Systems Engineering: Monolithic vs Microservices Architecture

Introduction to Software Architecture

The software architecture of a system represents the design decisions related to overall system structure and behavior.

Architecture helps stakeholders understand and analyze how the system will achieve essential qualities such as modifiability, availability, and security.

The most common architectural patterns practiced in modern software engineering are Monolithic and Microservices architecture. We'll explore these two to learn about their pros and cons and how to choose which architecture is suitable for your next project.

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Monolithic Architecture

A monolithic architecture is a traditional software architecture approach, built as a unified unit that is self-contained and independent from other applications.

A monolithic architecture is a singular, large computing network with one codebase that couples all of the business concerns together.

Pros

  • Single codebase - In monoliths, all actions happen in one codebase, so you don't have to shuffle between multiple codebases during development.
  • Easy to develop - It's easier to develop because it's just a single codebase using traditional forms of development.
  • Easy to deploy - It's easier to deploy because it's just a single directory.
  • Easy debugging - Due to the fact that all code is in one place, it's easier to trace any bug and fix issues quicker.
  • Simplified testing - E2E testing can be carried out faster because of the single-unit nature of monoliths.
  • Single technology stack - Monoliths are usually built with one programming language, which makes it easier to maintain and update.

Cons

  • Slow development - It's difficult to understand and maintain due to its size and complexity.
  • Single tech stack - One tech stack might not be suitable to solve some specific problems because of the limits of the technology.
  • Scalability - No possibility of scaling individual components.
  • Fault tolerance and reliability - An error in one component can affect the entire app.
  • Inefficient deployment - A small change to the system will require a redeployment of the entire system.

Microservices Architecture

Microservices architecture, also known as microservices, is an architectural approach that structures a server application as a collection of small, independently deployable services. The microservices architecture is mainly oriented to the back-end.

Each service has its own data and domain logic, and services communicate using protocols such as HTTP/HTTPS, WebSockets, or Advanced Message Queuing Protocol (AMQP).

API communication could be direct client-to-server or via an API gateway pattern.

Microservices Communication Types

sync-async
  • Synchronous: Microservices can use HTTP, a synchronous communication protocol that uses the request-response pattern.
  • Asynchronous: Microservices can also use events/message queue patterns to communicate between services. The client sends a request but does not wait for a response from the services. It uses AMQP with message brokers like Kafka and RabbitMQ, and communication can be one-to-one or one-to-many.

Communication Styles

Request-response communication:

req-res

This traditional form of communication happens over HTTP/HTTPS protocols. In this style, the client sends a request to the server, which processes the request and sends a response back. This form of communication is synchronous.

Push or real-time communication:

req-res

This form of communication is implemented by WebSockets, where the server sends a response to the client without waiting for the client to initiate a request.

Pub/sub communication:

req-res

This form of communication is implemented by Advanced Message Queuing Protocols (AMQPs). This style allows asynchronous communication between decoupled services with latencies on the order of 100 milliseconds. Pub/Sub lets you create systems of event producers and consumers, called publishers and subscribers. Publishers communicate with subscribers asynchronously by broadcasting events rather than by synchronous remote procedure calls (RPCs).

Pros

  • Scalability - Microservices allow each service to be independently scaled to meet demand for the application feature it supports.
  • Faster development - Teams can work separately on individual services, leading to faster development cycles.
  • Flexibility in technology stack - This allows teams to choose the best tools for specific tasks.
  • Fault isolation - Microservices are designed to be fault-tolerant. One service can go down without bringing down the whole app.
  • Maintainability - Smaller services are easier to maintain, understand, and update.
  • Isolation of data - Microservices can have their own data storage solutions, leading to improved data isolation, security, and database performance.
  • Enhanced monitoring and observability - Each service can have its own monitoring and observability tools, making it easier to track performance, troubleshoot issues, and gather insights into the system's behavior.
  • Easy deployment - Microservices enable continuous integration and continuous delivery.

Cons

  • Operational complexity - Managing many microservices can be complex in terms of infrastructure, tools for deployment, monitoring, etc.
  • Interservice communication - Microservices rely on network communication, which can have issues with latency, reliability, and data consistency between services.
  • Testing and debugging - It can be time-consuming and difficult.
  • Security and compliance - Security can be more challenging in a microservices environment due to potential entry points for security breaches.
  • Monitoring and observability - Monitoring and tracking the behavior of multiple services can be complex.

Making Software Architectural Decisions

Choosing between monolithic and microservices architectures depends on the specific requirements and context of your application. Here are some considerations to guide your decision:

  • Size and complexity of the system: If your system is going to be small, developed by a small team, uses one technology, and won't grow to become bloated, then a monolithic architecture may be suitable. Monolithic architectures are straightforward and can be easier to manage for small-scale applications.

  • Development team size: Smaller teams might benefit from the simplicity of monolithic architecture, while larger teams can leverage the modularity of microservices to work on different services independently.

  • Technology stack: If you plan to use a single technology stack and don't foresee the need for multiple technologies, a monolithic architecture can be simpler. However, if you need to use different technologies for different parts of the system, microservices offer the flexibility to choose the best tools for each task.

  • Scalability requirements: Microservices allow for independent scaling of services, making them a better choice for applications with varying loads on different components. Monolithic architectures can be challenging to scale as the entire application needs to be scaled together.

  • Fault tolerance and reliability: Microservices are designed to isolate faults, so an issue in one service doesn't necessarily bring down the entire system. Monolithic systems, on the other hand, are more prone to cascading failures.

  • Deployment and delivery: Microservices enable continuous integration and continuous delivery, allowing for more frequent updates and faster time-to-market. Monolithic systems might require longer deployment cycles.

  • Evolving requirements: If you anticipate that your application will evolve significantly over time, starting with a monolithic architecture and later refactoring to microservices can be a practical approach. This allows you to manage initial complexity while setting the stage for future scalability and flexibility.

Most modern enterprise applications tend to use microservices because their benefits often outweigh their cons. However, not all applications need microservices. You can start building a great, scalable app with a monolithic architecture and later decide to refactor into a microservices architecture as the project becomes more complex.

Summary

Monolithic and microservices architectures each have their strengths and weaknesses. Monolithic architecture offers simplicity and ease of development, making it suitable for small to medium-sized applications with limited scalability needs. In contrast, microservices architecture provides scalability, fault isolation, and technological flexibility, making it ideal for complex, large-scale applications. The choice between the two should be guided by the specific needs and context of your project, keeping in mind factors such as team size, scalability requirements, and future growth.

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