Dropins

Drop-ins are modular configuration pieces or small software components designed to be added to an existing system to change behavior without editing the core code. They are typically stored in a designated directory or repository and loaded by the host system at startup or on demand, merging with the base configuration. The merge rules and the order of loading vary by system, but most implementations apply drop-ins in a defined sequence, with later files able to override earlier settings.

A widely cited example is systemd drop-ins. Systemd allows per-unit configuration fragments placed in a directory named unit.d, for example /etc/systemd/system/myservice.service.d/*.conf. Drop-ins can modify unit sections or add new ones, and the final effective configuration is the result of merging the main unit file with its drop-ins.

Other contexts use drop-ins in web servers, application frameworks, and configuration management tools, where include or override files are loaded to extend behavior without touching the primary configuration.

Advantages of drop-ins include safer experimentation, easier updates, and targeted overrides. Limitations include potential conflicts, debugging difficulties, and the need for consistent loading order across environments.

See also: plugins, extensions, include files, and configuration overrides.