Service tags in Symfony are far more than tools for event listeners. They enable true architectural decoupling, letting you build systems open for extension without modifying configuration files. This guide moves beyond basics to construct a Document Processing Pipeline using PHP 8.3 and modern attributes for a modular, maintainable codebase.

The core pattern uses #[TaggedIterator] to inject typed service collections. By implementing an interface with #[AutoconfigureTag], new document processors like PDF or CSV are added by creating a single class—no YAML edits required. The DocumentManager selects the correct processor at runtime, creating a clean strategy pattern that simplifies adding support for new formats.

For performance in large applications, #[TaggedLocator] enables lazy loading. Services are only instantiated when requested, conserving memory. You can also create custom attributes like #[AsDocumentProcessor] to attach domain-specific metadata (e.g., priority). Compiler Passes enforce rules during container compilation, like preventing duplicate types, catching errors before deployment.