another concern is sharing common dependencies between engines without duplication.
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The approach is similar: we maintain shared gem files for synchronizing Rails versions across engines.
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In the root gem file, we reference the engine gem files and maintain them consistently.
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But what about lock files? Should the versions in the root application and engines be the same?
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Initially, we didn’t care about consistency, but we kept encountering inconsistencies.
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We realized that most were either tied to development dependencies or batch version differences.
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We started thinking about a better way to maintain synchronization, resulting in a unified lock file.
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Now, instead of having separate lock files for engines, we use a root-level lock file, encompassing all engines.
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This presents its limitations and potential complications, particularly in managing development and test dependencies.
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We didn’t want development test dependencies to load in production.
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Moreover, while testing engines, we often load everything from all applications, which is not ideal.
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We resolved this using a helper method in the gem file, called 'component,' which serves two portfolios.
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First, it adds a gem to the root gem file; the second part allows adding development dependencies under a named group.
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This way, we ensure that the engine’s dependencies don’t pollute the main application.
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Next, let’s discuss database management. What makes databases interesting within engines?
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Well, we need a way to utilize migrations efficiently within the main app and how to handle seeds.
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The standard way with migrations usually involves copying them into the main app's migrations directory.
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However, in a component-based architecture, we would rather avoid the extraction and instead keep the codebase together.
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We prefer to make the main app recognize engine migrations by tweaking two parameters in the Rails migration system.
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Essentially, we need to point the main app to look for migrations in both its own directory and the engines' directories.
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This allows us to conduct seamless migrations without copying any files.
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The same approach can be applied for seeds, as engines already come with 'load_seed' methods that can be utilized.
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This allows us to run the engine’s seeds within the main application effectively.
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Now, let's address testing: How do we ensure engines are tested in isolation?
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To do this, we rely on a tool called 'combustion,' which streamlines testing engines within their gem.
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Combustion generates a minimal Rails app, enabling tests without unnecessary components.
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Another relevant question is how to run engine tests on CI.
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This architecture can complicate CI configuration, but on the other hand, it allows us to skip tests for unchanged engines.
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Engines with unchanged code or dependencies can be safely skipped during testing, saving time.
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To implement this, we developed a simple script that identifies dirty engines or changed dependencies.
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The script builds an inverted index and checks for modifications compared to the previous state of the engines.
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When integrating this into CI, it executes the script first; if it returns zero, we run the tests.
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If it returns one, we skip the tests.
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Next, we face the question of accessing entities across engines or the main app.
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How do we deal with different entities across engines?
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We came up with an idea called base and behavior convention that allows for flexible entity handling.
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For example, an application controller in one engine can inherit from a configurable base controller defined outside.
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This base controller can specify required behaviors, offering flexibility in your engine.
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In practical terms, this translates to a custom application controller in the main app implementing additional behavior.
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Another common scenario is extending models from other engines using concerns.
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We utilize ActiveSupport's loading hooks to ensure that when a model is accessed, any registered concerns are included.
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Now, let’s discuss communication between engines.
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How do we manage notifications when a user registers in one engine that affects another?
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In this scenario, we want newly registered users to join a default chat within the chat engine.
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Here lies the crux: the registration process occurs in the connect by engine, while the chat logic belongs in another.
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Our solution was to use an event-based system and a publish/subscribe mechanism.
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Though many tools exist for these purposes, we chose Rails as our base framework, leveraging its features.
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This integrates seamlessly with ActiveRecord, allowing synchronous and asynchronous event consumption.
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The result is improved event-handling capabilities that are encapsulated within custom gems.
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Moving on, let’s address the critical topic of keeping the main app intact while embracing microservices.
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As we migrated towards a component-based system using engines, it raises the question of what should remain in the main application.
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It’s impractical to transform the main app into a ‘zero code’ structure even when components are independent.
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You still need configurations for the database, certain generative processes, and maybe other library integrations.
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In ideal scenarios, you might function from a zero-code perspective, especially for applications built from scratch.
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Still, common applications will tend to retain essential logic directly within the main application.
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What definitely resides in the primary app includes feature/system tests. You should always write integration tests for the system as a whole.
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Instrumentation, error handling, and basic configuration for the Rails framework must remain.
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Lastly, let’s discuss gems and local gems, which we widely use to tackle different application problems.
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By organizing code unrelated to business logic into reusable components, we enable efficient functionality across applications.
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This becomes especially valuable with our event store gem, which initially served us internally before evolving into an open-sourced solution.
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The benefits of local gems include not only better structure but also isolated tests that can significantly reduce CI build times.
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That’s a quick overview of engines and gem self-rejection. If you noticed, we’ve only covered half of the map, but I hope I provided valuable insights.
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In conclusion, using engines can prevent monoliths from becoming monsters. They come with plenty of benefits.
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One significant benefit is the option to extract engines into microservices in the future.
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It's essential to acknowledge that third-party gems often lack sufficient support for these practices.
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Here’s a link to the resources, scripts, and other useful materials from this talk. Feel free to use them.
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Thank you, and remember, don't let your monoliths turn into monsters!
