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Code Reuse

Common business domain and infrastructure functionalities are typically shared across multiple components.

There are several ways in which these components can be shared, especially in distributed architectures.


Code Replication​

  • Shared code is copied into each service
  • Recommended to use it only in exceptional cases

Advantages:

  • Preserves the bounded context

Disadvantages:

  • Difficult to apply code changes
  • Code inconsistency across services
  • No versioning capabilities across services

Useful for:

  • Simple static code

Shared Library​

  • Code is shared in an external artifact (library)
  • Typically bound to the service at compile time

Dependency Management:

  • Sharing code into big common libraries (coarse-grained) is easier for dependency management, but it is not good for change control and maintainability
  • Breaking shared code into small functionality-based libraries (fine-grained) is better for change control and maintainability, but makes dependencies management difficult
  • It is generally recommended to favor change control over dependency management

Versioning:

  • Always use versioning
  • Version changes must be coordinated and communicated
  • Create a deprecation strategy
  • Serious defects or breaking changes to shared code causes all services to adopt the latest version of a shared library at once
  • Avoid the use of the LATEST version in service requirements

Advantages:

  • Ability to version change
  • Shared code is compile-based, reducing runtime errors
  • Operational characteristics (e.g. performance, scalability, fault tolerance) are not impacted
  • Risk of breaking other services with a change to common code is low because of versioning

Disadvantages:

  • Dependencies can be difficult to manage
  • Code duplication in heterogeneous codebases
  • Version deprecation and communication can be difficult

Useful for:

  • Homogeneous environments where shared code change is low

Shared Service​

  • Places shared functionality in a separately deployed service

Versioning:

  • Use API endpoint versioning for RESTful APIs
  • May be harder to implement for other protocols (e.g. gRPC, messaging)

Performance:

  • Adds network latency
  • Use of gRPC and messaging can help mitigate some of the performance issues

Advantages:

  • Good for high code volatility
  • No code duplication in heterogeneous codebases
  • Preserves the bounded context
  • No static code sharing

Disadvantages:

  • Versioning changes can be difficult
  • Performance is impacted due to latency
  • Fault tolerance and availability issues due to service dependency
  • Scalability and throughput issues due to service dependency
  • Increased risk due to runtime changes

Useful for:

  • Highly polyglot environments
  • When shared functionality tends to change often

Sidecar​

  • Decouples the domain logic from the technical logic
  • Same principles may be applied to service mesh

Advantages:

  • Consistent way to create isolated coupling
  • Consistent infrastructure coordination

Disadvantages:

  • Sidecar component may grow large/complex

Useful for:

  • Operational coupling