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Shared Library Pattern

4 min readStatus:Seedling

Core Idea

The Shared Library Pattern involves extracting common functionality into compiled libraries (JAR, DLL, NPM packages) that multiple services or applications include as dependencies.

Definition

The Shared Library Pattern involves extracting common functionality into compiled libraries (JAR, DLL, NPM packages) that multiple services or applications include as dependencies. This pattern enables code reuse across distributed systems by packaging cross-cutting concerns, utility functions, or domain models into versioned artifacts consumed at compile-time or runtime. While conceptually appealing for reducing duplication, shared libraries introduce compile-time coupling that can undermine the independence and agility microservices architectures seek to achieve.

Key Characteristics

  • Compile-time dependencies: Services include library code directly, creating version coupling between consumers and the library
  • Version coordination challenges: Library updates require all dependent services to rebuild, retest, and redeploy to adopt changes
  • Temporal coupling: Teams cannot deploy independently when library changes affect multiple services simultaneously
  • Appropriate for infrastructure concerns: Well-suited for cross-cutting technical concerns like logging, monitoring, security, and serialization
  • Problematic for domain logic: Including business logic or domain models creates tight coupling and breaks bounded contexts
  • Version management complexity: Multiple library versions may coexist, requiring semantic versioning (MAJOR.MINOR.PATCH) to manage compatibility
  • Deployment cascade risk: Single library change can trigger synchronized deployments across multiple teams and services
  • Trade-off between DRY and coupling: Tension between Don’t Repeat Yourself principle and maintaining loose coupling in distributed systems

Examples

  • Logging frameworks: Log4j, SLF4J, or custom logging libraries providing standardized logging across all services
  • Serialization utilities: JSON/XML parsers, protocol buffer libraries, data transformation helpers
  • Security libraries: Authentication tokens, encryption utilities, authorization helpers shared across services
  • Monitoring and observability: Distributed tracing libraries, metrics collection, health check implementations
  • Client SDKs: Shared client libraries for calling other services (often considered an anti-pattern when containing domain logic)
  • Data validation: Common validation rules, though domain-specific validation should remain in bounded contexts

Why It Matters

The Shared Library Pattern represents a critical architectural trade-off in distributed systems. While it enables efficient code reuse for genuine cross-cutting concerns, overuse creates the very coupling microservices architectures aim to avoid. Understanding when shared libraries strengthen architecture (infrastructure concerns) versus when they create anti-patterns (domain logic sharing) is essential for maintaining service autonomy. The pattern’s coupling implications directly impact deployment independence, team autonomy, and system evolution speed—core benefits organizations seek from microservices. As Sam Newman emphasizes in Building Microservices, shared libraries introduce “subtle coupling” that erodes the boundaries between services, making seemingly independent services actually interdependent through versioned artifacts. Architects must weigh code duplication costs against coupling costs, often accepting strategic duplication to preserve service independence.

Sources

Note

This content was drafted with assistance from AI tools for research, organization, and initial content generation. All final content has been reviewed, fact-checked, and edited by the author to ensure accuracy and alignment with the author’s intentions and perspective.

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