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Example Decision

ADR-0001: Example Architecture Decision

Status

Status: proposed | Date: 2023-12-15

Deciders: @lead-architect, @tech-lead, @senior-developer

Technical Story: [TECH-123](https://jira.example.com/browse/TECH-123)

Business Story: [BIZ-456](https://jira.example.com/browse/BIZ-456)

Context and Problem Statement

Our application currently uses a monolithic architecture which is becoming difficult to scale and maintain as the system grows. We need to decide on an approach to break down the monolith into more manageable components.

  • The current monolithic application handles user authentication, business logic, and data storage in a tightly coupled manner
  • Development velocity has decreased as the codebase grows
  • Scaling the application requires scaling the entire monolith, even when only specific components need additional resources
  • Deploying changes is becoming risky and time-consuming

Decision Drivers

Business Drivers

Business factors influencing this decision:

  • Cost Efficiency: Need to optimize infrastructure costs by scaling components independently
  • Team Structure: Multiple teams need to work on different parts of the application simultaneously
  • Time to Market: Need to reduce deployment cycles and enable faster feature delivery

Technical Drivers

Technical factors influencing this decision:

  • Scalability: Need to scale different components independently based on load
  • Data Management: Need to isolate data access patterns for different components
  • Cloud Migration: Planning to move from on-premises to cloud infrastructure

Constraints

Constraints that limit our options:

  • Budget: Limited budget for the transformation project
  • Timeline: Need to complete the initial phase within 6 months
  • Legacy Integration: Must maintain compatibility with existing legacy systems

Objectives

What we aim to achieve with this decision:

  • Improved Scalability: Enable independent scaling of application components
  • Enhanced Maintainability: Make the codebase more manageable and easier to understand
  • Faster Deployments: Reduce deployment time and risk

Forces

Forces at play that influence the decision:

  • Technical Debt: Accumulated technical debt in the current monolith
  • Team Expertise: Current team expertise in different architectural patterns
  • Operational Complexity: Trade-off between development simplicity and operational complexity

Considered Options

Options considered to address the problem:

Option 1: Microservices Architecture

Decompose the application into small, independently deployable services that communicate over a network.

Pros

Advantages of this option:

  • Independent Scaling: Each service can be scaled independently
  • Technology Diversity: Different services can use different technologies
  • Team Autonomy: Teams can develop, deploy, and scale their services independently

Cons

Disadvantages of this option:

  • Operational Complexity: Increased complexity in deployment, monitoring, and management
  • Network Latency: Communication between services introduces network latency
  • Learning Curve: Steep learning curve for the development team

Mitigation Strategies

Strategies to mitigate the cons:

  • For Operational Complexity: Invest in robust DevOps practices and tools
  • For Network Latency: Implement caching and asynchronous communication where appropriate
  • For Learning Curve: Provide training and start with a small pilot project

Option 2: Modular Monolith

Restructure the monolith into well-defined modules with clear boundaries while keeping it as a single deployable unit.

Pros

Advantages of this option:

  • Simplified Operations: Single deployment unit is easier to manage
  • Lower Complexity: Avoids the distributed systems challenges of microservices
  • Easier Transition: More gradual path from the current monolith

Cons

Disadvantages of this option:

  • Limited Scaling: Still need to scale the entire application
  • Technology Constraints: All modules must use compatible technologies
  • Deployment Coupling: Changes to one module require redeploying everything

Mitigation Strategies

Strategies to mitigate the cons:

  • For Limited Scaling: Design modules to be stateless where possible
  • For Technology Constraints: Establish clear interfaces between modules
  • For Deployment Coupling: Implement feature toggles and robust testing

Option 3: Service-Oriented Architecture (SOA)

Organize the application into medium-grained services that share a common communication layer.

Pros

Advantages of this option:

  • Balanced Approach: Middle ground between monolith and microservices
  • Reuse: Services can be reused across different parts of the application
  • Governance: Centralized governance and standards

Cons

Disadvantages of this option:

  • Complex Middleware: Often requires complex middleware (ESB)
  • Potential Bottlenecks: Shared services can become bottlenecks
  • Integration Challenges: Service integration can be complex

Mitigation Strategies

Strategies to mitigate the cons:

  • For Complex Middleware: Use lightweight integration approaches where possible
  • For Potential Bottlenecks: Design for appropriate granularity and scalability
  • For Integration Challenges: Establish clear service contracts and versioning policies

Trade-off Analysis

Technical Trade-offs

Analysis of technical trade-offs between options:

Criteria Microservices Modular Monolith SOA
Scalability High Medium Medium-High
Deployment Independence High Low Medium
Development Complexity High Medium Medium-High
Operational Overhead High Low Medium
Technology Flexibility High Low Medium

Business Trade-offs

Analysis of business trade-offs between options:

Criteria Microservices Modular Monolith SOA
Cost High Low Medium
Time to Market Slow initially, faster later Medium Medium
Business Value High Medium Medium-High
Risk High Low Medium
Scalability High Medium Medium-High

Decision Outcome

Chosen option: [Modular Monolith]

Technical Justification

Why this option is technically superior:

  • Balanced Approach: Provides a good balance between modularity and operational simplicity
  • Evolutionary Path: Offers a clear evolutionary path toward microservices if needed in the future
  • Team Readiness: Better aligns with the current team's expertise and capabilities

Business Justification

Why this option aligns with business goals:

  • Cost-Effective: Requires less investment in new infrastructure and operational tools
  • Faster Delivery: Can be implemented more quickly with less disruption
  • Risk Management: Presents lower risk compared to a full microservices transformation

Consequences

Positive Consequences

Positive outcomes we expect:

  • Improved Code Organization: Clearer boundaries between different parts of the application
  • Better Maintainability: Easier to understand and modify specific modules
  • Gradual Evolution: Ability to evolve toward microservices incrementally if needed

Negative Consequences

Negative outcomes we accept:

  • Limited Independent Scaling: Still need to scale the application as a whole
  • Deployment Coupling: Changes to one module still require full redeployment
  • Technology Constraints: Limited ability to use different technologies for different modules

Implementation

How we plan to implement this decision:

  • Step 1: Identify module boundaries based on business capabilities
  • Step 2: Refactor the codebase to respect these boundaries
  • Step 3: Implement internal APIs between modules
  • Step 4: Establish governance for module interfaces and dependencies

Other ADRs related to this decision:

Notes

Additional notes and references: