Dependency

Dependency is a relationship in which one component relies on another for input, functionality, or fulfillment of its purpose. In software engineering, a dependency is a library, framework, module, or service that a program requires to compile, build, or run. Dependencies can be direct (explicitly declared by the project) or transitive (brought in by other dependencies). They can be required at build time, at runtime, or for testing, and may be mandatory or optional.

Dependency management involves declaring constraints on versions, resolving compatible combinations, and producing a closed set of components for a given build. A dependency graph represents the relationships among all involved components; cycles or conflicts in this graph can cause dependency hell, complicating upgrades and leading to duplicated code or incompatible behavior.

Versioning schemes, such as semantic versioning, help communicate compatibility. Tools known as package managers or build systems use lockfiles or repository indices to ensure reproducible builds by fixing exact versions. Practices include pinning exact versions, avoiding broad version ranges, and regularly updating dependencies, while monitoring for security vulnerabilities and licensing issues. Techniques like dependency injection focus on decoupling components and improving testability by deferring bindings to runtime.

In everyday use, dependencies are ubiquitous in software ecosystems. Managing them effectively—minimizing unnecessary dependencies, controlling transitive footprints, and enforcing clear licensing and security practices—helps maintain reliability, performance, and maintainability.